Teleconnections modulating inter-annual climate variability over northern Namibia

INTERNATIONAL JOURNAL OF CLIMATOLOGY

Int. J. Climatol. 19: 1459–1475 (1999)

TELECONNECTIONS MODULATING INTER-ANNUAL CLIMATEVARIABILITY OVER NORTHERN NAMIBIA

MARK R. JURYa,* and SUSANNE ENGERTb

a Geography Department, Uni6ersity of Zululand, KwaDlangezwa 3886, South Africab Geography Department, Wuerzburg Uni6ersity, Germany

Recei6ed 29 January 1998Re6ised 16 No6ember 1998

Accepted 24 No6ember 1998

ABSTRACT

Regional teleconnections that anticipate and sustain anomalous climatic conditions in northern Namibia are studied.A rainfall index is formulated using 11 stations in the area 17–20°S, 15–19°E. The three wettest and three driestsummers are selected: 1974, 1976, 1978 and 1981, 1983, 1988 to produce composite maps of environmental fields atlags −6 months, −3 months and zero. Sea surface temperatures are warmer in dry years across most of the Atlanticand western Indian Ocean at all lags. Convection is more active in the central Indian Ocean during the OND seasonpreceding a drought, according to satellite OLR. Upper westerlies are strong over the tropical Atlantic in the JASseason preceding a dry year, and a low level anticyclonic vortex is present in the South Atlantic. Together theseindicate a Walker cell anomaly over the tropical Atlantic with lower easterly/upper westerly flow contributing todescending motion over Namibia and adjacent parts of southern Africa. The results help identify mechanismsunderlying inter-annual climate variability over northern Namibia. From the analysis, a multiple linear regressionmodel is formulated to predict the late summer rainfall. Its predictors and potential skill are outlined. Copyright© 1999 Royal Meteorological Society.

KEY WORDS: northern Namibia; inter-annual climate variability; teleconnections; sea surface temperature; rainfall

1. INTRODUCTION

Summer rains in the January–March season in northern Namibia offer a limited opportunity for dry landcrop farming. Because of high evaporative losses and the limited holding capacity of the soils, agriculturalactivities require careful management. The study area is within the domain 17–20°S, 15–19°E. It includesOwambo tribal lands, Etosha game park and the Otavi triangle where commercial farming is important.Significant crop yields are derived from millet, maize, cotton and sunflower. Grazing of livestock alsocontributes to agricultural productivity. Higher population density in the Owambo district has led todegradation owing to outdated methods of land cultivation, overstocking of animals and the stripping ofnatural vegetation cover. Regional dam levels have been in decline since the mid 1970s. In the period1992–1996, the established dams in Namibia were B20% full, and grave concerns were expressed aboutthe continued ability of the country to provide water to its people. The drought in 1992 was linked to theIndo-Pacific El Nino and was widespread, whereas the drought in 1995–1996 was associated with theAtlantic El Nino, and more confined (CAC, 1995, 1996).

The landscape in northern Namibia is composed of flat bushlands interspersed with small pans that actas focal points for development and resource utilization. Heavy downpours cause the pans to overflowand inter-link, causing flooding in the absence of vegetation to buffer run-off. In 1974 and 1997,widespread rainfall was observed, dam levels rose above 80%, groundwater was recharged and agricultureoutput was boosted, despite flooding.

* Correspondence to: Geography Department, University of Zululand, KwaDlangezwa 3886, South Africa.

