PART I: INTRODUCTION AND OVERVIEW 2.1. Introduction · PDF filecomparisons of timeseries of rainfall and dust concentrations over the Atlantic (Prospero and Nees, 1986) or the frequency

Chapter 2: Climate change in the Sahel - the scientific context

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2. Climate change in the Sahel – the scientific context

PART I: INTRODUCTION AND OVERVIEW

2.1. Introduction

The primary objective of this thesis is to investigate the links between the production and

distribution of atmospheric mineral dust and climate in the Sahel-Sahara zone of Africa.

Climatic regulation of atmospheric dust concentrations and the subsequent impact of dust on

the structure and motion of the atmosphere are investigated. The emphasis of the thesis is on

the potential of dust to modulate the conditions which are necessary for rainfall generation

over the Sahel, the role of rainfall in modulating dust production via its effects on the land

surface, and the extent to which variability in atmospheric dust concentrations is determined

by atmospheric processes rather than land surface conditions.

Observed changes in rainfall indicate that the Sahel may have undergone a climatic shift in

the last half of the twentieth century (Chapter 1). Although increased dust levels over the

region during the same period have generally been ascribed to drought and land degradation,

the hypothesised mechanisms linking these phenomena have not been well studied (Jacob et

al., 1995). Investigations into the relationship between dust and rainfall have been limited to

comparisons of timeseries of rainfall and dust concentrations over the Atlantic (Prospero and

Nees, 1986) or the frequency of dust events in West Africa (Middleton, 1985; N’Tchayi et

al., 1994, 1997). It is plausible that at least some of the observed increase in atmospheric

dust loadings is the result of changes in the atmospheric mobilisation, transport and removal

mechanisms associated with the postulated regional climate change (Middleton, 1985;

Prospero and Nees, 1986; N’Tchayi et al., 1997). As mentioned in Chapter 1, the changes in

dust loadings over the Sahel may further modulate the regional climate, for example via a

weakening of the regional and/or local scale convection arising from alterations to the

radiative structure of the atmosphere.

The mechanisms which mobilise and transport dust, and any subsequent modulation of the

climate by atmospheric mineral aerosols, will operate within the context of the regional

general circulation. Any changes in these processes must therefore be considered within the

context of regional climate change, which must in turn be understood in relation to changes

in the global climate. For these reasons, this chapter reviews the research into Sahelian

climate variability on a range of timescales and as a response to various different processes.


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Much of the work reviewed has been carried out as a direct result of a desire to understand

processes related to drought in the Sahel, whereas some has been undertaken independently

of studies of Sahelian climate.

The development of ideas concerning mechanisms of rainfall modulation is discussed in the

Overview (Section 2.2). Part II of the chapter addresses possible mechanisms of rainfall

modulation in the Sahel. The palaeoclimatic and historical context of the recent dry episode

is summarised, after which the issue of land surface change is discussed. The research into

changes in global temperature patterns and their correlations with Sahel rainfall is

summarised, with particular attention being paid to sea surface temperature patterns and

variations in the inter-hemispheric temperature contrast. The impact of sulphate and other

aerosols is discussed briefly in this section, due to the possible effect of such particles on

hemispheric-scale temperature patterns, which are associated with variations in Sahelian

rainfall. Relationships between Sahel rainfall and the regional and global scale atmospheric

circulation is broadly reviewed. Following a summary of the known and postulated patterns

and processes of climate change related to Sahelian desiccation, the relation of tropospheric

dust aerosols originating in the Sahel-Sahara region to the regional climate is addressed in

Part III. Sources, transport, distribution and physical and radiative properties of dust aerosols

are discussed, as is their potential role in modifying the tropospheric radiation budget and

regional circulation over northern Africa and the tropical North Atlantic.

2.2. Overview

The nature of the recent period of reduced rainfall in the Sahel has been discussed in Chapter

1. The duration of this dry episode has given rise to concern that the Sahel region may be

experiencing a “climatic step” towards a more arid regime (e.g. Kidson, 1977; Rognon,

1987). It was speculated as early as 1975 that aridification in the Sahel might represent a new

mean regional climate, perhaps as the result of anthropogenic influences on the global

atmosphere or regional land-atmosphere interaction (Dyson-Hudson and Dyson-Hudson,

1975; Kelly, 1975; Newman, 1975). Such concerns have encouraged various authors (e.g.

Nicholson, 1976, 1978) to investigate the historical precedents for such a dry episode.

However, such studies suffer from the scarcity of historical and palaeoclimatic data, as well

as from the poor temporal resolution inherent in these data and the difficulty in quantifying

changes in rainfall before the beginning of the instrumental period. (These issues are

discussed in detail in Chapter 4.) An understanding of the nature of the dry episode must

therefore be based on an appreciation of the mechanisms behind the change in the rainfall

regime.


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Early studies of rainfall changes in semi-arid regions such as the Sahel emphasised the

potential of changes in vegetation cover to lead to changes in the processes governing heat

and moisture exchanges between the land and atmosphere (Charney et al., 1975). Such

studies assumed that changes in ground cover, arising from drought and/or the removal of

vegetation by humans and animals, would be widespread enough to cause changes in the

atmospheric heat and moisture budgets on a large scale. However, many authors have

regarded the presumed extent of such hypothetical changes as unrealistic (Hulme and Kelly,

1993; Mortimore, 1998). Indeed, assumptions of widespread land degradation throughout

the Sahel are largely unfounded, as data are available for only a limited number of locations

which are not necessarily representative of the region as a whole (Mace, 1991; Stocking,

1996; Williams and Balling, 1996). The issue of land degradation will be addressed in more

detail in Section 2.4, and also in Chapters 5 and 6, as it is directly relevant to the analyses

presented in these chapters.

Other early investigations into the recent period of desiccation associated dry conditions in

the Sahel with large-scale atmospheric circulation patterns (Kidson, 1977; Lamb, 1978).

Kidson (1977) linked drought-related changes in the regional circulation with global

atmospheric changes. Such large-scale changes in the atmospheric circulation appear to have

persisted throughout much of the period of desiccation (Shinoda, 1990). The implication of

this is that the multi-decadal scale dry episode witnessed recently in the Sahel is at least

partly the result of a (perhaps temporary) shift in the global circulation. Associated with such

atmospheric circulation changes are changes in patterns of sea surface temperatures (SSTs).

Many investigations have concentrated on the association of dry years in the Sahel with

particular modes of SST variation both within particular ocean regions and on a hemispheric

and global scale (e.g. Folland et al., 1986; Ward, 1998). The scenario in which drought in the

Sahel is linked with changes in atmospheric circulation and particular SST patterns is

supported by work such as that of Newell and Hsiung (1987), Street-Perrott and Perrott

(1990), and Fontaine and Bigot (1993), who report different modes of SST variation and

changes in wind fields before and after the onset of the dry episode. It may also be

hypothesised that the onset of dry conditions was due to large-scale atmosphere-ocean

dynamics, but that the continuation of drought conditions has been due to regional feedback

processes, which will continue to reinforce drought conditions until a large-scale circulation

anomaly occurs which is of sufficient magnitude and of the appropriate nature (i.e. opposite

in sense to that which triggered the dry conditions) to overcome the feedback processes and

re-establish a wetter regime (e.g. Nicholson, 1995; Shukla, 1995).


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As discussed in Chapter 1, annual rainfall in the Sahel exhibits far more persistence after

1950 than in the previous half of the twentieth century (Nicholson, 1995; for a further

discussion, see Chapter 4). This suggests that either (i) the mechanisms modulating Sahelian

rainfall are less variable on a year-to-year timescale in the latter half of this century

compared to the former, (ii) regional feedback processes have acted so as to reinforce

drought conditions, or (iii) a different regional or global climatic regime acts in concert with

regional feedback mechanisms (perhaps themselves directly dependent on the changes in

circulation) to sustain the dry episode. This thesis will investigate the possible role of

atmospheric Saharan and Sahelian dust as such a feedback mechanism.

Since the onset of drought conditions in the Sahel in the late 1960s, the amount of dust in the

atmosphere over the region and exported from northern Africa over the Atlantic has

increased dramatically (Middleton, 1985; Prospero and Ness, 1986; Goudie and Middleton,

1992; N’Tchayi et al., 1994, 1997). Regardless of the nature of the mechanisms associated

with the onset and continuation of dry conditions, it is to be expected that the presence of

atmospheric dust will have a significant effect on the regional climate via the modification of

the atmospheric radiation budget (e.g. Fouquart et al., 1987; Chiapello et al., 1997; Alpert et

al., 1998). The presence of atmospheric dust over continental North Africa is expected to

result in warming of the troposphere and cooling of the underlying land surface, although the

net effect on the regional atmospheric system may be one of warming due to decreased

reflectivity of the land-atmosphere system (Carlson and Benjamin, 1980). Tropospheric

heating and surface cooling in desert regions will weaken the vertical temperature gradient

and reduce convection: Carlson and Benjamin (1980) state that significant dust events are

often accompanied by suppression of convective development of cloud. If heating of the

continental land surface is reduced, the temperature contrast between continental West

Africa and the tropical Atlantic Ocean will also be reduced. Reduced land-ocean temperature

contrast just before or during the wet season will act to reduce the land-ocean pressure

contrast, moisture convergence, precipitation and precipitation minus evaporation over areas

of North Africa (see Coe, 1997, for a discussion of the implications of changes in heating

due, in this case, to changes in orbital parameters for Holocene North Africa).


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PART II: MECHANISMS OF SAHELIAN RAINFALL MODULATION

2.3. Historical and palaeoclimatic context

Much research has been carried out into the palaeoenvironment and palaeoclimate of

northern Africa in the recent geological past, based on geomorphological evidence (e.g. dune

activity) and the dating of lake sediments and archaeological, faunal and floral remains (e.g.

Maley, 1977; Pachur and Kröpelin, 1987; Lezine, 1989; Petit-Maire and Kröpelin, 1991).

Rognon (1987) describes the Sahel and southern Sahara as experiencing rapid changes in

climate over the last 20,000 years. Aridity is associated with the last glacial maximum

(LGM) at around 18,000 years before present (BP), when the Sahara occupied a much larger

area than it does at present (Gasse et al., 1990; Thomas and Thorp, 1995). After the LGM,

rainfall and vegetation cover in the Sahel and Sahara fluctuated considerably until about

10,000 BP, by which time conditions in the region were much wetter than at present. Much

of the Sahara was covered by semi-arid and seasonal vegetation (Ritchie and Haynes, 1987;

Jolly et al, 1998). Lézine (1989) describes a 400-500 km northwards shift in vegetation

zones in the central Sahara for around 8000 BP. Lake levels were at their highest during the

Holocene between 9500 and 7000 BP (Jolly et al., 1998). Lake Chad occupied several times

its present area for most of the period from 9000 to 4000 14C years ago, during which time

inter-dune lakes were widespread throughout the Sahel and southern Sahara (Maley, 1977).

(For such periods, radiocarbon dates underestimate the age of the dated material by the order

of a thousand years (Renfrew, 1973).)

The pluvial phase described above was interrupted by a period of aridity lasting the order of

centuries sometime around or before 8000 BP, although the timing of this arid phase may not

have been synchronous throughout northern Africa (Gasse and Van Campo, 1994; Alley et

al., 1997). Between about 8400 and 8000 BP (7650-7200 14C years ago), a widespread

cooling signal is evident in oxygen isotope records from Europe and Greenland (von

Grafenstein et al., 1998). Barber et al. (1999) argue that this cooling was the result of the

collapse of a remnant of the Laurentide ice sheet, injecting cold, fresh water via Hudson Bay

into the Labrador Sea. This would have resulted in a freshening of the North Atlantic, a

suppression of deep water formation and a reduction in North Atlantic SSTs (Street-Perrott

and Perrott, 1990), with consequences for oceanic heat transport and atmospheric moisture

content.


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Street and Gasse (1981) identify two wet periods from 12,500 to 7500 BP and from 6500 to

5000 BP. The latter of these phases was more arid than the former, but was wetter than

present conditions (Ritchie and Haynes, 1987). Many indicators suggest a drying of northern

Africa (relative to conditions around 8000 BP) by 5000 BP (Lioubimtseva, 1995), with lake

levels reaching a minimum between 5500 and 4000 BP (Jolly et al., 1998). Modelling

studies by Claussen et al. (1999) indicate that the onset of the present aridity in the Sahel-

Sahara zone may have occurred in two stages, the first from around 6700 to 5500 BP and the

second some 4000 to 3600 BP. They attribute the onset of this millennial-scale desiccation to

changes in solar insolation over Africa resulting from cyclical variations in the Earth’s

orbital parameters.

Information relating to climatic variability in the region since the onset of the last period of

desiccation is relatively sparse, and information concerning changes in rainfall on a decadal

timescale is limited to very qualitative historical records of drought or inferred from other

historical data. Although comparisons between present-day conditions and those throughout

the Holocene and other periods may be instructive, it must be borne in mind that the climatic

boundary conditions in the past were different to those prevalent today. This is especially

true for epochs prior to the onset of the present Saharan aridity, when the prevailing regional

climate was quite different to that characterising the past several millennia. Although no

major climatic upheavals have occurred to dramatically change the general nature of the

Saharan or Sahelian climate, climatic boundary conditions have not remained constant since

the Saharan desiccation. One of the more obvious changes in the last five millennia is the

anthropogenically driven increase in atmospheric CO2 concentrations since the eighteenth

century. This has changed the external forcing of the climate system, and hence the global

climatic boundary conditions, in a fashion which has no analogy in the recent geological

past. This necessitates caution in any comparative historical or palaeoclimatic studies.

Nicholson and Flohn (1980) briefly address this issue, as well as changes in boundary

conditions due to differing land-surface conditions both within Africa and at high latitudes.

For example, the land-surface characteristics of the latter regions will vary with changes in

the extent of ice cover, and it is conceivable that resultant changes in temperature patterns

and atmospheric circulation may affect the climate of low-latitude regions via atmospheric

teleconnections.

It is clear that the Sahel has experienced extended periods of relatively dry or wet conditions

over the past few millennia (Nicholson, 1976; A. T. Grove - personal communication, 1996;

Holmes et al., 1997). Nicholson (1976) describes the major historical droughts as occurring

in the 1680s, from 1738-56, in the 1790s and 1830s and in the years 1913/14. The last of


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these episodes, Nicholson argues, was the culmination of a drying trend which commenced

in the 1890s and affected much of Africa. The above investigations notwithstanding, it is

very difficult to quantify the precise duration and severity of such dry episodes using the

proxy data sources available. Tarhule and Woo (1997) have used a comparison of recorded

twentieth century drought in northern Nigeria with rainfall records to interpret local

historical droughts prior to the period of meteorological records, although such a method has

obvious limitations. These limitations, and the problems of historical interpretation, are

discussed further in Chapter 4.

2.4. The land surface and Sahelian climate change

At the beginning of the period of twentieth century desiccation, many authors attempted to

explain the observed changes in the Sahelian climate in terms of land-atmosphere

interaction. The prevailing notion was that widespread changes in the land surface had

resulted in changes in the forcing of the regional atmosphere which led to conditions

favourable to enhanced aridity. The most well-known of these theories was the

biogeophysical feedback model of Charney et al. (1975). In this model, removal of

vegetation by human activity such as deforestation and livestock grazing increases the

albedo of the land surface and leads to a reduction in the radiative heating of the atmosphere,

causing enhanced subsidence and reducing the potential for convective rainfall. This model

was supported by modelling studies (Charney et al., 1977; Sud and Fennessy, 1982; Laval

and Picon, 1986). The removal of vegetative cover would result in increased wind and water

erosion of soils, leading to land degradation and desertification (Rapp, 1986; for a discussion

of, and definitions of, land degradation, see Imeson (1991) and Mortimore (1998)).

Such degradation would reduce the capacity of the soil to hold moisture, and the postulated

subsequent reduction in available surface moisture would also reduce the likelihood of

rainfall, particularly from the squall lines which bring most of the rainfall to the Sahel, and

whose development is partly dependent on surface moisture sources (Rowell and Milford,

1993). The importance of these processes was also supported by modelling studies (Sud and

Fennessy, 1984; Cunnington and Rowntree, 1986).

