Progress in Physical Ceography 16,3 (1992) pp.283-318

The Australian summer monsoon:a revtewR. SuppiahClimate lmpact Croup, CSIRO, Division of Atmospheric Research, Private BagNo. 1 , Mord ia l loc 3195, Aus t ra l ia

Abstract: The Australian summer monsoon influences the climate of the Australian tropics during the periodfrom December to March. During this period, interannual and intraseasonal vadations of rainfall associatedwith global-scale circulation anomalies strongly effect human life and economic activities in this region. Anychanges in the global-scale circulation pattems in relation to changes in the heat balance

"o-fo".ii, ""a..enhanced greenhouse condition could alter monsoonal circuladon characteristics and thus couli bring seriousimpacts to human life in the monsoon-dominated region. To provide a basis for looking at chinges inmonsoonal characteristics under enhanced greenhouse condition, iha arlrr.rrt understanding of the Australiansummer monsoonal circulation characteristics is reviewed here. Detailed information is givei on the formadonand the steady development of che Pilbara heat low over the nprhwestern pan of Australia and the importanceofthe location of the monsoon shear line, active and break cycles ofthe monsoon, influence of South Ciina Seacold surges on monsoon activity, 40-50 oscillation in monsoon rainfall and winds and their link to El Nifro/Southern Oscillatioh (ENSO) phenomenon, the influence of the ENSo phenomenon on rainfall on interannualtime scales and the link between monsoonal activity and tropical cyclones. The problems related to the abovementioned topics and their research priorities are highlighted.

Key words: Austtalian summer monsoon, Pilbara heat low, monsoon shear line, El Nifro/Southem Oscillation(ENSO) phenomenon, active and break cycles ofthe monsoon, cold. surges, crossequatorial flow, intraseasonaloscillation, interannual variation, monsoon depressions and tropical

"y"l.n.r.

I Introduction

The term 'monsoon' is derived from the Arabic word, mq.usim which refers to theseasonality of winds (Ramage, l97l). However, the term monsoon is presently used todistinguish various climatic, agricultural and land use phenomena. Based. on Ramage'scriteria that rely on atmospheric circulation differences between January and July, the areanorth of 25oS over Australia could be delineated as monsoonal. This monsoonal areaconsists of more than 400 000 square kms covering the nofihern halves of $TesternAustralia and Queensland and more than 90% of the Northern Territory. As the summermonsoon rainfall strongly determines the agricultural and industrial r"ctor. of this area,any strengthening or weakening of the Australian summer monsoon associated withchanges in general circulation would cause drastic changes in crop production and cattlenumbers, particularly in the inland regions. These impacts could be exacerbated byextreme climatic events' such as if the monsoon circulations were used to exhibit stronger

@ Edward Arnold 1992.

284 The Austral ian summer monsoon: a review

intraseasonal and interannual variability under enhanced greenhouse conditions. A reviewof the Australian summer monsoon was previously undertaken by McBride (1987).However, in this article special attention has been given to the active/break cycle of themonsoonJ influences of the South China Sea cold surges on monsoon activity, andintraseasonal rainfall variability during the monsoon season. The article is divided into 10sections. They are: Monsoon climate in northern Australia; The Australian monsoondefinition; Summer monsoon heat lows and the monsoon shear line; Australian monsoononset and withdrawal; Active, moderate and break phases of the monsoon; Surges andmonsoon activity; Inrraseasonal scale (40-50 day) oscillations in the monsoon circula-tions; Interannual variations of monsoon circulation, rainfall and the Southem Oscillation;Tropical cyclones and the monsoon; and Conclusions.

ll Monsoon climate in northern Australia

Although the northern Australian region is vast in size and shows diverse characteristics insoils and vegetation, a certain unity is observed due to the influence of the monsoon. Inparticular, this region is characterizedby a pattern of uniform high temperature and solarradiation and strong seasonal rainfall incidence (Fitzpatrick and Nix, 1970; Nix andKalma, 1972; Nix, 1983) and is demarcated as Koppen's Aw (tropical wet-dry climate),along the northern coast, as BSft (tropical steppe climate) and BlVh (tropical desertclimate) in the interior, and as Af (equatorial wet climate), Aut and Czu (temperate rainyclimate) on the east coast (Strahler, 197 5). The seasonal march of rainfall in tropical andsubtropical latitudes in Australia can be explained by seasonal movements of the equatorialtrough (Hobbs, 1973;1975; Riehl, 1979). The seasonal, poleward shift of the subtropicalhigh pressure belt from about 30oS in winter to about 40'S in summer and the southwardmigration of the equatorial trough following the sun, bring pronounced changes inatmospheric circulations and rainfall characteristics over north Australia. This regionexperiences a short wet season in the north and east, that is modulated by the proximity ofthe active equatorial trough (Nicholls et aI., L982i Kininmonth, 1983). It also experiencesa prolonged dry season during the rest of the year under the influence of the subtropicalridge and southeast trades. In the Southern Hemisphere summer, the monsoonal-typeweather, which is associated with violent thunderstorms, tropical depressions and severecyclones, gives much rainfall to the northern half of Australia when the equatorial troughlies at its extreme southern limits.