CCC 0899–8418/99/131459–17$17.50Copyright © 1999 Royal Meteorological Society

M.R. JURY AND S. ENGERT1460

The climate of south-eastern Africa is modulated by the global ENSO phase and associated warm-ing of the tropical Pacific and Indian Oceans (Rocha, 1992; Pathack, 1993). Rainfall over Botswana,South Africa and Zimbabwe is typically 50–70% of normal during a warm event. Over northernNamibia, the link between rainfall and conditions over the tropical Pacific is more tenuous. ThePacific Southern Oscillation Index (SOI) accounts for &20% of summer rainfall variance. The Namib-ian rainfall spectrum is dominated by low amplitude 2–5 year cycles (Nicholson and Entekhabi, 1987;Nicholson, 1989) that have been linked to fluctuations in the Benguela sea surface temperature (SST).Jury (1996a,b) relates El Nino-like events in the tropical eastern Atlantic to drought over south-west-ern Africa, and proposes a convective dipole mechanism. Composite warm events are preceded bywesterly surface wind anomalies and Ekman downwelling over the central Atlantic (Hirst and Hasten-rath, 1983; Servain and Legler, 1986). Winds converge over the Angolan coast and adjacent ocean atthe expense of the interior continental plateau, where drought conditions ensue. In the extendedAtlantic El Nino of 1995–1996, summer rainfall over northern Namibia was B50% of normal.Historically, Atlantic warm events occur at decadal intervals and may not function as a dominantclimatic determinant at inter-annual time scales.

Namibia is a developing country with moderate infrastructure. Its economy is dependent on theuptake of resources to best advantage by a small population concentrated in Windhoek and along thenorthern border. However, the dry climate imposes limits on agricultural productivity and waterresources. The coefficient of rainfall variability is &40% (Nicholson et al., 1988) and about one-thirdof all years can be described as drought- or flood-prone (\ (0.5 S.D.). Sharp gradients in meanannual rainfall divide the Namib coastal desert (20 mm) from the Zambezi rainforest to the northeast(1200 mm, Gondwe and Jury, 1997). Longitudinal shifts in rain bands are likely to be modulated bybackground circulation and regional teleconnections. We intend addressing the following questions inour research:

(i) What is the character of year-to-year rainfall variability over northern Namibia? Are there significantrhythms?

(ii) How do droughts and floods evolve over a 6-month period prior to occurrence? Are rainfallfluctuations teleconnected to climatic patterns?

(iii) What SST and OLR patterns anticipate wet and dry summers? How is the summer circulationmodulated in dry and wet years?

(iv) Can statistical associations be exploited to enable climate prediction?

2. DATA AND METHODS

Rainfall from 11 stations in northern Namibia (square in Figure 1) were averaged for the rainymonths: January–March in the period 1962–1993. The three wettest and three driest summers wereidentified and composite responses were evaluated using regional-scale environmental fields. Griddedatmospheric data were available to determine the evolution of convection and circulation anomalies.Monthly averaged estimates of cloud depth, from outgoing longwave radiation (OLR) for the period1975–1994 and wind flow patterns at 3 and 12 km levels from 1968–1994 were obtained from theNational Centre for Environmental Prediction (NCEP) at 5° intervals. These data come from thepre-reanalysis period, as this research commenced prior to its availability. SST data were available at4° resolution from the COADS ships dataset for the period 1960–1992. Composites were constructedby averaging the fields according to the dry and wet seasons, and the historical mean was subtracted.Hence, \60 months of reference data were subtracted from a 9-month composite, to determine dryand wet anomaly patterns and their evolution from the preceding spring. Additionally, a case studycomparison is made using ECMWF weather data averaged over 90 days (JFM) in 1990 (wet) and1992 (dry) at a grid point 20°S, 17.5°E.

Copyright © 1999 Royal Meteorological Society Int. J. Climatol. 19: 1459–1475 (1999)

TELECONNECTIONS MODULATING INTER-ANNUAL CLIMATE VARIABILITY 1461

Figure 1. Historical mean 850 hPa dewpoint depression averaged over for December–February months (from Taljaard, 1972) usingradiosonde data

3. RESULTS

3.1. Mean fields

In this section the regional climatology is presented for selected meteorological fields using historicalanalyzes and mean ECMWF data for the period November–March 1986–1992. The 850 hPa dewpointdepression analysis of Taljaard (1972) clearly indicates an arid zone over the Benguela coast in thesummer season (Figure 1). Differences \14°C are found and indicate subsident conditions relating tocold underlying SSTs and divergent equatorward trade wind flow (Jury and Reason, 1989). The analysis