The above explanations for the Sahelian drought rested on the assumption of widespread and

significant land degradation and desertification. The idea that the Sahelian environment had

deteriorated in the manner described above was well established in the 1970s and 1980s, and

an influential paper by Lamprey (1975) claimed that the Sahara had advanced by some 90-

100 km in the north Kordofan region of Sudan between 1958 and 1975. Lamprey also cited

“…ecological degradation….largely due to past and current land use practices….accelerated


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during periods of drought.” Sand encroachment was “….the result of several thousand years

of abuse of the fragile ecosystems which formerly existed in the Sahara and Nubian areas.”

He concluded that there was a “….need to educate the rural population, particularly as many

of the problems are due to traditional and hitherto unquestioned practices.”

Lamprey’s conclusions echoed those of colonial authors. The notion of an advancing Sahara

originated in the 1930s and 1940s, when many European and North American observers

interpreted seasonal and interannual changes in Sahelian vegetation as being indicative of a

southward expansion of the Sahara (For a detailed discussion and critique of theories of

desertification and land degradation see Mortimore (1998)). Such changes are generally a

result of natural variability in the regional climate of the Sahel - high variability in rainfall

and hence in the physical environment is a feature of semi-arid areas in general (Thomas,

1993, 1997). On an interannual timescale in the Sahel, vegetation bands will shift northwards

and southwards during periods of high and low rainfall respectively. The dynamic nature of

the geomorphological processes operating in the region means that blown sand and mobile

dunes will penetrate southwards under dry conditions. However, such shifts in climatic and

ecological zones are as likely to represent an oscillation of the so-called “desert boundary”

(Tucker et al., 1991) in response to short-term changes in rainfall as they are to represent a

steady and systematic southwards encroachment of the Sahara. Studies of vegetation cover

using the Normalised Difference Vegetation Index (NDVI) have shown that vegetation

quickly recolonises “desertified” regions when rainfall permits (Hess, 1996; Tucker et al.,

1991). Whereas Lamprey (1975) interpreted the different locations of the “desert boundary”

in 1958 and 1975 to be the result of a systematic expansion of the Sahara, Tucker et al.

(1991, 1994) have demonstrated that the location of this boundary (defined in terms of

vegetation cover) varies from year to year in association with rainfall amounts. Figure 2.1

shows wet-season NDVI, averaged over the approximate 200-400 mm zone of the Sahel for

the years 1980 to 1994. The NDVI is a measure of vegetation cover, and is well correlated

with rainfall (Figure 2.1). The decline in NDVI up until 1984, and the subsequent rapid

recovery of NDVI values, demonstrates that vegetation quickly recolonised areas affected by

dry conditions in the early 1980s, suggesting that the impact of successive dry years on the

land surface is reversible. A study by Helldén (1988) also failed to find any evidence for

severe desertification in north Kordofan over the period investigated by Lamprey (1975), but

did demonstrate a short-term impact of drought on the vegetation in the region. Nicholson

and Tucker (1998) state that there has been “…no progressive change of either the Saharan

boundary or vegetation cover in the Sahel…, nor has there been a systematic reduction of

‘productivity’ ” between the early 1980s and late 1990s. Over the same period they suggest

little change in surface albedo has occurred, although they claim that a change in albedo of


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up to 10 % since the 1950s is “conceivable”. This should be compared with a maximum

seasonal variation between July 1989 and September 1990 of 6.5 % for fallow and 5.7 % for

tiger-bush as described by Allen et al. (1994).

Even if the idea that the Sahara is systematically expanding is no longer accepted wisdom

(although there are many references to desert expansion in both popular media and some

scientific articles), the notion of widespread land degradation as a result of human activity

has persisted. Modelling studies continue to suggest that regional-scale changes in land

surface properties such as albedo may significantly modify Sahelian climate (Xue, 1997;

DeRidder, 1998). Other such studies have simulated the effects of grazing on the land

surface (Stroosnijder, 1996). Tegen and Fung (1994, 1995) have explained increases in

Sahelian dust production in terms of land degradation, emphasising the effects of

overgrazing.

There is a tendency for such studies to presuppose that land degradation is a major feature of

the Sahelian environment on a regional scale, and modelling studies have tended to impose

large changes in land surface properties over large areas (Xue and Shukla, 1993; Polcher and

Laval, 1994; Zheng and Eltahir, 1997). Rowell and Blondin (1990) used a fully interactive

model of soil wetness to investigate the impact of changes in the surface hydrology on

Sahelian rainfall, and found a much smaller impact on the large-scale atmospheric flow than

did other researchers (Sud and Fennessy, 1984; Cunnington and Rowntree, 1986) who

imposed much larger initial changes over wider areas which were maintained throughout the

simulations. Taylor et al. (1997) also found that the modelled influence of surface moisture

flux on the evolution of the boundary layer was limited when soil moisture patterns were

generated from daily rainfall estimates in a mesoscale model with a high horizontal spatial

resolution.

The evidence that land degradation is severe and widespread in the Sahel is limited at best.

The most widely used estimates of the extent of degradation are from UNEP (1992), based

on the Global assessment of Soil Degradation (GLASOD) for the period from 1950 to 1980

(e.g. Feddema, 1998). However, this dataset has been constructed by extrapolating relatively

few data, which are generally concentrated around inhabited areas, where human impacts on

the land will be greatest. Wint and Bourn (1994) found livestock numbers (including the

livestock associated with pastoral activity) to be highly correlated with the distribution of

permanent human settlements, and only very weakly related to the extent of rangeland. This

suggests that widespread overgrazing of the Sahelian rangelands is unlikely away from

settled areas.


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Figure 2.1: Relationship between NDVI and rainfall (top), and interannual variabilityof NDVI (averaged over the Sahel for the wet-season) from 1980 to 1994 (bottom).Figures reproduced from Tucker (1995) [Elements of change 1995: Session I, AspenGlobal Change Institute; http:www.gcrio.org/ASPEN/science/eoc95/sessionI/Tucker.html.]


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Prince et al. (1998) used remotely sensed vegetation indices to calculate the rainfall use

efficiency (RUE) - the ratio of net primary production (NPP) to precipitation for the entire

Sahel for the period 1982-1990. They found NPP to be in step with rainfall (reflecting the

findings of Tucker et al. (1990, 1994)), with little variation in RUE, indicating a resilience in

the ability of the regional ecology to recover from drought which is not consistent with

widespread, subcontinental-scale desertification. The impact of livestock on the land surface

has also been investigated indirectly by Hanan et al. (1991), who found no consistent

relationship between primary production (determined by NDVI values determined by NOAA

AVHRR data) and proximity to wells at a resolution of 1.1 km in the North-Ferlo region of

Senegal.

The assumption that traditional land use practices have a detrimental effect on the Sahelian

environment which leads to land degradation and desertification is also questionable, even if

the lack of detectable regional-scale degradation allows for localised anthropogenic

vegetation clearance and soil disturbance. Many of the theories of anthropogenic land

degradation are based on the idea of carrying capacity, which has been questioned as being

inappropriate to agricultural and pastoral systems in the Sahel (Mortimore, 1998). Mortimore

(1998, pp150-151) also discusses evidence from Kano in northern Nigeria that soils remain

stable under traditional cultivation methods, even in a region with a high, and increasing,

population density. Timmer et al. (1996) describe indigenous management techniques of

trees in central Burkina Faso as contributing to a sustainable use of tree resources.

It appears that the susceptibility of the Sahelian environment to degradation and

desertification, and the role of human activity in initiating or enhancing such processes, has

been over-emphasised. While sand encroachment, overexploitation of natural resources and

severe soil erosion may well be problems in some areas, they are not as ubiquitous as

previously believed by many researchers. The type of large-magnitude, regional-scale

changes in the land surface that have been modelled might well have the potential to modify

the Sahelian climate. However, it appears increasingly unlikely that such changes have taken

place. Detecting any such changes is complicated by the high degree of interannual

variability in the regional environment and climate. Terms such as desertification and

degradation can often be misleading, and have been used to describe land surfaces exhibiting

the lack of vegetation cover characteristic of dry periods or multi-year dry episodes. Much of

the “detected” desertification and land degradation is likely to be associated with the

temporary southwards retreat of vegetation zones during the low-rainfall years which have

dominated the climatological record since the late 1960s. This is not to suggest that land-


27

atmosphere interactions are unimportant; strong wet-season feedback processes involving

greater convective activity and hence greater rainfall generation over wetter Sahelian

surfaces have been described by Taylor and Lebel (1998) and Lare and Nicholson (1994).

However, such processes are unlikely to be important on an interannual timescale, as the

moisture content of the Sahelian soils at the onset of the wet season will be very low.

Patterns of spatial persistence in rainfall due to moisture feedback are likely to be the result

of the atmospheric dynamics of particular wet seasons.

The lack of evidence for the kind of coherent, regional-scale changes in the Sahelian land

surface which are associated with climatic changes in modelling studies, suggests that the

origins of the Sahelian dry episode lie outside of the African continent.

2.5. The role of the oceans and global temperature patterns

2.5.1. Sahel rainfall and sea surface temperatures

The relationship between aridity in the Sahel and Sahara and large-scale changes in

glaciation was discussed briefly in Section 2.2. Past arid phases are associated with

widespread glacial conditions over the northern hemisphere land masses or with probable

cooling of the North Atlantic. Variations in the contrast in hemispheric temperatures have

been linked to Sahelian rainfall during the twentieth century, with a relatively cold northern

hemisphere being associated with drought (see below). The northern hemisphere will be

colder than the southern hemisphere during glacial conditions due to the greater extent of the

northern hemisphere land masses, which facilitates the growth of large ice-sheets which are

absent in the southern hemisphere outside Antarctica. Thus there is a possible analogy

between twentieth century temperature patterns associated with Sahelian drought and longer

timescale variations in temperature distributions linked to glacial aridity.

However, changes in glaciation cannot at present explain the entire Holocene climatic

history of northern Africa. For example, the causes of the onset of the present Saharan

desiccation some 5000 years ago are still unknown, although the global nature of the change

in climate in the northern tropics is evident from the contemporaneous desiccation of much

of Asia lying within the same latitudinal zone (Ji et al., 1992). This is consistent with the

orbital forcing described by Claussen et al. (1999). Shukla (1995) suggests that high rainfall

in the 1950s and early 1960s followed by low rainfall in the late 1960s and in the 1970s

characterised the African-Asian monsoon region as a whole, again suggesting causal

mechanisms operating on a global-scale.


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It is reasonable to assume that, in the absence of such major climatic upheavals as the rapid

growth and collapse of ice sheets, more subtle processes will act to modulate rainfall in

semi-arid regions such as the Sahel. Such processes may take the form of changes in surface

solar insolation (Coe, 1997; Claussen et al., 1999), manifestations of the internal variability

of the global climate or modifications of the radiative forcing of the atmosphere due to

increased CO2 and aerosol concentrations.

A large body of work links twentieth century drought in the Sahel with patterns of sea-

surface temperatures (e.g. Folland et al., 1986; Bah, 1987; Street-Perrott and Perrott, 1990;

Fontaine and Bigot, 1992; Lamb and Peppler, 1992; Ward et al., 1993; Semazzi et al., 1996;

Ward, 1998). In the present epoch, and on the interannual to decadal-scale timescales

involved, temperature distributions are more likely to be associated with patterns of heat

transport in the oceans than with variations in glaciation or orbital forcing (e.g. Newell and

Hsiung, 1987).

Different oceanic regions will exert varying degrees of influence on Sahelian rainfall,

depending on their proximity to the Sahel and the extent to which they are linked to the

Sahel via atmospheric teleconnections. A number of studies have used principal component

analysis (PCA) to assess the degree of correlation of Sahelian rainfall with sea surface

temperature (SST) for various oceanic regions (e.g. Ward et al., 1993; Shinoda and

Kawamura, 1994). Shinoda and Kawamura (1994) found that total rainfall over the Sahel

region is strongly correlated with SST anomalies (SSTAs) over the Indian Ocean, whereas

the latitude of the centre of gravity of the Sahelian rainbelt is most strongly correlated with

Atlantic anomalies.

Such anomalies in the Indian and Atlantic Oceans are a manifestation of an inter-

hemispheric oscillation of SSTs on a global scale. This oscillation is characterised by SSTAs

of the same sign throughout the southern hemisphere oceans and the northern Indian Ocean,

with anomalies of opposite sign characterising large areas of the remaining northern

hemisphere oceans. The hemispheric anomalies are more or less synchronous on interannual

and longer timescales, but lag relationships between different regions are observed on

timescales of months (Lazante, 1996). This pattern appears to be the most important mode of

variation of sea-surface temperatures within the context of Sahelian rainfall studies. A

relatively warm southern hemisphere and Indian Ocean, and a cool northern hemisphere, are

associated with reduced rainfall over the Sahel (Folland et al., 1986; Street-Perrott and

Perrott, 1990; Ward et al., 1993). This configuration of global SSTs has become more

prevalent since the onset of drier conditions in the Sahel (Folland et al., 1986; Newell and

Hsiung, 1987).


29

Relationships between Sahel rainfall and individual oceanic regions are important on shorter

timescales than those between rainfall and hemispheric SST patterns. The primary moisture

source for the Sahel is the tropical Atlantic Ocean, and the impact of this region on the West

African Monsoon has been investigated at length (Bah, 1987; Lamb and Peppler, 1992;

Fontaine and Bigot, 1993). SSTAs in the tropical Atlantic tend to reflect the global SSTA

patterns described above. Folland et al. (1986) found that the association between Sahel

rainfall and the hemispheric SSTA dipole dominated over the Atlantic SSTA dipole on

timescales longer than a year. Nonetheless, on timescales of a few months, relationships

appear to exist between parts of the Atlantic and Sahel rainfall. Bah (1987) states that

positive SSTAs in the Gulf of Guinea are coincident with reduced summer rainfall in the

Sahel. Opoku-Ankomah (1994) describes an opposite relationship between SSTAs in this

region and rainfall in Ghana, on a similar timescale.

The association of drought conditions in the Sahel with a relatively warm southern

hemisphere and cool northern hemisphere is supported by the work of Newell and Hsiung

(1987), who discuss different distributions of SSTs prior to and during the prolonged Sahel

drought. They describe SSTs in the mid-latitude North Pacific and the western middle-

latitude North Atlantic as being out of phase in the period 1949-1964, and in phase during

the period 1965-1979, when SST patterns were also more homogeneous in both the north-

south and east-west directions. In the former period the South Atlantic was out of phase with

the western North Atlantic; in the latter phase it was out of phase with the entire North

Atlantic. Wagner (1996) describes a strengthening of the northward SST gradient,

represented by SST gradient anomalies which persist from January until the period March-

April, in the tropical Atlantic over the period 1951-1990. It should be noted that the eight

years he describes as exhibiting the strongest northward temperature gradient (represented by

the temperature of the North Atlantic domain minus that of the South Atlantic) are not the

eight driest years in the Sahel. However, with the exception of 1968, these strong-gradient

years, if not synchronous with the driest Sahel years, do immediately succeed or precede

them. Wagner (1996) describes the strongest negative SST anomalies as occurring between

10°N and 20°N, and the strongest positive anomalies as lying between 10°S and 25°S. These

findings are consistent with those described by Lamb and Peppler (1992). However, the

latter authors noted that the Atlantic SST anomalies during the very dry year of 1983 did not

conform to this pattern.

Variations in SST in the Pacific, Indian and Atlantic Oceans appear to be related, as would

be expected given fact that SST anomalies throughout the northern and southern

hemispheres respectively appear to be synchronous on interannual and longer timescales.


30

Anomalies appearing in the eastern tropical Pacific are reflected in the subsequent evolution

of the central Pacific, Indian Ocean and, to a lesser extent, the North Atlantic (Lazante,

1996). Large changes in the Pacific are generally related to the El Niño Southern Oscillation

(ENSO) phenomenon, and the variable nature of the Pacific-Atlantic lag relationships

described by Lazante (1996) implies that ENSO events may influence the Atlantic and West

African Monsoon circulation in certain, though not all, ENSO event years. The likelihood of

ENSO-Sahel rainfall associations will also depend on the timing of ENSO-related changes in

the Pacific, and the subsequent timing of the Atlantic response. If the latter occurs in the

Sahelian transition or wet season months, it may be expected that rainfall might be affected.