In winter, the area located south of the subtropical high-pressure belt experiences mostof its rainfall from the passage of moist low-pressure troughs. The west coast of \TesternAustralia receives little rainfall from low-pressure systems approaching from the Indianocean during this season. The area north of the subtropical belt is generally fine in thewinter half year due to steady dry easterly winds.

The distribution of mean annual rainfall over tropical Australia is illustrated in Figurela. The heavy rainfall regions are concentrated along the coastal area, with rainfalldecreasing markedly towards the inland regions. The influence of topography upon rainfalldistribution is clearly seen, particularly in the eastern part of north Australia. During thesummer, moisture-laden, on-shore winds cross the warm Coral Sea, giving a substantialincrease in rainfall along the eastem flanks of the Great Dividing Range and the east coastof Queensland. On the other hand, the western slopes of the Great Dividing Range alsoreceive a considerable amount of rainfall due to orographic effects during westerly flow in

R. Suppiah 285

Figure la Mean annual rainfall of tropical Australia in mm. Contour interval is 200 mm.Source: Bureau of Meteorology (1988b).

summer. It is also evident from Figure la that the wettest parts of tropical Australia coverthe nonhwest, north and northeast coasts. On the northwest and norih coasts the annualrainfall exceeds 1600 mm (Snake Bay - 1638 mm; Darwin - 1659 mm). There are rwomaximum rainfall regions on the northeast coast of Queensland. One with above 3800 mmis located around Innisfail (3813mm) and another with above l700mm is aroundProserpine (L794 mm). In general, rainfall decreases rapidly toward the inland and reachesits minimum over the Simpson Desert, in South Australia with less than 150 mm. (SeeFigure I a for the location of stations.)

Figure lb depicts mean annual rainy days over tropical Australia. Mean rainy days alsoreveal a clear contrast in their spatial pattern which is strongly influenced by topographicalfeatures. They vary from under 30 over the Simpson Desert to above l6O on-the .u...r.,flanks of the Great Dividing Range over Queensland. In particular, a sharp increase inrainy days is observed from the coast towards inland on the eastern haf of northernAustralia.

The rainfall of tropical Australia is highly seasonal and heavily concenrrated in theperiod between December and March, the summer wet season. The distribution ofmedian summer rainfall and its contribution to annual median rainfall are illustrated in

1 10"E 1 200E 1 300E 1400E

Figure lb Mean annual rainy days of tropical Australia. Contour interval is l0 davs.Source: Bureau of Meteorology (1988b).

1 30"E

286 The Austral ian summer monsoon: a review

100s

Figure 2a December to March median rainfall in mm.

Source: Bureau of Meteorology (1988b).

100s

1 100E 1200E 130"E 140"E 150"E

Figure 2b Contribution (%) of December to March rainfall to total rainfall.

Source: Bureau of Meteorology (1988b).

Figures 2a and 2b. Here, the median rainfall varies from less than 100 mm over the

Simpson Desert to above 1600 mm over the northeast coast of Queensland. The 200 mm

isohyet lies near 20oS, notably, over Vestern Australia and Northern Territory. This

isohyet is located further south in Queensland, indicative of the influence of winter rainfall.

Apart from the maximum rainfall over northeast Queensland, two maximum rainfall zones

are observed in the northwest coast, one centred around Darwin and another located near

Kuri Bay. These two maximum zones also reveal the influence of topography on rainfall.

Occasional heavier falls in the interior are evident in the 90 percentile map (Bureau of

Meteorology, 1988a) and in the rainfall records of Alice Springs (Kininmonth, 1983)' As

indicated in figure 2b, over 80% of total rainfall is received within this four-month period

in the area north of 20"S in the westeffr and eastem parts of tropical Australia, and from 50

to 80% is received between 20 and 25"5.Similar to rainfall amounts, rainy days also indicate strong seasonality. During the

summer season, rainy days vary from below 10 in the interior to over 70 on the northwest

and east coasts. Figure 3 indicates a steady, southerly decrease in the number of rainy days

in the northeastern part and a gfadual decrease in the westr where the contribution of

winrer rainfall is higher than in summer. This figure also reflects the topographical effects

on rainfall.