Figure 2. Historical mean water vapour flux (g kg−1 m s−1) averaged over the November–March months from ECMWF data.Northern Namibia study region is indicated by open square. Vector scale is given



Figure 3. Historical mean 200 hPa divergence (upper, 10−6 s−1) and vorticity (10−5 s−1) averaged over the November–Marchmonths from ECMWF data. The upper level circulation over southern Africa is dominated by divergent anticyclonic conditions

stems from radiosonde profiles, for example at Alexander Bay (29°S, 16°E), where 850 hPa dewpointdepressions reach 40°C on occasion. An eastward shift of the subsident area can induce droughtconditions over southern Africa.

The water vapour flux (WVF, e.g. specific humidity weighted vector) describes the movement ofmoisture by winds in the lower troposphere. Figure 2 illustrates moisture advection around thesubtropical anticyclones, in the monsoon regions to the north and in the zonal westerlies to the south.Much of the moisture rounding the South Atlantic high is dispersed seawards and unavailable tocontinental rainfall systems. On the other hand, moisture influx from trade wind and monsoon sources tothe northeast provides some inputs during summer. A weak northeasterly stream from Zambia directsmoisture towards eastern Namibia. A portion of southerly marine flow is recurved over Angola and isavailable for uptake. Further south, southwesterly flow from the coastal zone brings dry air to theadjacent interior.

The upper atmospheric circulation is described in Figure 3 using derivatives of the 200 hPa wind field.It is known that convection in the late summer is more efficient with anticyclonic spin and divergence at200 hPa (Lindesay, 1988). Positive upper divergence spreads from the SW Congo across south-easternSouth Africa. Regions of upper convergence, hence sinking motions, are found over the marine areas off



Angola and in the southern Mozambique Channel. The upper vorticity is almost everywhere anticyclonic,owing to a semi-permanent upper ridge and the shearing action of the subtropical jet from the SouthAtlantic. When anticyclonic vorticity invades the lower troposphere, convection is suppressed, asdiscussed in the case study later.

3.2. Rainfall

The 30 year northern Namibian rainfall index for late summer is shown in Figure 4. Wet (1974, 1976,1978) and dry (1981, 1983, 1988) years are evident in the record. Statistically significant 2.3 year anddecadal cycles are prominent in the spectral power (not shown). The frequency distribution is skewedtoward mild drought conditions. Using this summer rainfall index, Jury (1996a,b) conducted a pair-wisecorrelation study at various lags. The results of this work are summarized in Figure 5. A number of keyteleconnections were found: OLR (cloud depth) and poleward surface wind flow in the southern IndianOcean at lag −4 months (September), SST anomalies in the South Atlantic Ocean during late summer(March +2) and upper level zonal winds over the equatorial Atlantic Ocean at lag −4. Subsequentstatistical analysis of these associations did not reveal a stable relationship with respect to fluctuations ofrainfall.

3.3. Dry and wet composites

Figure 6 illustrates intra-seasonal differences between composite dry and wet years using pentad rainfalldata. Sustained wet spells are absent during dry summers, particularly from mid-January to mid-March.Jury and Engert (in press) outline weather patterns supporting short-lived wet and dry spells in northernNamibia. The composite SST distribution for dry minus wet years is illustrated in Figure 7. SSTdifferences are positive in the late winter season (lag −6 months) almost everywhere, except immediatelyoff Angola. The axis of positive SST differences in the Indian Ocean lies along 60°E, whereas in theAtlantic, SST are warmer in dry years in the 5–10°N band along the coast of West Africa, and in the20–30°S latitude band. In the OND season, a spreading of the area of warm SST is noted. SST differencesare most positive in the 20–30°S band in both the Atlantic and Indian Oceans, areas found by Nicholsonand Nyenzi (1990) to influence east African rainfall variability as well. The dry minus wet scenario (0 lag)illustrates that both the Atlantic and Indian Ocean are�1.0°C warmer. Maximum differences occur in the0–10°S tropical band in the longitudes 25–5°W and 50–80°E, and in the subtropical South Atlantic along