Correlations between rainfall in the Sahel and the southern Oscillation Index (SOI), the low

phase of which is associated with El Niño events (Hastenrath, 1991), have increased over the

period of Sahelian aridity, and have been significant since the early 1970s (Janicot et al,

1996). Janicot (1994) suggests that any association of Sahel rainfall with SSTs in the Gulf of

Guinea is weak, and that ENSO impacts are generally a more dominant (though not

necessarily unrelated) factor in determining Sahelian rainfall.

2.5.2. Other ocean-atmosphere interactions

Newell and Hsiung (1987) postulate that warmer southern hemisphere oceans and cooler

northern hemisphere oceans may be the result of a decreased northward oceanic energy flux.

This flux may be controlled either by the energy received at the surface of the Indian Ocean,

or by changes in wind patterns in and towards the north of the tropics. Street-Perrott and

Perrott (1990) calculated a mean modulus of 15 Wm-2 (averaged over the world ocean) for

heating anomalies arising from differences in global July-September SSTs between five wet

and five dry years in the Sahel. They compare this with the figure of 4 Wm-2 estimated to be

the consequence of a doubling of atmospheric CO2. They suggest that the north-south SST

oscillation modulates the moisture convergence into the inter-tropical convergence zone

(ITCZ), and has some influence on the location of the zone of strongest convective activity

within it. Wagner (1996) claims that a cool North Atlantic is accompanied by an anomalous

southward pressure gradient, associated with weaker cross-equatorial flow from the south,

strengthened north-east trade winds and slightly weakened south-east trades. He states that

such a pattern is accompanied by increased rainfall in the Brazilian Nordeste. It is likely that

such a weakening of air and moisture transport across the equator from the south would also

serve to reduce the intensity of the West African Monsoon, so bringing drier conditions to

the Sahel.


31

Janicot (1994) presents evidence that since 1970 (but with the exception of 1987), ENSO

events have been associated with ascent over the eastern inter-tropical Pacific and descent

over the Gulf of Guinea and West Africa. This enhanced subsidence could certainly reduce

the rainfall over the Sahel if the timing of the ENSO event was such that the descending

motion in this region occurred during the months of June to September. A similar

mechanism of convection suppression has been postulated by Shinoda and Kawamura

(1994), who argue that an inter-decadal scale warming of the Indian Ocean might result in

enhanced upward motion over that region, with associated descent over West Africa, thus

reducing Sahelian rainfall. Such a hypothesis is interesting in light of the observed warming

of the Indian Ocean and southern hemisphere oceans, and the cooling of the northern

hemisphere oceans, in the latter half of the twentieth century (Folland et al., 1986)

2.5.3. The thermohaline circulation

The temperature of the North Atlantic may also be modulated by the strength of the

thermohaline circulation (Manabe and Stouffer, 1988; Street-Perrott and Perrott, 1990), with

cooling of the North Atlantic triggered by a reduction in the North Atlantic Deep Water

(NADW) formation. Manabe and Stouffer (1988) describe a GCM simulation with the

thermohaline circulation switched off. The North Atlantic surface waters that would

otherwise sink in and around the Norwegian Sea are not replaced by warmer, more saline

water from low latitudes. As a result, the surface salinity and temperature of the North

Atlantic Ocean are maintained at a substantially lower level than in the case with

thermohaline circulation. Studies by Schiller et al. (1997), using a coupled ocean-atmosphere

GCM, suggest that input of fresh water into the Labrador sea (albeit over a period of 250

years) can result in the reduction, and ultimately shutting down, of the Atlantic thermohaline

circulation, reducing northward heat transport and hence SSTs in the North Atlantic. Cai et

al. (1997) also describe the results of a modelled, imposed North Atlantic high-latitude

freshening, equivalent to about eight times the salt deficit observed in the “Great Salinity

Anomaly” of the late 1960s and early 1970s, and imposed over five model years. They report

initial cooling in the sinking region, and a reduction in the NADW formation, which

recovers within twenty years of the freshening being removed. However, warming occurs to

the south of the sinking region and subsequently aids the recovery of the NADW formation.

Other coupled ocean-atmosphere model studies and theoretical considerations suggest that

the collapse of the thermohaline circulation might be a consequence of large increases in

atmospheric greenhouse gases (Broecker, 1997).


32

Although such studies are not conclusive, the possibility that enhanced greenhouse warming

may lead to a cooler and fresher North Atlantic due to the melting of Arctic ice, or

freshening due to increased precipitation, should not be discounted. (McCartney et al., 1996)

report the spreading of unusually cold waters from the Arctic into the north-east Atlantic.

Such cold temperature anomalies may be due to the input of cold water resulting from the

melting of Arctic ice (Cavalieri et al., 1997) and permafrost (Hecht, 1997). Perez et al.

(1995) report, from observed data, some freshening of the North Atlantic along 42°N

commencing in the 1970s and persisting until 1990. Read and Gould (1992) have found that

the waters between Greenland and the United kingdom were 0.08°C and 0.15°C colder in

August 1991 than in 1962 and 1981 respectively, and slightly less saline than in 1962.

Melting of Arctic ice may also explain decreased salinity in the northern North Atlantic.

Cavalieri et al. (1997) report that the Arctic ice cover has decreased at a rate equivalent to

2.9 ± 0.4 % per decade from 1978-1996, although this decrease has not been smooth. The

two lowest summer extents prior to 1997 occurred in 1990 and 1995, and the four lowest

summer extents before 1997 occurred after 1990. The equivalent trend in the Antarctic was

of an increase in ice extent of 1.3 ± 0.2 % per decade. The authors of this study describe

these results as being consistent with the modelled response to CO2 induced climate

warming. Reid et al. (1998) link decreases in phytoplankton in the north-east Atlantic with

the negative SST anomalies described above, and suggest that this relationship constitutes a

vegetation response to climate forcing.

From 1973 to 1976 Arctic ice extent increased (Cavalieri et al., 1997), although the

behaviour of the ice sheet prior to this period is unclear. Street-Perrott and Perrott (1990)

describe the Great Salinity Anomaly as lasting from 1968-1982, and as being associated with

deep water cooling and freshening in the Norwegian and Labrador Seas. This event could

have been associated with a reduction in ice cover and consequent input of fresh water into

the ocean, with anomalous atmospheric circulation or with an increase in precipitation over

land areas between 35° and 70°N over the previous few decades (Bradley et al., 1987). Both

this freshening and cooling episode, and another outbreak of relatively fresh water from the

Arctic from around 1908-1914, coincided with drought in the Sahel. The existence of the

earlier episode suggests that any apparent associations between North Atlantic freshening

and enhanced greenhouse warming should be treated with caution, as the observed global

and hemispheric warming has occurred since 1920 (Parker et al., 1994).

The question of when the downward trend in precipitation in the Sahel commenced is

especially relevant to this hypothetical relationship between drought in the Sahel and North

Atlantic freshening. If freshening and cooling of this region is important as a drought-


33

inducing mechanism the implication is that the process of desiccation should be viewed as

commencing in the late 1960s, when Sahelian rainfall anomalies relative to the twentieth

century mean were negative for the first time since 1949 (Chapter 1, Figure 1.2). If other

evidence supports the view that reductions in rainfall in the Sahel prior to 1968 should be

interpreted as part of the same drying trend then changes in the Sahelian rainfall regime

preceded the North Atlantic freshening, making the latter an unlikely candidate for a drought

inducing mechanism. Nonetheless, the question is raised as to whether rainfall in the Sahel is

modulated to some extent by the behaviour of the North Atlantic Ocean, with dry conditions

being associated with a freshening and consequent cooling of the North Atlantic and perhaps

a reduction in the strength of the thermohaline circulation. If this is the case, it is also

pertinent to ask if such processes are occurring at the present time as a response to

anthropogenic greenhouse warming. If this is so, prospects for a return to generally wetter

conditions in the Sahel within the near-future may be remote.

2.5.4. The impact of sulphate aerosols

Another potential anthropogenic modulation of sea surface temperature could arise from the

impact of atmospheric sulphate aerosols produced by industrial processes. Over two decades

ago, Bolin and Charlson (1976) suggested that the scattering of solar radiation by sulphate

aerosols could be equivalent to a drop in the average temperature of the northern hemisphere

of 0.03°C to 0.05°C. Blanchet (1995) describes fine sulphate particles as the most effective

aerosol cooling agent when mixed with condensed water in moist air.

Sulphate aerosol particles have a short residence time in the atmosphere (a few days),

resulting in large spatial variability in their distribution (Charlson et al., 1992). As most of

the world’s industry is concentrated on the northern landmasses, atmospheric sulphate

aerosols are generally confined to the northern hemisphere. In modelling studies conducted

by Haywood et al. (1997), the radiative forcing due to sulphate aerosols in the northern

hemisphere was a factor of four greater than in the southern hemisphere, although forcing

was strongest over and downwind of sources, i.e. over land areas. An earlier assessment of

the empirical evidence by Charlson et al. (1992) suggested that this climate forcing was

twice as great in the northern hemisphere as was the global average. These authors also state

that aerosol forcing is greatest in the daytime in summer, and that concentrations of such

particles are an order of magnitude greater within 1000 km of the sources than in remote

regions. Charlson et al. (1992) report that tropospheric mass concentrations of non-sea-salt

aerosols sulphate over the northern hemisphere oceans appear to be enhanced by 30% or

more over the natural background. Erickson et al. (1995), hypothesise that sulphate aerosol-


34

induced changes in cloud albedo may result in cooling of the North Atlantic and North

Pacific, with the largest effects occurring in winter. This suggestion is the result of modelling

studies, in which circulation changes result from the different responses of the North

American and Eurasian landmasses, with a deepening of the trough over the former, and an

enhanced ridge over the latter.

Inclusion of sulphate aerosols in climate models has resulted in simulations reproducing the

observed record more accurately due to the sulphate aerosol cooling effect (Mitchell et al.,

1995a,b). However, the radiative processes involved are poorly understood and it is possible

that the aerosol representation in the simulations is not very realistic (Cane et al., 1997).

Inclusion of aerosol representation alongside greenhouse gases (GHGs) in a simulation using

the Hadley Centre climate model led to closer agreement of the modelled and observed

global changes in temperature, particularly in the most recent decades, when sulphate aerosol

forcing has been largest (Mitchell et al., 1995). In this simulation the largest decrease in

temperature, relative to the GHG-only case, due to the aerosol forcing occurred in the

northern mid-latitudes and the Arctic. The former regions are subject to the greatest aerosol

forcing, while cooling in the Arctic is amplified by increases in sea ice. A model study of the

effects of sulphate aerosols alone by Taylor and Penner (1994) resulted in the largest cooling

occurring not in the regions of greatest forcing, but over the Norwegian and Greenland seas.

Again, this cooling in the simulation was associated with increases in sea ice. It should be

noted that this particular modelled response of the climate system to increased sulphate

aerosol loadings conflicts with the observations presented by Cavalieri et al. (1997). They

describe decreases in the extent of the Arctic sea ice from the late 1970s, the period during

which Mitchell et al. (1995b) suggest the effects of sulphate aerosols (at least in terms of

radiative forcing) should have been greatest. Hulme (1998) found that modelling studies

incorporating the effects of sulphate aerosols, performed at the Hadley Centre (the HadCM2

simulations), did not simulate the Sahelian dry episode. These studies therefore do not

support the hypothesis that such aerosols reinforce the temperature anomaly dipole

associated with a dry Sahel. However, it is not clear that the simulations capture the

relationship between rainfall and global temperature patterns. These issues are addressed in

Chapter 4 of this thesis.

2.5.5. The effect of other aerosol types on global temperature patterns

It should be noted that some estimates of the magnitude of the global-mean radiative forcing

by mineral aerosols, and also by soot, are comparable to the forcings suggested for natural

sulphate aerosols of biogenic origin and also for anthropogenic sulphate aerosols (Duce,


35

1995; Andreae, 1996). Anthropogenic aerosol emissions and source regions of mineral dust

are concentrated in the northern hemisphere, and very little exchange of aerosol particles

occurs between the hemispheres due to the short residence times of aerosol particles and the

longer timescales required for mixing between the two hemispheres across the ITCZ (Duce,

1995). The total-aerosol forcing of the climate system, which is thought to be negative

(Charlson and Heintzenberg, 1995) is thus higher in the northern hemisphere (Roeckner et

al., 1995). Increases in the global aerosol budget in general are therefore most likely to act to

preferentially cool the northern hemisphere and lower SSTs in the northern hemisphere

oceans, reinforcing the global SST pattern which has been associated with dry conditions in

the Sahel.

Whereas many of the particles present in the atmosphere are a consequence of relatively

recent human activity, it may be assumed that mineral aerosols have been a more or less

constant characteristic of the atmosphere at least as long as the global climate has resembled

that of the present day. This will be particularly true of regions downwind of major dust

sources such as the Sahara. However, there is strong evidence that the amount of mineral

dust produced in northern Africa and transported across the Atlantic has increased

dramatically since the 1950s (Middleton, 1985; Li et al., 1996; N’Tchayi, 1994, 1997). The

likely effect of such increases in dust concentrations over the North Atlantic will be one of

negative solar forcing at the ocean surface, with a reduction in SST (Schollaert and Merrill,

1998) This would tend to reinforce the Atlantic component of the global hemispheric

temperature anomaly dipole pattern that is associated with Sahelian drought. A consideration

of aerosol optical depths suggests that local and regional forcing over the Atlantic due to dust

aerosols may be large (Duce, 1995). The potential of dust aerosols to modify the radiative

structure of the atmosphere and possibly SSTs in the tropical North Atlantic region, and the

observed increase in such aerosols over the past few decades, strongly suggests that the

climatic effects of mineral dust in this region may be significant. These issues are addressed

at greater length elsewhere in this thesis.

2.5.6. Sahel rainfall and twentieth century global and hemispheric air

temperatures.

It is interesting to compare the above SST-related studies with those of global surface air

temperatures whose principal goal is the detection of global warming signals. Jones and

Briffa (1992) state that the warming which is apparent on a global scale over the course of

the twentieth century occurs mostly in winter and spring and exhibits a notable spatial

variability in the northern hemisphere, whereas in the southern hemisphere warming is


36

evenly distributed throughout the year and generally more spatially homogeneous. Annual

warming in the twentieth century is greatest in the southern hemisphere, and cooling is

apparent over the northern Pacific Ocean and northern Atlantic Ocean for the decade 1981-

1990, which contained several highly rainfall-deficient years. It may be speculated that the

reduction in Sahelian rainfall is the result of a global warming which has resulted in a

preferential heating of the southern hemisphere, due in part to the greater negative solar

forcing due to aerosols in the northern hemisphere (Roeckner et al., 1995).

Figure 2.2: Combined annual land air and sea surface temperature anomalies relativeto 1961-1990, based on the datasets of Jones (1994) and Parker et al. (1995) (updated)Graphic obtained from Phil Jones at the Climatic Research Unit.

Variations in global and hemispheric temperatures over the twentieth century have also been

described by Parker et al. (1995). Figure 2.2 shows the series of annual average global and

hemispheric combined land-air and sea-surface temperature anomalies of Jones (1994) and

Parker (1995).


37

A global and hemispheric warming commences around 1920, after the dry year of 1913 in

the Sahel. The increase in temperature ceases in the 1940s, and in the early to mid 1940s

there is a slight cooling of both hemispheres. The shape of the anomaly curves is similar for

both hemispheres until the early to mid 1960s, exhibiting no general trend, and with the

southern hemisphere exhibiting mostly negative anomalies and the northern hemisphere

mostly positive anomalies. Parker et al. (1994) describe a slight fall in global average

temperatures, with a relatively warmer northern hemisphere in the decade 1951-1960, when

the Sahel experienced abundant rainfall. After 1965 the southern hemisphere warms, but the

northern hemisphere anomalies do not increase until the late 1970s. The decade 1971-80 was

characterised by relatively warmer southern hemisphere, and coinciding with the Sahelian

drought of the early 1970s. So a change in the inter-hemispheric contrast (IHTC) to the

pattern associated with drought in the Sahel appears to have occurred in the decade 1961-70.