R

R. Suppiah 287

1 10"E 1200E

Figure 3 December to March rainy days.Source: Bureau of Meteorology (1988b).

1 300E

lll Australian monsoon definition

Early studies on the Australian monsoon began with the works of Hunt ( I 908), Hvnt et al.(1913) and Hunt (1929). In this series of studies, the influence of the temperaturedifference between the Australian landmass and the surrounding oceans on the formationof monsoon flow was emphasized. This idea had, of course, emerged from the early worksof Halley (1686) and later of Hadley (1735) who attempted to explain the origin of theIndian monsoon with the differential heating process and earth rotation. However, basedon an analysis of air masses around the Australian region and circulation pattems over theeast Indian Ocean and heating process of north Australia, Gentilli (l97ia; l97lb; I7TZ)classified the monsoonal flows into three types. These are displayed in Figure 4. bentilliargued that westerly winds in the northwest of Australia are rhe Southem Hemispheretrade winds deflected at their northem edge toward the northem Australian heat low. Henamed the truncated westerly flow as 'pseudo-monsoon'. A pseudo-monsoon occurs overthe northern part of Western Australia, where the northwesterlies are shallow and arclargely dependent on southeast trade winds deflected by the semi-permanent hear lowlocated over the Pilbara area of Vestern Australia. Another flow is named as ,quasi-monsoon'J and related to the pattern in which the flow is directed towards eastcentral andsoutheast Queensland' as a response to the inland heat low over western Queensland. Heconsidered the 'true monsoonal' region as the area that lies between pseudo- and quasi-monsoons, north of 20oS. These winds include westerly or northwesterly winds of tropicalmaritime origin and equatorial air masses. In recent studies, however, Webster (19gla;1987a) stated that together with differential heating and earth rotation, moist processesshould be considered to understand the structure of the monsoon circulation. \xrhur.rr.,processes drive the Australian monsoon, it is evident from previous works on rainfall andwind field analyses that the area north of 25'S undoubtedly experiences a summer wet andwinter dry climate. Further, the monsoonal characteristics of north Australian climare arecomparable with other tropical monsoon regions of Asia and Africa where the seasonalmigration of equatorial trough dominates the alternation of wet and dry seasons. Theseregions also show a seasonal reversal of winds between winter and summer.

2BB The Austral ian summer monsoon: a revlew

Figure 4 Classification of the Australian monsoon wind system'

Source: Gentilli (l97lb).

tv summer monsoon heat lows and the monsoon shear line

Fleat lows form under hot clear skies where surface albedo is large' As Ramage (1971)

points out, well-developed heat lows are noticed only where the sun's rays reach the

irorrna surface under cloudless skies. Although the radiational cooling at night reduces the

i"ntensity of the heat low, it still persists under summer atmospheric conditions. Increases

in temperature, due to an increase in insolation over the period between October and

Decem^ber preceding the monsoon season, are a pai'ticularly striking feature- in the

northwestern pu.t of 'Western Australia. Here the maximum temperatures reach above

39"C in Octoler (Bureau of Meteorology, 1988a). The temperature increase,near the

ground surface results in a very large heaf flux from the surface to the atmosphere and

Iurr.", net heating within a deep mixed layer. This process initiates a Iow pressure afea at

the lower part of ihe air column with cyclonic circulation below 800 mb level'

Moriarry (Ig55) demonstrated that two heat lows exist over the Australian desert

region, orre ".ntred

near the Pilbara area in the northwest of Western Australia and the

other near cloncurry in western Queensland. observational and modelling studies (Falls,

l97Oa;1970b; Leiglton, 1979) ionfirmed that the two lows are apparently related to

cyctonic circuiations where maximum heating occurs. Of these, the Pilbara low is the

,irorrg", due to the large land-sea tempefature gradient on its nofthwestern to western

flank (Ramage, 19?lj. the i.rt.nsity ofltre Pilbara heat low may be increased due to the

arrival of warm-dry'sorrth."st traie-winds, which cross the inland desert region of

Australia. The steady development of these heat lows is perhaps the most important factor

in the formation of ilie Austialian summer monsoon circulation. These heat lows also act

as a triggering mechanism for the formation of westerly winds and organized convection

during the monsoon season. Figures 5a and 5b illustrate typical midsummer synoptic

p"rr"ri. for surface heat lows arrd.rpper level anticyclonic circulations (Leslie, 1980)'

R. Suppiah 289

110 'E 130.E 150.E

Figure 5a Mean sea-level pressure analysis for 23rd700 mb level for 23 December. 1973.Source: Leslie (1980).