Figure 4. Late summer (JFM) rainfall departures for the period 1963–1993 based on 11 stations in northern Namibia within theopen square shown in Figures 2 and 3



Figure 5. Pair-wise correlation maps in respect of the Namibia JFM rainfall time series shown in Figure 4 and environmental fields:map for OLR and surface wind (upper panel) at lag −4 months (September); SST correlation map at lag +2 months (March); and

200 hPa wind correlation map at lag −4, adapted from Jury (1996a,b)



Figure 6. Comparison of the seasonal distribution of pentad rainfall averaged over the study area for ‘dry’ and ‘wet’ years chosenin the composites, and also for a different mix of dry and wet years. Contrasts occur after the beginning of January

25°S. A more detailed analysis near the continent reveals maximum SST differences of 1.8°C west ofAngola at 0, 2°S. In contrast, differences in the western Indian Ocean are small. It is hypothesized thatoceanic warming west of Angola reduces surface wind divergence and land–sea temperature contrasts,which in turn shifts convective uplift from land to ocean.

The sequence of events leading to drought or flood years is traced using OLR in Figures 8 and 9 (dryand wet, respectively). Largest departures during JAS occur in the Atlantic Ocean 5–20°N, 20–40°W, andrefer to increased cloud depth prior to a dry summer. The OND map indicates that drought builds uplocally with positive OLR departures. Increased clouds occur over the central Indian Ocean from50–90°E, 5°N–10°S. During the January–March season, the ocean areas to the east and west experienceabove normal cloud along the 20°S axis, while the north Indian Ocean is drier than usual (+OLRdeparture). In the wet scenario, the JAS pattern illustrates negative OLR departures locally over Namibia,whilst the Indian Ocean east of 80°E has reduced convection. In the OND map (−3 month lag) a largearea of increased cloud depth extends over east Africa and the adjacent west Indian Ocean between 5 and20°S. The area of increased convection retreats towards Mauritius, 60–80°E, 10–25°S during summer.Concommitantly negative OLR departures develop over Namibia and Botswana, signalling sustainedconvection.

The circulation pattern underlying drought evolution is illustrated in Figures 10 and 11. Upper level(200 hPa) wind departures are predominantly westerly (easterly) over the South Atlantic (western IndianOcean), in the preceding JAS season. Easterly anomalies persist over the western Indian Ocean into theOND season. By late summer the intrusion of northern hemisphere westerlies across the equatorialAtlantic is a key feature of the drought circulation.

The low level circulation preceding dry years (Figure 11) is dominated by the South Atlanticanticyclone. Mid-latitude westerlies are stronger than normal across the South Atlantic and Indian Oceansduring JAS. Southeasterly wind anomalies are strong NW of Angola in the OND season. The SouthAtlantic anticyclonic anomaly persists through the JFM drought period. Equatorial easterly 700 hPa windanomalies are sustained over the Atlantic and also over the west Indian Ocean. In the latter case, theeasterlies appear to be driven by a strengthened Arabian ridge from the northern hemisphere. To the eastof Madagascar, poleward wind anomalies are sustained next to a persistent cyclonic vortex centred on25°S, 45°E. Although NE flow anomalies are sustained over central southern Africa, these appear to be



Figure 7. Composite maps of SST anomalies for dry years minus wet years, running in sequence through the seasons JAS, OND,JFM. Values are multiplied by 10. Largest differences occur in the South Atlantic and central Indian Oceans where dry years are

1°C warmer than wet years



Figure 8. Composite maps of OLR anomalies for dry years minus the historical mean (1974–1994). Positive values are dashed.Relatively weak signals are present



Figure 9. Composite maps of OLR anomalies for wet years minus the mean (as in Figure 8). Indian Ocean convection is opposedto that over Namibia



Figure 10. Composite maps of 200 hPa wind anomalies for dry years minus the historical mean (1968–1992). Vector scale is given(m s−1). Upper westerlies over the tropical Atlantic characterize the drought circulation