This again suggests that the downward trend in rainfall should be viewed as commencing in

the late 1960s, and that the reduction in rainfall which is apparent from the mid-1950s

represents merely a return to “normal” conditions after a wet episode related to a particular

mode of SST variation.

The northern hemisphere series exhibits negative anomalies in 1984 and 1985, with southern

hemisphere anomalies remaining positive. The two largest rainfall deficits in the Sahel were

in 1983 and 1984. Parker et al. (1994) state that during the most recent warming period (i.e.

since 1980) much of the central and western North Atlantic north of 50°N and much of the

central and mid-latitude North Pacific have cooled. It is plausible that the cooling of these

regions is related to changes in ice cover and freshwater input, as has been observed in

modelling studies (Mitchell et al., 1995a). An important question concerning the association

between Sahel rainfall and the IHTC is whether rainfall is directly modulated by circulation

changes which are driven by the SST patterns or whether both Sahelian rainfall variations

and changes in the IHTC are manifestations of more fundamental climate processes.

2.6. Atmospheric circulation and drought in the Sahel

Early theories concerning the causes of drought in the Sahel concentrated on the mean

position of the ITCZ, and the northward extent of its migration in the northern hemisphere

summer. H. H. Lamb (1977) suggested that drier conditions in the region were the result of

changing patterns of atmospheric pressure in mid- to high-latitudes, with an expanded region

of cyclonic activity in the former resulting in the displacement of the tropical climatic zones

towards the equator by a few degrees. P. J. Lamb (1978) describes the near-equatorial

pressure trough as being some 200 km south of its mean position from July to September in


38

dry years, and anomalously deep, with an associated anomalous southward extension of the

North Atlantic high pressure region. He states that this pattern is not seen in earlier months,

although such periods exhibit a stronger than normal horizontal pressure gradient, separating

higher than normal pressure north of 20°N from lower than normal pressure south of 12°N,

with a reduced northward extension of the South Atlantic high pressure region. Citeau et al.

(1994) suggest that the strengthening of the South Atlantic anticyclone and the associated

enhanced subsidence, extending further east, will produce stronger south-easterly trade

winds which will strengthen the West African Monsoon.

During periods exhibiting the cold northern hemisphere and warm southern hemisphere SST

anomaly pattern, the equatorial Atlantic experiences stronger northerly winds to the north of

the equator, and weaker southerly winds to the south of the equator. The latter weaken the

upwelling of cold water off the African coast (Nobre and Shukla, 1996). The largest wind

anomalies occur between 10° and 20°N, and are associated with strong sea-level pressure

(SLP) anomalies further north. Nobre and Shukla (1996) suggest that during the period

March-May, wind stress anomalies are a response to the meridional SST gradient, which is

strongest in this period. The SST anomaly pattern moves westwards from the December-

February to the March-May period, with the leading eigenvector of the meridional wind,

whose amplitude is at a maximum in this season, over the western tropical Atlantic, being

directed towards the warmer waters of the southern hemisphere. Thus the meridional

component of the wind is determined by the meridional SST gradient, but the subsequent

development of the SST anomalies is a response to variations in the strength of the trade

winds.

Shinoda (1990) attributes poor rains in the Sahel primarily to a reduction in the total rainfall

of the monsoonal rainbelt rather than displacement of the belt towards the equator, although

such a southward displacement of the centre of gravity of the rainbelt is observed during

August in dry years. An analysis of atmospheric water vapour content (AWVC) at Bidi

(northern Burkina Faso; 15º50’N) over the period January 1987 to September 1989, and over

the 1985/86 and 1986/87 dry seasons at Ouangofitini (Côte d’Ivoire; 9º36’N) by Faizoun et

al. (1994) found good agreement between the AWVC values for these periods and

climatological values obtained before the onset of the Sahelian dry episode. They infer from

this that there is no significant link between monthly AWVC values and the low rainfall

values experienced in the region since the onset of desiccation. This is a significant result,

although the extrapolation to such a conclusion should be treated with caution as long as the

data are restricted to only three wet seasons. The 1987 wet season was characterised by a

large rainfall deficit (approximately 30% relative to the twentieth century mean). Rainfall in


39

1988 was close to the mean for the twentieth century. 1989 was characterised by a rainfall

deficit of intermediate magnitude, some 10% below the mean. Conclusive evidence that

rainfall and AWVC are not generally related would require comparisons of these two

quantities for many years of varying rainfall, including several years with large rainfall

deficits. Nonetheless, these findings beg the question as to whether drought conditions arise

from a lack of convective activity rather than from lack of precipitable moisture. Such a

hypothesis is consistent with the conclusions of Bell and Lamb (1994), who have found that

the decline in the monsoonal rainfall in the Sahel results from a general progressive decrease

in the size and intensity of the disturbance lines (DLs) which generate most of the rainfall

over the Sahel. These are organised lines of westward propagating convective cells arising

from baroclinic and barotropic instabilities in the African Easterly Jet whose generation and

development also depend on orography, the availability of surface moisture and surface

temperatures (Peters and Tetzlaff, 1988; Rowell and Milford, 1993).

Shinoda (1990) presents evidence that the recent dry episode in the Sahel is the result of a

weakening of the African Hadley circulation, and that continuous intensification of the

northern subtropical high has contributed to this weakening. He describes increases in 700

hPa heights and temperatures throughout the zone 30°N-30°S in August, about east-west

axes over the Sahel, and attributes such increases to enhanced subsidence heating.

Differences in the 850 hPa geopotential height between Niamey and Abidjan (defined as the

former minus the latter) for August increased from negative to near-zero or positive from the

early 1960s to the mid-1980s, implying a reduction in the northward gradient of the 850 hPa

surface towards the ITCZ. 200 hPa height differences decreased over the same period,

indicating a reduction in the southward gradient of the 200 hPa surface from upper levels

above the ITCZ to the equatorial trough. 200 hPa heights increased, mostly in the northern

hemisphere; 200 hPa temperature changes were more localised than in the 850 hPa case,

with changes occurring near a north-east directed axis originating in the central Sahel. Weber

(1997) analysed northern hemisphere 300 hPa temperatures over the period 1966-1993 and

found increased values at low and middle latitudes, with low latitude temperature increases

exceeding those at mid-latitudes. Changes at 300 hPa were non-continuous and synchronous

with temperature increases throughout the 300-1000 hPa layer, although the former were

greater in magnitude. The lack of significant changes at high latitudes means that the

temperature gradient at 300 hPa between mid- and high-latitudes has increased.

Other studies have also linked dry conditions in the Sahel/West Africa with a weakened

northern hemisphere circulation. Eltahir and Gong (1995) compare the years 1958 and 1960,

and find weaker August-September meridional circulation, particularly with respect to


40

southerly flow, in the drier year of 1960. By contrast, the meridional circulation in 1958 is

strong, characterised by southerly winds from the surface to 300 hPa, and northerly winds

from 300 hPa to 100 hPa. The August-September zonal wind at the equator is stronger in

1958. Landsberg (1975) described rainfall over the Sahel as being essentially determined by

the strength of the southern Hadley cell. Thus a reduction in Sahel rainfall is associated with

a weakening of the northern hemisphere Hadley circulation, which has been synchronous

with a large scale tropospheric warming throughout the northern hemisphere. Such a trend is

consistent with the expected enhanced greenhouse warming resulting from anthropogenic

emissions of greenhouse gases (IPCC, 1996). It is possible, therefore, that a spatially

heterogeneous anthropogenic warming of the troposphere has modified the atmospheric

circulation in such a fashion as to reduce the strength of the West African Monsoon.

However, such theories are highly speculative at the present time.

Many studies have also connected rainfall deficits in the Sahel with more localised elements

of the atmospheric circulation. Kidson (1977), performing an analysis based on rainfall data

at 15°N, reports that the 850 hPa trough and the 200 hPa jet are more pronounced in wet

years, and that the 850 hPa trough completely disappears in dry years. 200 hPa easterlies

(overlaying the monsoon flow) are much weaker in dry years. He suggests that these

phenomena are part of a global trend towards weaker meridional circulation. Lamb (1983)

suggests that the tropospheric easterlies above the monsoonal air layer derive much of their

moisture from the lower south-westerly airstream. He found that the dry year of 1972

coincided with particularly shallow south-westerly flow across the Gulf of Guinea, although

the monsoon layer depth was not always greater in wetter years. This study also identifies

the wind direction above Dakar (on the West African coast near 15°N) as being closely

linked with the nature of the wet-season rainfall over the Sahel. Dakar appears to be located

close to the northernmost limit of the present day south-westerly surface monsoon flow. In

dry years, the northerly extent of this flow may lie several hundred kilometres south of

Dakar, above which the lower tropospheric flow will probably be from the north or north-

west. Besler (1983) suggests that the cross circulation in the exit region of the Tropical

Easterly Jet (TEJ) may affect rainfall. She cites the TEJ as being the major cause of the

anomalous aridity of northern Africa as compared with Africa south of the equator, and

argues that a stronger exit region cross circulation over North Africa may suppress rainfall in

the Sahel region due to enhanced subsidence.


41

2.7. Summary of potential rainfall modulation mechanisms: the relative

importance of “internal” and “external” processes.

The Sahel has been subject to a change in the nature of the rainfall regime which is

determined by the behaviour of the West African Monsoon. This has resulted in a

desiccation of the region since the late 1960s and a higher interannual rainfall persistence

than prevailed in the earlier part of the twentieth century. The desiccation appears to be

largely synchronous with large-scale changes in atmospheric circulation, characterised by a

general warming of the troposphere and a reduction in the intensity of the northern

hemisphere meridional circulation. Also associated with these trends is the establishment in

recent decades of a north-south dipolar sea surface temperature pattern, with anomalies in

the northern and southern hemispheres generally being out of phase (but with the whole

Indian Ocean exhibiting the same behaviour as the southern hemisphere oceans). It is

therefore reasonable to say that the recent Sahelian desiccation is associated with large-scale

changes in the global coupled ocean-atmosphere system.

Dry conditions in tropical North Africa are not necessarily synonymous with a shallower

monsoon layer, or with a reduced northerly penetration of the Inter-Tropical Convergence

Zone, but are associated in a more general fashion with the intensity of the monsoonal air

flow. Weakening of the monsoon may be viewed as being due to the general weakening of

the Hadley circulation on a hemispheric or global scale. Lag relationships between

temperatures in the Atlantic and Pacific Oceans, and the correlation of Sahel drought with

particular large-scale patterns of SST, support the notion that conditions in the Sahel are a

response to global-scale processes. However, the relationship between Sahelian rainfall and

hemispheric oscillations in SST anomalies is not simple, and does not hold on an interannual

time-scale over the entire period of desiccation, implying that other mechanisms also affect

rainfall in the region. Such mechanisms could be the result of atmospheric teleconnections

which are relatively independent of the mean large-scale circulation, or could result from

local and regional scale feedback processes. If the breakdown of the rainfall-IHTC

relationship on short time-scales is the result of persistence in the regional climate system,

then feedback processes operating on a synoptic scale over tropical North Africa appear to

offer the most likely explanation for such discrepancies.

One of the principal factors in determining the strength of the West African Monsoon will be

the distribution of regions of convergence and divergence throughout the troposphere, and

the intensity of the resulting convection and subsidence. For example, variations in the

strength and extent of the sub-tropical highs will impact on monsoonal airflow. The strength


42

and extent of subsidence over North and West Africa will also be important, and it is highly

plausible that regional factors such as land-atmosphere interaction may modulate

atmospheric stability. Modelling studies of changes in soil moisture and vegetation cover

were discussed briefly in Section 2.4, and doubts were raised about the degree to which such

studies accurately represent changes in these quantities. It was seen that it is highly likely

that the modelling studies have greatly exaggerated the extent and severity of changes in the

land surface.

One land-atmosphere interaction which has not been widely studied, particularly from a

modelling perspective, is the mobilisation and transport of atmospheric dust and its

subsequent effect on the radiative and dynamical properties of the atmosphere. Mineral dust

from the Sahara and Sahel is mobilised in vast quantities and transported over very large

distances, extending over considerable areas of North and West Africa (Maley, 1982; Goudie

and Middleton, 1992). This dust will therefore have an impact on the thermal structure of the

troposphere over wide areas which may be considerable distances from the dust source

regions. The influence of dust on atmospheric stability may be sufficient to modify the

regional circulation, and any studies of the climate of the Sahel-Sahara zone should consider

dust as a potential agent of climate modification.

Dust impacts, and other suggested land-atmosphere interactions (such as the influence of

changes in vegetation cover, soil moisture and land albedo), may be viewed as “internal”

mechanisms of climate forcing: they exist within continental Africa and provide a means of

feedback between the North African land mass and its prevailing climate. The influence of

global SST patterns and changes in the global or hemispheric atmospheric circulation in this

sense can be viewed as “external” to the region, or not necessarily dependent on conditions

within the African continent. Of course “internal” and “external” processes will interact,

with the latter exerting a greater influence on the former than vice versa (although a dust-

induced cooling of the tropical North Atlantic as postulated in Section 2.5.5 provides a

possible mechanism by which internal processes could influence the external forcings). A

conceptual model can be described in which large-scale changes in the behaviour of the

atmosphere and oceans precipitate drought in the Sahel. The increased aridity in the Sahel

will result in changes in the local and regional land-atmosphere system, which may act to

reinforce and sustain drought conditions even after the removal of the external factors which

initiated drought. In order for a return to wetter conditions to occur, atmospheric or oceanic

anomalies of the similar magnitude and opposite sign to the original drought-inducing

anomalies must occur. Such a scenario has been postulated by Nicholson (1995).


43

This thesis investigates the hypothesis that atmospheric dust may provide an “internal”

mechanism through which the recent drought has been sustained or enhanced. It also looks at

the potential of atmospheric dust to act as a diagnostic of changes in the regional climate of

the Sahel and Sahara. The remainder of this chapter reviews the potential impacts of airborne

Saharan and Sahelian dust on the radiative properties of the atmosphere over and in the

vicinity of sub-Saharan North Africa, and ultimately on the climate of the region, based on

the information concerning this topic available in the scientific literature to date. Sources of

mineral dust are discussed (including the impact of drought on the Sahel as a dust source), as

is the nature of dust aerosols and their transport and concentration in the atmosphere. The

impact of dust on the thermal structure of the atmosphere, and on climate, is also addressed.

PART III: ATMOPSHERIC MINERAL DUST IN THE SAHEL-SAHARAZONE

2.8. The importance of North African dust emissions

Airborne mineral dust from the world’s deserts makes a major contribution to the global

tropospheric aerosol budget, with dust concentrations being particularly high over arid

regions (D’Almeida, 1987). Williams and Balling (1996) describe the Sahara as the world’s

most important dust source; it is thought that northern Africa is responsible for up to half the

global mineral dust emissions. Estimates of the amount of dust exported annually from

northern Africa, effectively the Sahara-Sahel region, range from 260 MT to some 1500 MT

(Schütz et al., 1981; N’Tchayi et al., 1997). Schütz et al. (1981) describe a relatively steady

dust transport away from Saharan regions by the mean regional circulation. They suggest

that transport from the Sahara to the Caribbean is “almost continuous”. Large quantities of

dust from Saharo-Sahelian sources travel over the Atlantic Ocean in the “Saharan Dust

Plume,” which carries aeolian material to the Cape Verde Islands, Bermuda, Florida and the

Amazon Basin (Prospero et al., 1981; Karyampudi and Carlson, 1988; Arimoto et al., 1995;

Ellis and Merrill, 1995; Chiapello et al., 1997). Saharan dust is also transported eastwards in

summer (Newell and Kidson, 1979) and northwards over the Mediterranean to Europe (Reiff

et al., 1986; Moulin et al., 1998a). Analyses of land based (Goudie and Middleton, 1992;

N’Tchayi et al., 1994, 1997) and marine data (Prospero and Nees, 1986) demonstrate that

atmospheric dust concentrations over the Sahel have increased dramatically since the 1950s.

The potential impact of mineral dust aerosols on the earth’s radiation budget has been

discussed at some length (Coakley et al., 1983; Tegen et al., 1996; Andreae, 1996). Li et al.

(1996) estimate the mass-scattering potential of mineral dust in the trade wind layer over


44

Barbados as being some four times greater than that of non-seasalt sulphate aerosols, and

conclude that dust provides the dominant light scattering mechanism over the tropical and

subtropical North Atlantic. The influence of dust aerosols on the regional and local climate

will very probably be greatest over the eastern tropical Atlantic and north-west Africa, closer

to the source regions, although such impacts will depend in a complex fashion on the size

distribution of the aerosols, the altitude range of the dust layer, cloudiness and the properties

of the underlying surface. Dust layers in the troposphere alter the radiative structure of the

atmosphere by cooling the Earth’s surface and heating the atmosphere in the vicinity of the

dust layer (Alpert et al., 1998; Schollaert and Merrill, 1998), enhancing atmospheric stability

and reducing the potential for convection throughout the atmospheric column. The overall

effect on the lower troposphere may be one of heating or cooling, depending on factors such

as surface reflectivity and cloud cover (Lacis and Mishchenko, 1995). Several authors have

speculated that dust may act as a positive feedback mechanism as far as rainfall is concerned,

reinforcing drought (Kellogg and Schneider, 1977; N’Tchayi et al., 1997).

It has been suggested that mineral dust accounts for a significant fraction of the direct

radiative forcing of the global climate due to natural aerosols (Andreae, 1996). Duce (1995)

suggests that the direct radiative forcing due to mineral aerosols is of the same order as that

caused by natural sulphate from biogenic gases, and comparable to estimates of the forcing

from anthropogenic sulphate, and from biomass burning without black carbon. These

comparisons are based on a global mean optical depth of 0.023 for mineral dust. Regional

and local optical depths characteristic of dust events lie in the range 0.3 to 2.5 (Fouquart et

al., 1987; Tanre and Legrand, 1991), implying regional and local forcings orders of

magnitude greater than the global average sulphate aerosol forcing. Any increases in dust

loadings, such as those that have been described for the Sahel-Sahara region (N’Tchayi et

al., 1994, 1997) are likely to result in a sizeable contribution to the regional aerosol forcing.

Negative solar forcing by dust aerosols (Schollaert and Merrill, 1998) will add to the

negative forcing by other aerosol types which, on a regional scale, are thought to partially

mask increases in temperature due to an increase in the greenhouse gas forcing of the global

climate (Karl et al., 1995; Mitchell et al., 1995a,b). The transport of iron in mineral dust may

also play a role in the formation of atmospheric dimethylsulphide over the oceans, and hence

in cloud formation (Zuang et al., 1992, Duce, 1995).

2.9. General climatology of North African Dust

The vast quantities of dust produced in and exported from North Africa are distributed

widely both within and outside the region (Goudie and Middleton, 1992; N’Tchayi et al.,


45

1994; Li et al., 1996). They are responsible for the dust hazes which characterise much of

the continent in the vicinity of the Sahara, and for the presence of atmospheric dust over the

Mediterranean, Europe, the Atlantic and the Americas (Westphal et al., 1988; Swap et al.,

1992; Moulin et al., 1998a,c). Winter transport to the south of the Sahel-Sahara zone and

over the Atlantic is the result of the north-easterly trade winds (the Harmattan circulation)

(McTainsh and Walker, 1982; Pinker et al., 1994). Summer transport across the Atlantic is

determined by easterly waves and the associated convective disturbances (N’Tchayi et al.,

1997); Atlantic dust trajectories also depend on the position of the Azores high-pressure

region, and on the intensity and phase of the North Atlantic Oscillation (NAO) (Moulin et

al., 1997c). Transport over the Mediterranean towards southern Europe occurs as a result of

eastward travelling Mediterranean and North African cyclones (Moulin et al., 1998a).

Dust events over the Sahel-Sahara region persist for times ranging from hours to tens of days

(Jankowiak and Tanré, 1992), with the smaller particles being transported the largest

distances. Dust particles are removed from the atmosphere either by precipitation (wet

deposition) or direct uptake by the surface (dry deposition). Wet deposition is the dominant

removal process for dust particles in the size range 0.1-1 µm (Kiehl and Rodhe, 1995), while

dry deposition is most important for particles larger than about 5 µm (Duce, 1995), which

are either deposited near the source or disaggregated into smaller units by sandblasting

(Gomes et al., 1990).

Duce (1995) reports that particles which are transported long distances are generally

associated with diameters of the order of 1 µm. Upper limits on the diameter of particles

transported away from source regions have been placed at 20 µm (Gillette, 1979) and 16 µm

(Williams and Balling, 1996), although such limits depend on meteorological conditions and

are to a certain extent fairly arbitrary. For example, the strong vertical mixing over Saharan

and Sahelian regions can result in the transport of relatively large particles to high altitudes,

from where they are transported over the ocean in the Saharan dust layer. Carlson and

Caverly (1977) suggest that high concentrations of particles of the order of 4 µm diameter

are transported over the Atlantic. Prospero (1999) reports African dust events characterised

by particles less than 2.5 µm in diameter in the southeastern United States. Particles with

diameters of up to several hundred µm are mobilised during dust events. While most such

particles are deposited or disaggregated near the source, they have been observed over the

Pacific Ocean, some thousands of km away from their sources (Betzer et al., 1988).

One of the principle aims of this thesis is to assess the potential of atmospheric dust to

modify the thermal and dynamical properties of the atmosphere over northern Africa, and

thus to influence the climate of the Sahel, particularly via modulation of spring and summer


46

rainfall. The impact of dust on convective activity (associated with the development of rain-

bearing disturbance lines over Sahelian regions) is of great interest, as is the influence of

widespread atmospheric dust on the large-scale circulation which constitutes the West

African Monsoon flow. Any effects of dust over the Atlantic on SST patterns are of interest,

in the light of the apparent association of Sahel drought with reduced SSTs in the North

Atlantic. These factors suggest that, from the point of view of investigating links between

drought conditions in the Sahel and atmospheric dust loadings, the influence of dust on

circulation over Saharan and Subsaharan North Africa and the eastern tropical North Atlantic

is of most interest. The following discussion will concentrate on the spatial and temporal

distribution of dust over these regions.

2.9.1. Magnitude of North African dust emissions

The figure of 260 MT of dust produced annually by continental North Africa given by

Schütz et al. (1981) constitutes at least half of the northern hemisphere dust production as

estimated by Junge (1979). However, the uncertainties in such estimates are great. More

recent estimates by Swap et al. (1996) describe dust production in North Africa to be in the

range 400-700 MT yr-1. Chiapello et al. (1997) place a lower limit of 300 MT yr-1 on this

range. They cite D’Almeida (1986) as stating that about 45% of the global aerosol emissions

comprise mineral dust from arid and semi-arid regions. N’Tchayi et al. (1997) place global

dust production at between 1000 and 3000 MT yr-1, based on a range of estimates (Andreae,

1995; Duce, 1995), and suggest that around half of this originates in the Sahara-Sahel region.

Moulin et al. (1997c), citing D’Almeida (1986), agree broadly with this estimate, placing the

quantity of African dust transported by the atmosphere at 1000 MT yr-1.

2.9.2. Source regions

The distribution of dust in the atmosphere over the Sahara and sub-Saharan northern Africa

is complex, and data concerning distributions and characteristics of dust over northern Africa

are scarce. Indeed, dust concentrations and deposition at sites outside continental Africa

seem to have been more widely studied than have such phenomena within Africa itself

(Carlson and Prospero, 1972; Swap et al., 1992; Drees et al., 1993; Ellis et al., 1993).

It was accepted until fairly recently that most of the dust produced in and transported away

from northern Africa originates primarily in the Sahara Desert. A consideration of turbidity

values in north-central Nigeria led D’Almeida (1987) to write that “...the Sahel region is an

insignificant area for mineral dust production.” The most important source regions of

Saharan dust occur in northern Mauritania, Western Sahara, the central Sahara and in


47

northern Sudan (N’Tchayi et al., 1997). A source region lying between 20° and 30°N,

extending through Morocco, Algeria, Mauritania and Mali is also described by N’Tchayi et

al., 1997. Source regions have historically been associated with areas in the northern Sahel

and Sahel-Sahara margins such as the Bilma-Faya Largeau alluvial plain in north-eastern

Niger and northern Chad (McTainsh and Walker, 1982; Drees et al, 1993). McTainsh (1980,

1982) identified Harmattan dust deposits in northern Nigeria with source regions in the Chad

Basin, specifically the Faya Largeau - Borkou region (Figure 2.3). He also suggests the

Bodelé Depression as a source region.

Figure 2.3: Major dust source regions and summer and winter dust transport innorthern Africa, from McTainsh (1980).

2.9.3. Seasonality of dust production

Newell and Kidson (1979) describe predominantly Saharan sources of dust in summer and

Sahelian sources in winter. They identify the highest wind speeds as occurring between 10º

N and 15º N (Sahel) in winter, and at 25º N (Sahara) in late summer. Middleton (1985) states

that for Nouakchott and other Mauritanian stations, dust storms tend to be concentrated in

the dry season from January to May. Pye (1987) reports that the highest frequency of dust

storms in the Sahel occurs in the period January to March, while Littmann (1991) describes a

dust storm maximum in April for the Sahel in general, with a secondary maximum in the


48

winter sometimes occurring after one or two dry years. N’Tchayi et al. (1997) describe a

maximum in dust frequency during the Sahelian wet season for sites north of about 16.5º N.

They attribute this summer maximum to dust mobilisation by dry convection associated with

disturbance lines (DLs) which are intense, yet too weak to produce rainfall.

Differences in dust mobilisation and transport between the wet and dry seasons in the Sahel

are due to the different prevailing climatological situations arising from the seasonal

variation in the position of the ITCZ. The northeasterly hot, dry Harmattan winds prevail

north of the surface front between moist monsoonal air and Saharan air, which migrates

between about 6ºN in the boreal winter and 20º-25º N in August in years of maximum

northwards penetration (Hastenrath, 1991; N’Tchayi et al., 1994) (Figure 2.4).

Over the Sahara the mixing layer, which may contain large amounts of dust, extends from

the surface to an altitude of some 5-7 km (Prospero, 1981). Where the Saharan air rises over

the cooler monsoonal air to form the Saharan air layer (SAL), dust of Saharan origin is

confined above the discontinuity between the Saharan air and the moist southwesterly

monsoonal air from the tropical Atlantic. Hence in the northern hemisphere summer, when

the ITCZ penetrates to the southern fringe of the Sahara and the monsoon air layer attains its

maximum depth, dust of Saharan origin above Sahelian and sub-Sahelian regions will only

occur above a certain height. Several authors (Prospero and Carlson, 1972 and Carlson and

Prospero, 1972; Chiapello et al. 1997) describe such an elevated dust layer as occurring

between about 1.5 km and 5-7 km above sea level.

Dust originating in the Sahel in the summer months will tend to remain in the moist

monsoonal air mass. Therefore the atmosphere above the Sahel may contain two distinct dust

layers, a lower layer of locally generated dust, which will contain particles with a wide range

of sizes (including particles with large radii) and a layer aloft (the Saharan Air Layer (SAL))

containing particles which have travelled some distance from their sources and which will

therefore tend to have smaller radii.

In the Sahelian dry season (approximately November-May), the ITCZ lies to the south of the

Sahel, which is subject to the Harmattan circulation. Saharan dust may therefore occur

throughout the air column over the Sahel, and dust is exported over sub-Sahelian regions and

the eastern tropical Atlantic at lower altitudes than in the summer. The Saharan dust aerosols

will be mixed with any locally generated Sahelian particles, resulting in a single mixed dust

layer with a population that is not unimodal in terms of size distribution, due to the different

natures of the Saharan and Sahelian soils and the presence of particles that have travelled

different distances. Such an interpretation is supported by analyses of particle size


49

distributions during dust events in Sahelian regions (e.g. Gillies et al., 1993).

Figure 2.4: Prevailing winter (January) and summer (August) synoptic situation overWest Africa, reproduced from N’Tchayi et al. (1994).

2.9.4. Westward dust transport over the Atlantic

Dust that is not removed by rainfall washout or gravitational settling is transported to the

south and west of the Sahel and Sahara, towards the Gulf of Guinea and across the West

African coast (Drees et al., 1993; Gillies et al., 1996). Dust transported westwards affects

wide areas over the North Atlantic (Prospero et al., 1981; Arimoto et al., 1995; Ellis and

Merrill, 1995; Li et al., 1996; Chiapello et al., 1997). This material remains in the northern

hemisphere, as the residence time of the dust aerosols is much shorter than the time required

for the exchange of air across the ITCZ (Junge, 1979): the maximum boundary-layer dust

concentration measured over the tropical South Atlantic is 7.5 µgm-3, compared with 240

µgm-3 at Barbados, 160 µgm-3 at Bermuda, 380 µgm-3 at Tenerife and 20 µgm-3 Mace Head

in Ireland (Duce, 1995).

Long-range transport of dust to the western Atlantic occurs in the SAL above about 850 hPa,

or some 1.5 km (Carlson and Prospero, 1972; Prospero and Carlson, 1972; Bergametti et al.,

1989, Chiapello et al., 1997). The SAL often lies between a temperature inversion at 1-1.5

km (due to warm Saharan air lying over cooler oceanic air) and a subsidence inversion at 5-7

km (Prospero, 1981). There is relatively little mixing between the Saharan and oceanic air


50

layers, and the lower air layer remains relatively dust free (Rhiel, 1979; Prospero, 1981).

Over Barbados the SAL is still clearly defined, with mean concentrations of dust some three

times greater than those at the surface (Prospero and Carlson, 1972). The vertical structure of

the atmosphere over the Atlantic thus facilitates the long-range transport of mineral aerosols

by keeping them confined above some 1.5 km. The height of the base of the SAL increases

westwards, with the dust plume over Barbados typically centred at 2-3 km above sea level

(Talbot et al., 1986). Transport times for dust crossing the Atlantic are of the order of a week

(Kiehl and Rodhe, 1995).

Westphal et al. (1988) state that some 200 MT of dust cross the West African coast each

summer, with around 37 MT reaching the Caribbean. Prospero and Carlson (1972) estimated

that 25 - 35 million tons of Saharan dust are transported through the longitude of Barbados

every year. Swap et al. (1996) give an indication of interannual variability in the westward

transport over the Atlantic: they calculate an annual westward dust flux of 130 MT in 1990

and 460 MT in 1991, with most dust transported across the West African coast in the period

February-April and a minimum in transport around the November-December period. They

state that in March 1992 some 67 MT of dust was transported across the Atlantic; the figure

for December 1990 was 0.75 MT.

Optical depth measurements indicate that concentrations of dust aerosols can be extremely

high over the eastern tropical North Atlantic, close to the source regions (Carlson and

Caverly, 1977; Lacis and Mishchenko, 1995). Swap et al (1996) state that the frequency of

dust outbreaks over the eastern tropical Atlantic and the spatial extent of the aerosol particles

is greatest from January-June, with a peak in activity occurring in the period February-April.

They describe westwards transport as occurring in a relatively well defined band (the

“Saharan dust plume”) of some 10º - 20º latitudinal extent, which is confined within the

latitudinal zone between about 0º and 30ºN. Husar et al. (1997) have prepared global maps

of seasonally averaged aerosol optical depth (AOT) over the oceans derived from NOAA

AVHRR data. These maps illustrate the dominance of the Saharan dust plume, and show the

seasonal variation in its extent and position (Figure 2.5). The northwards movement of the

plume in spring and summer is evident, (Figure 2.5, a to c) as is the minimum extent of the

plume in autumn (September-November). This migration of the plume is determined by the

position of the ITCZ. A strong signal in the AOT fields is also evident off the coast of west-

central Africa and Angola in the boreal summer and autumn, when the southern hemisphere

regions are dry. This signal is likely to be due to smoke aerosols from biomass burning; it is

also likely that combustion aerosols contribute to the high AOT values over the Gulf of

Guinea in the period December-February (Husar et al., 1997). High values are also apparent


51

over the Arabian Sea in June-August, related to the export of material from Africa and

Arabia by the Indian Monsoon circulation.