Figure 11. Composite maps of 700 hPa wind anomalies for dry years minus the mean, as in Figure 10. A lower level anticycloniccirculation persists in the South Atlantic



Figure 12. Composite maps of 200 hPa wind anomalies for wet years minus the historical mean, as in Figure 10. Upper westerlyanomalies persist over the Indian Ocean prior to rainy summers



Figure 13. Composite maps of 700 hPa wind anomalies for wet years minus the mean, as in Figure 12. Low level westerly windanomalies develop over the equatorial Atlantic whilst an anticyclone over South Africa feeds moisture towards Namibia



drawn into the Atlantic anticyclone and deprive weather systems over northern Namibia of rain-produc-ing potential.

In the flood scenario upper level flow is anomalously westerly across much of the tropical band duringthe JAS season (Figure 12). This pattern is maintained over the Indian Ocean during OND, whilst southof Africa upper westerlies increase. By late summer, a deep anticyclonic vortex is centred on 30°S, 30°E,resulting in upper easterly wind anomalies over Namibia and Botswana. Equatorward upper windanomalies are sustained over Madagascar through the flood sequence. In the lower level (Figure 13),easterly wind anomalies remain over the central Indian Ocean, however, over the tropical Atlanticwesterly anomalies are prominent during OND and JFM seasons. Cyclonic anomalies occur over theSouth Atlantic and South Indian Oceans where semi-permanent high pressure cells are normally situated.This suggests a weakening of the subtropical anticyclones prior to a wet summer over Namibia. Polewardrecurvature of westerly flow from the tropical Atlantic is promoted over Namibia. Southwesterlyanomalies are persistent to the east of Madagascar. Together these patterns sustain an anticyclonicanomaly during late summer that brings easterly flow in the low levels from Madagascar.

4. CASE STUDY COMPARISON

Considering the ECMWF dataset that offers useful insights to processes underlying dry and wet summers,a comparison is made for the JFM seasons of 1990 (wet) and 1992 (dry). Seasonal means are comparedfor the two years using grid values at 20°S, 17.5°E. Differences between two seasons are listed in TableI.

Largest differences are noted for: upper level divergence (1990\1992), mid-level vorticity (moreanticyclonic in 1992), upper meridional winds (more northerly in 1992), vertical uplift (greater in 1990).Surprisingly, some of the smallest differences are found in the low level moisture and wind flow patterns.Dewpoint temperatures at 3 km are only 0.2°C lower during the 1992 drought. It is clear that dry and wetyears depend more on the properties of the circulation above 5 km than on the near surface moisturesupply. Largest regional scale differences in upper flow patterns in 1990 and 1992 occur over the tropicalSE Atlantic, where in the dry summer NW wind flow is considerably stronger, in agreement with Figure10 (lower). In wet summers, northern and southern hemisphere westerlies are decoupled over the tropicalAtlantic, and the South Atlantic ‘wave’ remains undeveloped.

5. DEVELOPMENT OF A FORECAST MODEL

Predictability of Namibian summer rainfall is possible using predictors after the preceding June, when theglobal El Nino and regional monsoons begin strengthening. Mattes and Mason (1998) discuss thedevelopment of a similar statistical model. Potential predictors include:

(i) the Southern Oscillation Index (SOI) and 30 hPa quasi-biennial oscillation (QBO) from Mason(1992) and Jury et al. (1994)

(ii) tropical Atlantic upper zonal wind anomalies over the Atlantic

Table I

Td700Vr500D6200D6850W500V200 Td500U200V700U700T850

1.31990 −16.622.2 −3.1 −1.5 4.9 −0.9 −0.11 −3.6 7.4 2.51992 6.3 1.1 −17.225.2 −2.6 −1.0 5.8 −2.3 +0.01 −2.7 −0.9

The subscript refers to pressure level; T, temperature; U, zonal wind; V, meridional wind, W, verticalmotion (10−2); D6, divergence (10−6); Vr, vorticity (10−5); Td, dewpoint.