It is in the spring and summer that the largest amounts of dust are transported to the western

Atlantic, with high concentrations of northern African dust reported at Barbados (Prospero

and Nees, 1986). Such transport is associated with the development of the westward

travelling disturbance lines associated with easterly wave activity. The approximately north-

south orientation of the easterly waves, and the maximum in the development of the

associated DLs at the latitudes characterising both the Sahel and Caribbean (Rowell and

Milford, 1993), explain the maximum in dust transport to Barbados in summer. Prospero and

Carlson (1972) state that July has a tendency to exhibit the largest dust loadings over the

tropical Atlantic, even though easterly wave activity is more intense in August. It is likely

that the rainfall maximum in August is associated with increased wet deposition, and thus

reduces the amount of material available for transport.

During the dry season, dust is often transported to sites further to the east and north than

Barbados. Geochemical analyses by Chiapello et al. (1997) of dry season (October - April)

dust samples from Sal, Cape Verde, for three years from December 1991, highlight 47 dust

events, 37 of which can be classified as originating predominantly in (i) the Sahel (south of

20ºN, including Senegal, southern Mauritania and southern Mali), (ii) the south and central

Sahara (north of 20º, including Mauritania, northern Mali and southern Algeria), or (iii) the

north and west Sahara (Western Sahara, Morocco and western Algeria). Sahelian dust

reaches Sal only in December and January. The south-central Saharan sources are evident

from November to February; the northwest Saharan sources are apparent throughout the

sampling season, but are strongest in March. The Sahelian events are described as the most

“intensive”. The authors suggest that the temporal variations in the source regions of the

samples reflect the variations in source region activity. Bergametti et al. (1989) detected dust

from the Sahel at Fuerteventura in the Canary Islands during summer, stating that this

material is transported to the region only under rather specific meteorological conditions.

Transport over the North Atlantic, particularly to locations north of the latitude of Barbados,

is governed to a large extent by the behaviour of the quasi-permanent sub-tropical Azores

high pressure region (Chiapello et al., 1997; Moulin et al., 1997c), and the phase of the

North Atlantic Oscillation.

Figure 2.5 (overleaf, a-d): Seasonal maps of aerosol backscattering over the oceansdetected by polar orbiting satellites, prepared by Husar et al. (1997), for the period July1989 to June 1991. Units are in equivalent aerosol optical depth, ranging from 0 to 0.3,with red indicating values towards the top of this range. The region shown is a sub-region of the global map, containing the Atlantic and Indian Ocean setors.


52

a) Dec, Jan, Feb

b) Mar, Apr, May


53

c) Jun, Jul, Aug

d) Sep, Oct, Nov


54

2.10. Recent increases in dust concentrations

Dust concentrations and distributions over the Sahel-Sahara appear to have undergone

significant modification over the past several decades, with atmospheric aerosol loadings

increasing dramatically over the Sahel region and possibly decreasing over some Saharan

regions. N’Tchayi et al. (1994, 1997 - hereafter referred to as N94 and N97 respectively)

have analysed the frequency of low-visibility events associated with dust haze at a number of

Saharan and Sahelian sites over the period from the late 1950s to the late 1980s. They

observe large increases in dustiness frequency at several stations throughout the Sahel,

especially west of 5°W, where some of the largest rainfall deficits have occurred since the

onset of the Sahelian dry episode. Figure 2.6 reproduces their series of low-visibility

frequencies for four sub-Saharan sites, demonstrating a general increase in dust frequency,

upon which is superimposed considerable interannual variability. The frequency of dust haze

increases notably after the dry years in the early 1970s and early-mid 1980s.

N97 also compared the frequency of dust events for the periods 1957-61, 1970-74 and 1983-

87. The largest increase in dust levels over northwest Africa for the latter two periods when

compared with 1957-61 occurred in the period October-December, immediately after the

Sahel wet season, although increases in dust frequency were widespread for all times of year

except the period January-March. N97 describe a shift in the period of maximum dust

frequency at Gao from the dry winter season in 1957-61 to the wet summer season in 1983-

87.

Similar increases in the number of days experiencing dust haze at sites throughout Sahelian

regions are described by other authors. Middleton (1985) states that dust storm activity

increased by a factor of six in Mauritania and up to a factor of five in Sudan after 1968.

Goudie and Middleton (1992) note that the number of days characterised by dust haze at

Bilma increased from 20-30 in 1955 to over 200 in 1973. It should be noted that Bilma lies

in the northern Sahel-Sahara transition zone, in what N97 term the “central Sahara of Niger,”

where rainfall is low. N97 also found a large increase in dust occurrence from 1957-61 to

1970-74, but a slight decrease in dust event frequency from 1970-74 to 1983-87 at the

extreme north of the Sahel (Agadez) and the southern limits of the Sahara, especially at

Bilma. They suggest that the decrease in dust frequency between the latter two periods is due

to enhanced aridity followed by rapid removal of fine material by deflation (the mobilisation

of soil particles by wind (Middleton, 1997)) between 1957-61 and 1970-74, which then

resulted in a weak potential source.


55

,

Figure 2.6: Evolution of frequency of dust haze events resulting in visibilities below 5and 10 km at four sub-Saharan sites, reproduced from N’Tchayi et al. (1994). Units arehours per year.

The dry episode which occurred in the Sahel in the early to mid-1980s was also apparent as a

peak in dust concentrations over Barbados (Prospero and Nees, 1986), which are at a

maximum in the summer months. Prospero and Nees (1977) observed that dust


56

concentrations at Barbados in the dry years of the early 1970s rose to three times the pre-

drought levels, and attribute such increases to enhanced dust production in Sahelian regions

arising from aridification. Prospero and Nees (1986) describe summer dust concentrations

over a 30-year period at Barbados as being well correlated with Sahelian rainfall in the

preceding year, while winter dust concentrations are poorly correlated with rainfall.

Long range transport of dust to sites in the Atlantic is highly dependent on the synoptic

meteorological conditions, which determine the nature of the atmospheric transport, and so is

not necessarily indicative of source strength as a general rule (Moulin et al., 1997c; T.

Jickells, 1998 - personal communication). However, a large increase in concentrations of

atmospheric dust in the Atlantic air over Barbados coincided with the observed increase in

dustiness over West Africa described above, and also with a high frequency of dust

outbreaks along the coast of West Africa (Prospero and Nees , 1986).

Enhanced dust concentrations over the Sahel, and increased dust transport across the

Atlantic, indicate a general increase in the northern African-Atlantic dust budget throughout

the year. It is generally accepted that this increase is a consequence of the recent desiccation

of the region (Prospero and Nees, 1986; Tegen and Fung, 1995; N97). The identification of

quite localised areas (e.g. the Niger Bend region) of increased dustiness lead N97 and others

to speculate that dust production is occurring in new regions as a result of enhanced aridity.

N97 conclude that the importance of the Sahel as a source of atmospheric dust now exceeds

that of the Sahara. However, the relative magnitudes of Sahelian and Saharan sources are

still uncertain. Because of the importance of northern Africa as a global dust source, the

possibility that much of the so-called “Saharan” dust may originate in the Sahel has

important implications for the role of the Sahel as a major contributor to the global dust

budget (Tegen et al., 1996). In order to fully understand the interactions of the dust sources

with the regional climate, dust loadings over continental North Africa must be analysed. A

clear identification of source regions is necessary if the question of the relationship between

the recent drought episode and the observed increase in dust production is to be properly

addressed. While the evidence points to a modulation of dust production by rainfall, this

explanation implies widespread modification of the land surface, and has led some authors to

conclude that dry conditions have resulted in severe land degradation, probably with a

significant anthropogenic component (Tegen and Fung, 1995; Tegen et al., 1996). As

discussed earlier in this chapter, such an explanation is speculative, and has been widely

challenged.

The relative importance of the Sahara and Sahel as source regions over the period 1984-1993

will be assessed in Chapter 5 of this thesis using the Infra-red Difference Dust Index (IDDI),


57

a proxy dust-loading dataset. Individual source regions will be identified by locating regional

maxima in the IDDI fields. Such an analysis should illuminate the relationship between

atmospheric dust concentrations, source activity and climatic conditions.

2.11. Reasons for changes in dust production

2.11.1. Land-surface properties and dust production

Generally, soil degradation as the result of land use practices, combined with a climatic

desiccation, has been cited as the cause of increased atmospheric dust loadings both over

Sahelian regions and in Atlantic air masses (Prospero and Nees, 1986; N94; Tegen and Fung,

1994, 1995; Tegen et al., 1996). The correlation of dust concentrations at Barbados with

Sahelian rainfall deficits (Prospero and Nees, 1986; N97) demonstrates that dust transport to

the Caribbean has increased in low rainfall years.

Soil moisture content becomes negligible some time prior to the onset of the rainy season. It

can be hypothesised that a lack of rainfall alone will only increase the likelihood of soil dust

mobilisation in months when the soil would usually be moist. Acting in isolation, rainfall

deficits may therefore be expected to produce anomalously high dust transport only in and

immediately after the wet season months. If soil moisture were the only determining factor,

late dry season increases in dust production would not necessarily point to aridification as a

cause of enhanced deflation. However, other factors, such as vegetation cover,

disaggregation of soils and the replenishing of aeolian material which can act as a source of

windblown dust (McTainsh, 1982) are also important. Results from north-eastern Nigeria

(Hess, 1996) show a rapid decline in the Normalised Difference Vegetation Index (NDVI)

from the end of the wet season until a time between the beginning of October and the end of

December which depends on the location of the measuring site. The decline is more rapid at

sites with lower rainfall. Once the rapid decline in NDVI has terminated, values resemble

those of the previous January, after which they remain more or less constant, or decrease

very slightly until the onset of the wet season. The implication is that vegetation returns to

close to its pre-wet season state within some three months or sooner after the end of the

summer rains, at which point soil moisture will be around the wilting point of the dominant

vegetation. Gillette (1979) cites Chepil and Woodruff (1957) as concluding that soil

moistures near the wilting point of plants are ineffective in holding soil particles together,

implying that the importance of soil moisture in cementing soil particles together and thus

reducing the likelihood of wind erosion is minimal for most of the dry season.

Soil disturbance by humans or animals may well act so as to make dry soils more susceptible


58

to deflation (Tegen and Fung, 1995). Soils may also tend to become more susceptible to

erosion over several years if vegetation growth is reduced in successive wet seasons, due to

the removal of the stabilising influence of the surface organic material and the input into the

soil of organic decay products after the departure of the rains. Removal or reduction of

vegetation by overgrazing would have similar effects (Lundholm, 1979). Oral accounts

(Panos/SOS Sahel, 1994) indicate severe deforestation and adverse changes in soil and land

characteristics in and around settled regions. N97 suggest that the shift in the time of

maximum dust occurrence at Gao is due to the cumulative effects of drought and land use on

the local soils, combined with a lack of summer rainfall and the seasonal importance of

disturbance lines (DLs). DLs act so as to mobilise soil dust (Pye, 1987), which will remain in

the atmosphere in the absence of rain. Bell and Lamb (1994) found the size and intensity of

DLs in Subsaharan West Africa has undergone a progressive decrease over the period 1951-

1990 and that, since the mid 1960s, the absence of DLs over wide areas has become more

frequent. Since such disturbances are a wet season phenomenon, the implication is that the

DLs are often too small or weak to produce rainfall, but may still be sufficient to raise soil

dust into the troposphere. Rainfall removes dust from the atmosphere (Newell and Kidson,

1979), so low rainfall as a result of weak DLs will result in prolonged/enhanced dust

suspension.

Although land degradation due to enhanced aridity and, particularly, due to land use by

humans has been given a high profile in the literature, the spatial extent, severity and

reversibility of such changes are still uncertain, as discussed in Section 2.4. The effect of

such changes in land-surface characteristics on dust production has not been studied

quantitatively and the extent to which such a phenomenon dominates the region as a whole

is debatable (Jacob et al., 1995; Thomas, 1997). A question which should be addressed is

whether such land degradation is widespread enough to produce new Sahelian source areas

of sufficient extent and severity to explain the very large increases in atmospheric dust

loadings which have been observed. The impact of humans and domesticated animals, and

the replacement of previously dominant vegetation types with those more typical of arid

regions, on factors such as soil integrity and soil moisture need to be investigated more

thoroughly before the relationships between drought, land use and dust mobilisation are

comprehensively understood.

2.11.2. Dust emission and wind

The measured concentrations of North African dust aerosols at Atlantic sites depend heavily

on the transport mechanisms, i.e. on the mean Tropical Atlantic circulation. The wealth of


59

evidence to suggest that the Sahelian desiccation has been associated with large-scale

changes in atmospheric and oceanic circulation (see Chapter 2: Part I) makes it entirely

plausible that changes in circulation may have contributed to changes in the concentrations

of Saharan and Sahelian dust at such sites. Although the association of measured Atlantic air

mass aerosol concentrations with Sahelian rainfall deficits is sound, factors other than the

Sahelian desiccation may also contribute to changes in such concentrations. The potential

impact of any changes in the transport mechanisms involved should be investigated and

accounted for. The activity of the dust source regions exhibits a high degree of seasonality

(Newell and Kidson, 1979; Middleton, 1985; Moulin et al., 1998) so circulation changes in a

particular season may preferentially enhance or decrease long-range transport from particular

regions or latitudes. Newell and Kidson (1979) postulated large-scale changes in circulation

over the whole of North Africa north of the equator as a factor influencing summer Atlantic

dust concentrations.

Enhanced deflation may occur due to factors other than soil desiccation and degradation due

to a combination of reduced rainfall and human-animal impacts. It is possible that enhanced

production of airborne dust in North Africa may be at least in part due to climatic changes

which have resulted in a higher frequency of winds in excess of the required threshold

velocity for dust mobilisation. Changes in wind strength which had this effect in Saharan

regions would necessarily involve a strengthening of the northerly and/or easterly component

of the surface wind, as the prevailing Harmattan is north-easterly. Increases in the strength of

northerly winds have been observed over the equatorial Atlantic, and are associated with the

patterns of SST which are linked with drought in the Sahel (Nobre and Shukla, 1996).

Such an enhanced northerly flow would act so as to oppose the West African Monsoon flow

as well as to increase the likelihood that the dust mobilisation threshold velocity will be

reached, provided the changes in wind strength extend over West Africa. Thus, the

correlation between rainfall deficits and dust concentrations may be partly due to their

independent modulation by a common factor, and the role of the causal mechanism linking

drought to increased dust production via soil desiccation may have been overstated. Goudie

and Middleton (1992) have speculated that the strength of the north-easterly dust-bearing

Harmattan winds has increased, based on the fact that dust was reported to have reached as

far south as Libreville, just north of the equator, in 1983.

N97 describe changes in dust event frequency over the Sahara itself, with relatively low

loadings during the period 1970-74 when compared with the periods 1957-61 and 1983-87,

but find a general increase in dustiness in the central and northern Sahara between 1957-61

and 1983-87. Since the Sahara is arid to hyper-arid, generally devoid of vegetative cover and


60

unaffected by human-animal-land interactions, it is unlikely that changes in the frequency of

dust events or the severity of dust loadings are due to changes in the surface properties of the

desert. N97 suggest that the observed changes in these parameters may be due to alterations

in advection patterns, and they “do not rule out” changes in mobilisation and transport

processes. They suggest that some Saharan stations may be influenced by enhanced Sahelian

dust production; this would necessitate a transfer of the dust from the monsoon air mass to

the air mass dominated by the Harmattan air flow, or a southward encroachment of the

Harmattan surface wind into the Sahel region with a subsequent northward movement of

dust into the Sahara. Enhanced dust loadings at the Saharan sites from 1970-74 suggest (i)

changes in wind patterns which either advect more dust to, or do not transport dust away

from, these regions or (ii) an increase in local or upwind deflation, implying stronger winds

over the Sahara. The reduction in observed Saharan dustiness between the periods 1970-74

and 1983-87 could be due to the rapid earlier removal of light material with the onset of a

stronger wind regime. An increased strength of the Harmattan wind could also explain at

least some of the increase in dry season Sahelian dust production. Such an interpretation is

highly speculative at present, but is worthy of consideration.

2.12. Dust aerosol characteristics and radiative forcing

Dust aerosols affect the atmospheric radiation budget through absorption and scattering of

solar radiation, and emittance and absorption of long-wave radiation (Tanré and Legrand,

1991; Adeyfa and Holmgren, 1996). The relative importance of these processes, and the

effect on the radiative and climate forcings is highly dependent on the particle size

distribution, concentration, composition, optical properties and vertical distribution of the

aerosol. The time-averaged solar forcing by dust aerosols is damped due to its modulation by

the diurnal cycle (Blanchet, 1995). Temporal variations in aerosol concentrations at a given

location are great, with order-of-magnitude changes in concentration occurring on short

time-scales (of the order of days or shorter) as a result of the episodic nature of dust events

(Blanchet, 1995).

2.12.1. Dust-radiation interactions

Attenuation of solar radiation by dust results primarily from scattering. Scattering by an

individual particle depends on the particle’s size, geometry and refractive index, as well as

on the wavelength of the incident light. The wavelength of visible light is around 0.5 µm,

and the most efficient scatterers of these wavelengths are aerosols with diameters of the

same order as the wavelengths in the optical range, from about 0.1 to 1 µm (Friedlander,


61

1977). Junge (1979) suggests that aerosol particles with sizes in the wider range of about 0.1

to 2 µm have the greatest direct influence on short-wave solar radiation.

Absorption of sunlight by desert aerosols is significant, contributing to a regional greenhouse

warming, and perhaps also to a warming of the global climate (Lacis and Mishchenko,

1995). However, dust may result in either a cooling or warming of the regional surface

climate (Lacis and Mishchenko, 1995), and the overall climatic effects of mineral dust

aerosols are unclear. For absorbing aerosols such as desert dust and soot, the aerosol-

radiation interactions are complex, depending strongly on the distribution of particle size and

the altitude of the aerosol layer. The thermal greenhouse forcing of desert dust is a factor of

six greater for particles of equivalent radius of 2 µm than for particles of 0.5 µm (Lacis and

Mishchenko, 1995). As well as absorbing solar radiation and reradiating long-wave

radiation, mineral dust absorbs strongly in the 8-14 µm infra-red region, adding to

greenhouse warming by trapping upwelling long-wave radiation (Duce, 1995).

The greenhouse forcing of dust is proportional to the contrast between the local temperature

of the aerosol layer and the ground surface temperature, and is therefore greater for higher

altitude dust layers. This effect is also highly dependent on the cloud cover; Lacis and

Mishchenko (1995) calculate that the ratio of clear-sky to cloudy-sky radiative forcing

exceeds 13 for dust particles with equivalent radius of 0.5 µm. They also calculate the

instantaneous radiative forcing for aerosols of equivalent radius 0.5 µm, evenly distributed

throughout the atmosphere between 0 and 3 km and representing an increase in optical depth

(see Section 2.12.3) of 0.1. The resulting values are -2.73 Wm-2 (cooling) for clear-sky

conditions and +2.29 Wm-2 (warming) for cloudy-sky conditions, rising to +4.32 Wm-2 when

the aerosol is mixed to an altitude of 6 km. These values should be compared with a

calculated forcing of approximately +4 Wm-2 for a doubling of CO2. The aerosol affects the

cloud single-scattering albedo (the ratio of scattering to extinction, where the extinction is

the fractional energy removed from an incident beam of radiation by scattering plus

absorption) and also absorbs part of the upwelling radiation which is otherwise reflected by

clouds. The value of the single-scattering albedo may determine the sign of the aerosol

forcing: Hansen et al. (1980) calculated that values below about 0.85 will tend to have a

warming effect, whereas albedos above this value will cause cooling. Lacis and Mischenko

(1995) state that the value of single-scattering albedo at visible wavelengths for the desert

dust as described above is 0.95, suggesting that the overall effect of dust aerosols on a

regional scale is likely to be a net cooling.


62

2.12.2. Size distribution of North African dust aerosols

The aerosols which have the largest effect on incoming solar radiation lie at the lower end of

the spectrum of particles of diameter up to 20µm which have the potential for transport over

large distances by soil eroding winds (Gillette, 1979). Williams and Balling (1996) place this

threshold at 16µm. Duce (1995) presents evidence that over oceanic regions far from dust

sources, typical aerodynamic mass median radii of aerosols lie in the range 0.5-1.5 µm. The

lower half of this range represents particles which are efficient scatterers of visible light,

according to the findings of Friedlander (1977) and Junge (1979).

Closer to the source regions, the concentrations of strongly light-scattering particles will be

higher, but the increased quantity of larger particles will result in a higher mean or median

radius. Particle sizes before a strong dust haze event in the inland delta region of Mali in

April 1990, described by Gillies et al. (1996) exhibited a bimodal distribution, with peaks at

around 6 µm and 50 µm; during the most intense phase of the event the population was

unimodal with a peak at 2.5 µm. Fallout from the event was unimodal with a peak at about

25 µm. The implication of these figures is that particles larger than 25 µm consisted of

aggregates of smaller grains and underwent disaggregation in the atmosphere during the

event, while particles smaller than this value were transported large distances, with the larger

grains perhaps experiencing some disaggregation. Although large particles (with diameters

greater than around 20 µm) will generally fall out of the atmosphere in the early stages of

transport, Betzer (1988) describes relatively high concentrations of mineral aerosol particles

with equivalent radii above 50 µm (and one particle of equivalent radius of more than 150

µm) over the central North Pacific, some 8000 km from the source region in China. The

processes by which such large particles are transported such distances are not understood.

The incidence of sub-micrometer aerosol particles is reviewed briefly by Duce (1995), who

cites the following work. D’Almeida and Jaenicke (1981) found a peak in the number

distribution of aerosols at radii of 0.05-0.1 µm for a dust event in Senegal, and identified

particles with radii as low as 0.02 µm. Gomes et al. (1990) found that fine particles with

radii centred around 0.1-0.15 µm were associated with heavy dust loadings in the northern

Sahara. Duce (1995) reports that high fine-particle concentrations tend to be characteristic of

dust events generated by high wind speeds. It is suggested that such fine particles arise from

the disaggregation of larger particles as a result of sandblasting (D’Almeida and Schütz,

1983; Gomes et al., 1990). D’Almeida and Jaenicke (1981) describe a bimodal dust

distribution from measurements in Senegal, the larger mode varying with local wind

conditions and having short atmospheric residence times, and the smaller mode most likely


63

representing a well-mixed population which provides the aerosols which are characteristic of

long-distance transport. Particles with radii above 5 µm are thought to consist of loose

aggregates of the parent soil, with smaller particles resulting from disaggregation due to

sandblasting (Gillette, 1979; Gomes et al., 1990).

2.12.3. Optical depth as an indication of aerosol concentration

An important parameter in the study of aerosol distributions is the aerosol optical depth

(AOD) δδδδ(λλλλ). This wavelength-dependent quantity is defined as the extinction per unit length

over a specified path. The vertical optical path is the vertical distance from the Earth’s

surface to the top of the atmosphere. The slant-path optical depth along a path having an

angle θ from the vertical is equal to the vertical optical depth multiplied by sec θ (Charlson

and Heintzenberg, 1995). An equivalent definition is the integral in the vertical dimension of

the aerosol light extinction coefficient or volume extinction cross-section at wavelength λλλλ

(Ogren, 1995).

The AOD can be related to the aerosol number density given a knowledge of the particle

characteristics. Ogren (1995) presents ranges of optical depths of 0.02-0.1 for clean

continental air and 0.05-0.1 for clean marine air. Clean in this context refers to minimal

anthropogenic perturbation over continental regions and negligible anthropogenic

perturbation over marine regions. The global-mean optical depth due to mineral dust alone

has been estimated at 0.023 (Andreae, 1995; Duce, 1995), or less than the amplitude of

variation of the optical depth of air which is approximately aerosol-free.

The relatively low value for the globally averaged AOD given above should be compared

with regional and local optical depths for visible wavelengths characteristic of dust events,

which lie in the range 0.3 to 2.5 (Fouquart et al., 1987; Tanre and Legrand, 1991). Although

the optical depth is wavelength dependent, Ben Mohamed and Frangi (1986) found that

seasonal variations in AOD at Niamey were similar for wavelengths within the range 0.35-

1.61 µm. Faizoun et al. (1994) have tabulated monthly median values of AOD at Bidi in

northern Burkina Faso for the years 1987-1989. Values range from 0.24 and 0.30 (December

and November values respectively) to 1.40 and 1.26 (June and April respectively of 1988).

The highest values in this period occur in the summer months, with values between 0.85 and

0.94 in July and August of all three years. Weekly mean AOD values at 0.5 µm measured

from February 1986 to June 1987 and described by Ben Mohamed et al. (1992) at several

sites throughout Niger exhibited minimum values around 1.5 and maxima between 1 and 1.5.


64

Fouquart et al. (1987) report values of 1.5 at 0.52 µm during dry haze conditions during

November-December 1980 at Niamey. D’Almeida (1987) found that AOD values at

Boutilimit (Mauritania) reached 2 for periods of the order of a day, before falling to 0.5

within 24 hours. Measurements taken at Ilorin, Nigeria (south of the Sahel in the Guinea

Savannah zone) from 1987 to 1989 peaked at 1.6-1.8 on January 10, 1989, and often

exceeded 0.8 (Pinker et al. 1994). AOD values from the Middle East have been reported as

reaching 2, (rising to 3 just after a frontal passage), with typical values under Khamsin wind

conditions being around 1.5 (Levin et al., 1980). During a dust storm in Tadzhikistan in

September 1989 measured AOD at 0.55 µm reached values between 3 and 5 (Sokolik and

Golitsyn, 1993). Otherwise typical values in this central Asian desert region were 0.15-0.25,

rising to 0.8-1.5 for dusty atmospheric conditions.

The highest values of AOD (in excess of 0.27) over the oceans described by Husar et al.

(1997) occur over the Gulf of Guinea during the northern hemisphere winter, and in the

eastern tropical Atlantic to the west and south-west of the Mauritanian coast in the northern

hemisphere summer (Figure 2.3). High optical depth values are more widespread over the

Atlantic in the latter period, with large values occurring in the eastern tropical Atlantic

between the equator and about 15º S; large values are also present over the Arabian Sea. In

the northern hemisphere winter the high optical depths associated with westward propagating

Saharan dust overwhelmingly dominate the global distribution.

Carlson and Caverly (1977) associate measured atmospheric optical depths greater than 0.7

with conditions of high turbidity during the summer of 1974 at Sal in the Cape Verde

Islands. Their measurements indicate values of 0.9 to 1.0 for days dominated by haze, and of

0.5 for non-hazy days. Villevalde et al. (1994) report that values of AOD are generally a

factor of two greater in the North Atlantic than in the Pacific Ocean, and they present

evidence that the North Atlantic atmosphere is characterised by a greater range of aerosol

particle sizes.

The above AOD values should be compared with the theoretical optical depth increase of 0.1

which produces a radiative forcing comparable, on a regional scale, with that associated with

a doubling of atmospheric CO2 concentrations. While the comparison is not straightforward,

due to the many factors which determine the net radiative effect of aerosols, it is clear that

large dust events have the capacity to have a dramatic impact on the radiative structure of the

atmosphere.


65

2.13. Thermal impact of tropospheric dust

The potential importance of tropospheric aerosols in determining the atmospheric heat

budget has been recognised for some time (Mörikofer, 1941; Gunn 1964; Kellog and

Schneider, 1974, 1977). The observed increase in transport of North African dust, concerns

about desertification, and the increasing sophistication of climate models have encouraged

interest in the role of dust aerosols in the climate system (Andreae, 1996; Li et al., 1996;

Overpeck et al., 1996; Tegen et al., 1996). Many studies have investigated the optical

properties of Saharan and other desert aerosols, and have addressed the interaction of such

particles with solar and longwave radiation (Ben Mohamed and Frangi, 1986, 1992;

Fouquart et al., 1987; Debo Adeyfa and Holmgren, 1996).

Tropospheric dust such as that observed to be widespread in extent over northwest Africa

and the tropical North Atlantic (Chiapello et al., 1997; N’Tchayi et al., 1997) affects the

atmospheric radiation budget by absorbing and scattering both shortwave and longwave

radiation, and emitting longwave radiation (Cautenet et al., 1992). The presence of a dust

layer in the atmosphere will reduce the amount of shortwave radiation reaching the earth’s

surface, predominantly by backscattering of incoming solar radiation. Thermal emission by

the dust will increase the downward longwave radiation, and redistribute longwave radiation

within the troposphere. The former negative solar/radiative forcing of the atmosphere will

act so as to cool the Earth, whereas the latter thermal forcing, which is always positive

(Tegen et al., 1996), will tend to warm the atmosphere and near-surface. The overall effect

on the surface-atmosphere system will depend on the relative importance of the solar and

thermal forcings. The magnitude and sign of the net solar forcing will depend on the

properties of the underlying land or ocean surface and on the presence or absence of cloud

and the cloud heights and properties, as discussed above.

2.13.1. Dependence of thermal impact of dust on the underlying surface

The impact of tropospheric dust aerosols on the atmospheric radiation budget is determined

to a large extent by regional factors. As well as the characteristics of the dust layer, the

nature of the underlying surface will play an important part in determining the net effect on

the earth-atmosphere energy balance. Over bright surfaces, where the land surface is highly

reflective, a less reflective dust layer will reduce the planetary albedo and result in a net

increase in absorption of solar radiation, increasing the energy input into the earth-

atmosphere system and resulting in a net warming. Cautenet et al. (1992) place shortwave

(solar) radiation in the wavelength range 0.25 to 4µm.


66

If an airborne dust layer is more reflective (lighter) than the underlying surface, as is the case

over dark land surfaces and oceanic regions free of cloud, the planetary albedo will be

increased, and proportionately more short-wave solar radiation will be reflected back to

space by the surface-atmosphere system (Overpeck et al., 1996; Tegen et al., 1996). The dust

layer will generate a positive thermal forcing by emitting longwave radiation. The net effect

may be one of warming or cooling, or of no overall change, depending on the characteristics

and height of the dust layer, and the relative reflectances of the dust layer and the land or

ocean. Clouds below the dust layer may result in the dust causing a decrease in the planetary

albedo even over a dark surface such as the ocean, with a consequent increase in solar

forcing, due to the dust being less reflective than the cloud. Thus the presence of reflective

clouds below the dust layer may reverse the effects of the dust as compared with clear-sky

case.

2.13.2. Thermal impact of dust over the ocean

Carlson and Benjamin (1980), using a radiative transfer model and assuming that the

properties of Saharan dust are fairly uniform and constant, found a heating rate of around 1

Kday-1 over the eastern equatorial Atlantic, averaged between 1000 hPa and 500 hPa, below

the top of the Saharan Air Layer and including the lower troposphere where dust

concentrations are low. Their calculated heating rates for the atmosphere over desert regions

were greater, between 1 K and 2 K. Such results may not be entirely realistic, due to

simplifications in the model. Later studies, using a combination of modelled data and

observed satellite data over the Arabian Peninsula and Persian Gulf (Ackerman and Chung,

1992), conclude that dust outbreaks result in a net radiative cooling over the ocean, and

heating over the desert.