Figure 14. Jack-knife predicted (dashed) and observed JFM rainfall for northern Namibia. The model lead time is 3 months

(iii) sea surface temperature (SST) principal component time scores for the global oceans, extending to40°S

(iv) pressure, SST and surface winds in the tropical SE Atlantic to the west of Angola: 0–15°E, 0–10°S;and in the Indian Ocean region from 15°N–30°S, 40°–100°E

(v) satellite OLR data for the Indian Ocean as above, from 1971–1992 with gaps filled using cloudalbedo (HRC)

(vi) southern ocean pressure, SST and surface winds averaged over the area 40°–55°S, 30°W–60°E

Environmental targets and candidate predictors, such as upper winds and satellite estimates ofcloudiness (OLR), overlap from 1971 to 1992. The extraction of predictors from global gridded data setsis guided by the above composite maps together with a number of factors discussed by Jury et al. (inpress). In addition to SST patterns, key areas for atmospheric fields are extracted for use as predictorsfollowing spatial averaging over areas \106 km2 and temporal smoothing over 3-month periods, as donefor the composites.

The multivariate regression model is constructed in step-wise fashion to retain only significant,uncorrelated (rB0.4) predictors to a maximum of three to reduce artificial skill. The model shouldachieve a hindcast correlation of 57% for 17 df to achieve statistical significance at the 99% confidenceinterval. The model algorithm for northern Namibia late summer rainfall is:

−0.80(oCIst)+0.51(aEolr)+0.34(oSTang)

The model accounts for 52% of JFM rainfall variance at a lead time of 3 months. The jack-knife skillscore is 65% and the tercile hit rate is 68%. The predictors include the central Indian Ocean SST (5°S,62°E), east Indian Ocean OLR (5°S, 77°E) and SST west of Angola. All predictors are for the SON seasonexcept for the OLR (JAS). Model jack-knife skill validation is graphed over the 1971–1992 trainingperiod in Figure 14. The model suggests that cooler waters in the central Indian Ocean, below normalconvection west of Indonesia and above normal SST off Angola precede rainy summers.

6. SUMMARY

In dry years Namibia becomes a desert unable to support even limited farming operations. Once every2–10 years, good rains enable self-sufficiency in food production and water replenishment at a nationallevel. It is therefore useful to establish what mechanisms underlie inter-annual rainfall variability. In thispaper we have outlined composite regional teleconnections that anticipate and sustain anomalous climaticconditions. The inclusion of years in the composite was based on a rainfall index averaged for stations



scattered across Namibia north of 20°S. Wet summers include 1974, 1976 and 1978; dry summers are1981, 1983 and 1988. Sea surface temperatures are warmer in dry years across most of the Atlantic andwestern Indian Ocean. Convection is more active in the central Indian Ocean during the OND seasonpreceding a drought. Upper westerlies are strong over the tropical Atlantic in the JAS season precedinga dry year, and a low level anticyclonic vortex is present in the South Atlantic. Together these indicate aWalker cell anomaly over the tropical Atlantic with lower easterly/upper westerly flow contributing todescending motion over Namibia and Botswana.

It should be noted that some of the composite patterns for dry and wet summers are not opposing andtherefore offer ambiguous forecast signals. The composites compare the wet 1970s with the dry 1980s andit is assumed that historical analyzes will indicate common features of future dry and wet scenarios. Thecomposite analysis highlighted the Atlantic Walker cell as a determinant of Namibian climate. In amultivariate formulation, SST are dominant in a model, which accounted for 52% of the rainfall varianceat a lead time of 3 months.

Further studies are recommended to determine the extent of influence of Indian Ocean convection andcirculation, in light of the patterns illustrated in Figure 5. An understanding of how changes in SouthAtlantic SST affect the local anticyclonic circulation is needed. Climate impacts should be assessedthrough crop yield and water balance studies, including the predictability of temperature.

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Teleconnections modulating inter-annual climate variability over northern Namibia