Li et al. (1996) report that the annual mean dust concentration in the trade wind layer at

Barbados (based on measurements since 1965) is sixteen times greater than that of non-

seasalt sulphate. They state that the mass scattering efficiency of the dust is about a quarter

of that of the sulphate, resulting in a net scattering of solar radiation by the dust of about four

times that by the non-seasalt sulphate aerosol. Consequently, mineral dust of North African

origin is the dominant light scattering mechanism throughout the tropical and subtropical

North Atlantic. The effect of airborne mineral dust is likely to be much greater near the

African coast, where dust aerosol concentrations will be higher. Considering the emphasis

which has been placed on the role of sulphate aerosols as agents of climate forcing

(Erickson, 1995), it is highly appropriate to consider the potential forcing due to mineral

dust, the direct effect of which, on local and regional scales, is likely to be at least as


67

important as sulphate forcing. However, it should be noted that the indirect effects of mineral

aerosols (for example their effect on cloud formation and development) are thought to be

minimal, whereas sulphates and other aerosols may play a significant role in the climate

through their capacity to act as cloud condensation nuclei (Duce, 1995; Möller, 1995). The

work presented in this thesis is concerned with radiative and dynamical interactions between

dust and climate, rather than aerosol chemistry or the role of dust particles (if any) in cloud

formation.

Concentrations of Saharan and Sahelian aerosols will be at their greatest over north-west

Africa, closest to the source regions. The above findings suggest that dust aerosols could

play a very significant role in modifying the atmospheric heat budget over Saharan and

Sahelian regions, although the impacts will not mirror those over the Atlantic, due to the

differing character of the desert surface and the different elevations of the dust layers and

distribution of particle sizes.

2.13.3. Thermal impact of dust over Northwest Africa

Over desert regions, the combined positive solar and thermal forcings resulting from the

presence of a dust layer will cause a net warming of the surface-atmosphere system. This

warming will be augmented by an enhanced local greenhouse effect arising from the

absorption by the dust of long-wave radiation emitted by the land surface.

At the surface, however, incident solar radiation is reduced by the higher-level absorption

and reflection of incoming solar radiation by the dust layer. This reduction in energy input at

the surface leads to a cooling of the land and near-surface regions. Cautenet et al. (1992)

describe the thermal impact as being “...located primarily at the ground surface.” Williams

and Balling (1996) cite Wen Jun (personal communication, no date given) in stating that

desert dust may reduce the solar radiation reaching the ground surface by up to 50%.

(Moriköfer (1941, cited in Bach, 1976) reported attenuation of direct solar radiation by

desert dust of between 10% and 30% at Davos, Switzerland). Geleyn and Tanré (1994)

report modelled reductions in solar radiation reaching the land surface of up to 60 Wm-1 for

Saharan regions, with a surface cooling of 3 K at 20º N; 30º E. To a certain extent the

cooling of the land surface will be offset by downward fluxes of long-wave radiation

resulting from the heating of the dust layer. These issues are addressed in a number of papers

(e.g. Carlson and Benjamin, 1980; Coakley et al., 1983).

The combination of surface cooling and heating of the dust layer aloft reduces the vertical

temperature gradient, increasing atmospheric stability and reducing convection (Carlson and


68

Benjamin, 1980). Such a mechanism is not dissimilar in impact to that postulated by

Charney et al. (1975), in which enhanced subsidence was held to be the consequence of

increased albedo due to the removal of surface vegetation.

2.14. Dynamical impact of North African dust

2.14.1. The Saharan Air Layer (SAL)

The impact of the dust layer over north-west Africa is closely related to the dynamics of the

SAL, in which the dust is contained. The following description of the SAL is based on that

of Karyampudi and Carlson (1988), hereafter referred to as KC88.

The SAL is formed when maritime air from the Mediterranean region flows across North

Africa and is subject to a strong sensible heating by the underlying Sahara Desert. This

results in a deep isentropic mixed layer of high potential temperature extending to a height of

about 6 km or 500 hPa. The SAL is thus a characteristic of the Harmattan circulation, which

is dominant to the north of the south-westerly monsoon flow, whose northern limit is

approximately coincident with the position of the ITCZ. The height of the base of the SAL

increases as the underlying monsoonal airmass thickens to the south, or with time as the wet

season progresses at a given location subject to the monsoonal flow. The base of the layer

also increases in height towards the west, reaching some 1.5 km at 25ºW (over the Atlantic)

and 2 km or more over the Caribbean. The SAL is confined within a latitudinal band

between about 25º-30º N and 10º-15º N and is associated with the Saharan dust plume.

Where the hot dry air of Saharan origin meets the cooler, moist monsoon air a temperature

inversion occurs, at the base of the SAL. A weak temperature inversion also exists at the top

of the SAL.

Within the SAL, a strong vertical shear in the zonal wind gives rise to an easterly jet between

850 and 650 hPa between 15º and 20ºN, with maximum wind speeds occurring between 600

and 700 hPa. Wind maxima at 700 hPa were observed near 20ºN during the July 1974 GATE

program, although a narrow band of strong winds also appear to be located near the southern

limit of the SAL, coinciding with a maximum in the temperature gradient (KC88).

2.14.2. Dust and convection

Over the desert regions of northwest Africa, radiative heating of the troposphere due to

absorption of shortwave radiation and absorption and emission of longwave radiation by the

dust layer, coupled with the surface cooling resulting from the negative solar forcing of the


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dust, acts to redistribute heat from the surface and near surface to higher levels of the

atmosphere. This will strengthen the inversion at the base of the SAL and will increase the

tendency of the SAL to suppress convection. Such a mechanism was suggested by Kellogg

and Schneider (1977). Radiative warming will also occur to a certain extent below the base

of the SAL; it is possible that this will further suppress convection by reducing the altitude of

the inversion. In the absence of a well defined SAL and associated inversion, the presence of

dust will lead to a slackening of the vertical temperature gradient and enhanced atmospheric

stability and stratification.

In a modelling study of the short-term impacts of several different aerosol types, Geleyn and

Tanré (1994) found that, when dust was present over desert regions, cooling dominated near

the ground, while an ascending circulation aloft resulted from mid-tropospheric heating of

the dust layer. This ascent occurs within the SAL, which has too low a relative humidity to

allow the formation of clouds (KC88). Alpert et al. (1998) estimate a tropospheric heating of

some 0.2 Kday-1 over the eastern tropical Atlantic resulting from the presence of dust

exported from West Africa.

Using a limited area model and case studies based on results from the GATE program of

July 1974, KC88 found that large amounts of dust in the SAL reduced ascent in the

equatorial region, decreasing convective rainfall. Within the SAL itself, ascent increased in

the simulations under conditions of diabatic heating corresponding to dust loading, with

maximum total ascent occurring to the west of Africa in the centre of the dust plume. In the

adiabatic (dust-free) case, ascent occurred to the north of the windspeed maximum, while

sinking was present along the southern edge of the SAL. The authors invoke radiational

warming of dust over the tropical eastern Atlantic as a major influence on the SAL. The total

vertical motion in the region of the SAL is generally characterised by descent except near its

southern limit where the authors describe frequent lines of convection which are related to

strong confluence near 700 hPa.

The windspeed maximum in the summer easterly flow at the altitude of the SAL is

associated with the African easterly jet (AEJ) (Peters and Tetzlaff, 1988). Although the

interaction of the AEJ with the regional climate is not well understood, it is believed to play

a role in the generation of easterly waves and convective disturbances (DLs) (Tetzlaff and

Peters, 1988; Rowell and Milford, 1993). Dust-induced heating of the SAL, and associated

increased ascent in the vicinity of the TEJ, may therefore have an impact on Sahelian

climate. Thorncroft and Blackburn (1999) suggest that meridional circulation arising from

dry convection (such as would arise in the SAL from dust heating) plays an important role in

maintaining the AEJ. Fontaine et al. (1995) conclude that dry Augusts are associated with


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stronger than usual mid-tropospheric easterlies south of and under the axis of the AEJ. While

any interactions with dust are speculative, it is plausible that dust may play some role in

determining the strength of these winds via an impact on the regional or local meridional

circulation through heating of the SAL. In the study of KC88 described above, the mid-level

easterly jet embedded in the Saharan Air Layer was stronger and broader under conditions of

heavy dust loading. As DLs play a role in dust generation and transport, dust-modification of

the AEJ may result in feedback processes in the dust cycle.

2.14.3. Consequences for the regional circulation

It is reasonable to suppose that a modification of the temperature structure of the atmosphere,

with a consequent modulation of atmospheric stability and therefore convergence and

divergence, may well alter regional circulation patterns (Andreae, 1996). Of all the aerosol

types investigated in the short-term modelling study of Geleyn and Tanré (1994) only the

Saharan aerosols had a recognisable impact on large-scale atmospheric dynamics. Modelling

studies of dust loading impacts by Overpeck et al. (1996) indicated that the presence of

tropospheric dust was associated with changes in atmospheric pressure and circulation

patterns over regions with reduced planetary albedo. KC88 present evidence, based on their

modelling results, that dust in the SAL acts to reduce the travel time of the front associated

with the SAL over the Atlantic. Alpert et al. (1998) used discrepancies between NOAA

AVHRR data and a model/data assimilation system (with no parameterisation of dust) to

infer a dust heating of some 0.2 Kday-1 in the lower troposphere over the eastern tropical

North Atlantic. Moulin et al. (1998) describe the mobilisation and transport across North

Africa and the Mediterranean of Saharan dust by cyclones - it is likely that dust plays a role

in the development of these cyclones via heating of the troposphere and a consequent

reduction in atmospheric stability (e.g. Chen et al., 1995).

Very few studies have been undertaken with the specific intention of investigating the effects

of dust loadings over North Africa or the North Atlantic on regional circulation patterns. At

present the nature of such impacts is highly speculative. It is intended that this thesis will

illuminate the issue of the impact of atmospheric dust of Saharan and Sahelian origin on the

circulation in the northwest Africa-tropical North Atlantic region, with the specific aim of

identifying possible feedback effects linking circulation, dust production and rainfall in the

zone ranging from the Sahara in the north to the Guinea Coast in the south.


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2.15. Summary

The recent dry episode in the Sahel appears to be related to hemispheric and global changes

in the oceans and atmosphere, as evidenced by contemporaneous changes interannual rainfall

persistence, atmospheric circulation and patterns of sea surface temperature. There is very

little evidence to suggest that regional-scale climate change has been driven by widespread

changes in the land-surface, and it appears that the magnitude and extent of such changes

have been overestimated. While the northernmost extent of vegetation in the Sahel is highly

variable and much reduced in periods of low rainfall, it appears that vegetation quickly

recolonises desiccated areas when rainfall increases. Palaeodata also indicate a high degree

of variability in vegetation cover on century and millennial timescales, suggesting that the

land surface does not suffer irreversible degradation during protracted dry periods. The role

of man remains ambiguous; while problems of land degradation, deforestation and soil

erosion may be significant on the local scale, it is unreasonable to extrapolate such

phenomena to the entire Sahelian region. In particular, notions of widespread overgrazing

should be reassessed.

These conclusions have important implications for the question of attribution: while the

decline in rainfall may well fall within the range of natural climatic variability on long

timescales, it is highly plausible that it is the result of global changes in climate resulting

from an anthropogenic enhanced greenhouse warming. There has been a tendency for

European and American researchers to lay the blame for climatic and environmental change

in the Sahel at the door of indigenous land-use practices, despite the fact that such methods

have been employed successfully for millennia. Arguments that traditional methods of land

management are no longer appropriate because of population pressure and climatic

fluctuations are tenuous; on a local scale changes in population density and climatic

conditions are unlikely to be a feature solely of the late twentieth century. A movement

towards mechanised agriculture and cash cropping may well give rise to novel

environmental problems, but again the extrapolation of such problems to the Sahelian region

as a whole must be questioned. It is at least as likely that the Sahelian desiccation is a

manifestation of climate modification arising from the industrial practices of the northern

hemisphere developed nations, and of Europe nations and the United States of America in

particular.

The above considerations also question the notion that increases in dust production are

simply the result of widespread desiccation and land degradation in the Sahel. If such

changes in the land surface are of as limited an extent as argued above, alternative

explanations for the observed elevated dust concentrations over the Sahel and North Atlantic


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must be found. It is likely that rainfall modulates subsequent dust production to a certain

extent, as apparent from relationships between rainfall and dust storm activity in West

Africa, and dust concentrations in the atmosphere over Barbados. However, the possibility

that rainfall and dust production are determined independently by the atmospheric circulation

should not be discounted. The role of rainfall in dust production is not straightforward; while

reduced rainfall may make soils more susceptible to deflation by removing vegetation cover

and reducing soil organic content and soil cohesion, rainfall erosion and fluvial transport of

sediment also acts to generate erodible material. Thus, the effects of rainfall on dust

mobilisation are multiple, and will sometimes oppose one another. The degree to which one

effect dominates over the other will depend on the timescales in question and on the

prevailing aridity of the regional climate.

Changes in atmospheric dust concentrations may be the result of changes in mobilisation and

transport mechanisms, for example the frequency of strong winds or convection events

which carry dust particles to high altitudes. A change in the ratio of dust mobilised to dust

removed from the atmosphere, for example arising from changes in the intensity of

convective disturbances and/or wet deposition, may also explain some of the observed

increase in dustiness. This explanation seems plausible given the lower rainfall amounts in

recent decades. Such possibilities will be investigated in Chapters 5 and 6.

Considerations of solar and thermal forcing of the atmosphere by mineral aerosols indicate

that the potential of atmospheric dust to modify the radiative and thermal structure of the

atmosphere is large. Such considerations suggest that the net effect of such material over the

Sahel, Sahara and surrounding regions is likely to be one of cooling near the surface,

particularly if the dust is confined within an elevated Saharan air layer, while warming will

occur aloft due to longwave emission by the dust layer. Such a redistribution of atmospheric

radiation will act to enhance atmospheric stability, and may inhibit convective rainfall.

A conceptual model may be postulated in which enhanced aridity is associated with

increased dust production, although not necessarily through a desiccation of the land surface.

Elevated atmospheric dust concentrations then act to increase atmospheric stability, and

further reduce the likelihood of rainfall. Reduced rainfall results in a higher background

concentration of dust aerosols due to decreased wet deposition. It is not proposed that dust

has initiated aridity in the region, as there is insufficient evidence that changes in dust

production have resulted from mechanisms independent of the regional synoptic climatology

(e.g. land degradation). Also, the evidence that rainfall is modulated by hemispheric-scale

patterns of SSTs and atmospheric circulation is strong. It is likely that the weaker meridional

circulation associated with Sahelian drought (Shinoda, 1990; Fontaine et al., 1995) is part of


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a large-scale climatic mode involving the SST patterns also linked with reduced rainfall in

the Sahel. It is plausible that dust over the Sahel and Sahara causes a regional amplification

of the characteristics of the circulation associated with this climatic mode when it is

otherwise weak on a global or hemispheric scale. In this way, increased dust concentrations

may help to explain the increase in persistence of interannual rainfall in the drier regime, as

well as the fact that rainfall remains low even in years when the SST patterns associated with

drought are weak or absent. This model is different from the geophysical feedback of

Charney et al. (1975), in which removal of vegetation increases atmospheric stability,

reducing rainfall which further diminishes vegetation cover. In the conceptual model of

climate modification by dust, it is changes in the atmosphere which initiate dry conditions.

Dust levels then increase as a result of a change in the balance between dust mobilisation and

dust removal, which may be due to internal atmospheric mechanisms or land surface change,

or a combination of both. However, changes in the land surface are not a prerequisite for

changes in atmospheric dust concentrations. Although the land provides the mineral aerosols

which constitute the active mechanism of the feedback process, the feedback itself may be

entirely located within the atmosphere, with reduced deposition elevating dust concentrations

without the requirement for increased input of dust particles from the land.

This thesis tests the above conceptual model by examining associations between rainfall and

subsequent dust production, relationships between dust concentrations and the synoptic

climatology, and correlations between dust concentrations and atmospheric temperatures and

vertical motion at various altitudes. Meridional air transport across the Gulf of Guinea,

Guinea Coast and Sahel is related to dust loadings in order to identify any modification of

the regional circulation by dust. Dust concentrations and distributions are represented by the

Infra-red Difference Dust Index (IDDI). This dataset, and the other data used in the studies

described in the thesis, are described in the following chapter.

Documents

PART I: INTRODUCTION AND OVERVIEW 2.1. Introduction · PDF filecomparisons of timeseries of rainfall and dust concentrations over the Atlantic (Prospero and Nees, 1986) or the frequency