1100E 130.E 150.E

December, 1973. b Geopotential heights at

The application of a surface heat balance scheme (McBride, 1975; Leslie, 1980) and atwo-layer quasi-geostrophic model incorporating orography, surface heating and release oflatent heat in convectioq (Adams, 1986), has demonsrrared the existence of these heatlows during the summer season. Adams (1986) pointed out rhar a shallow easterly low-level air stream and weak vertical wind shear are favourable conditions for the formation ofa large amplitude trough during summer. These heat lows are parricularly well simulatedin the sea-level pressure patterns. Leslie (1980) has also pointed out that these heat lowsare more persistent than rapidly changing middle latitude low-pressure sysrems. Fur-thermore, these heat lows are apparently related to radiational and dynamical processes.They are very complex in their vertical and horizontal scales, and particularly sensitive tochanges in temperature over the desert regions of Australia. Such factors could beinfluenced by an increase in temperature under enhanced greenhouse conditions.

The line which separates the trade wind easterlies and the westerlies further north overthe Australian continent is called the monsoon shear line or monsoon rough and it isforced by continental heat lows (McBride and Keenan, L982). The climatological positionof the monsoon shear line at the gradient level is shown by McBride and Keenan (1982). Astriking feature is the concave shape of the monsoon shear line toward the main heat low inthe northwestern part of Vestem Australia (see Figure 4 of McBride and Keen an, 1982).This configuration indicates the influence of the strong land-sea temperature gradient onthe western flank of the monsoon shear line. Compared to the western margin, its meanposition over the eastern half of tropical Australia is found further norrh over Queensland,due to topographical effects. It runs parallel to the 27'C isotherm over the oceanic area, aprerequisite condition for the development of tropical cyclones (Palmen, 1956). Thewesterly winds north of the monsoon shear line are associated with high summer rainfall,while an easterly wind regime south of the monsoon shear line is linked to less rainfall. It isalso characterized by a large north-south gradient in rainfall. The monsoon shear line may,however, change its position and direction on day to day timescales due to the influence ofland-surface processes and circulation changes at upper and lower levels.

V Australian summer monsoon onset and withdrawal

A systematic investigation of the onset of the Australian summer monsoon and its related

2gO The Austral ian summer monsoon: a review

rainfall was begun with the work of Troup (1961), who used two criteria to define the

onser dares, orr. by using the area-avetaged rainfall of six stations within 300 km of Darwin

and the other with referlnce to westerly wind spells over Darwin' According to his rainfall

definition, monsoon onset occurs on any occasion after I November when the rainfall

record of four or more stations over Ndays exceeds 0.75(Nt 1) inches' This work was

mainly based on the criteria used to define the summer monsoon onset over India' where a

sharp and sustained increase in rainfall is used as a prime requisite for declaring the

monsoon onser over a station or a group of stations (Ramakrishnan and Narayanan, 1955).

Nicholls et al. (1982) and Nicholls dg3+a) defined the onset days by using Darwin's

rainfall alone and staied that Darwin's rainfall can be predicted three months in advance

from its antecedent pressure data. They also pointed out that the amount of rain received

during the wet season is weakly correlated with the onset dates. In addition, they found the

rainfall received in the middle and later parts of the season is totally unrelated to either the

onset or the amount of rainfall during the early part of the season' From their analysis it

was shown that a strong onset could give more than 500 mm to the northwestern \?estern

Australia and in the northern part of Northem Territory'

Secondly, Troup (1961) dimonstrated that a spell of moderate west wind at 3000 ft

occurs "nd

l".t for Ndays when the cumulative zonal wind component (eastward positive)

exceeds lg(N+ 1) knot. This criterion is satisfied when this component is greater than 5

knot (2.58 ms-t; otr two consecutive days. He also observed sffong easterlies at the upper

level i40 000 ft) during the time of strong westerlies and heavy rain spells. In another

study, Berson and Troup (1961) showed that the monsoon onset is associated with the

pole#ard displacement olth. Southern Hemisphere subtropical jet stream. Murakami and'Sumi

(t9g2i; lg}Zb) used the 850 mb level wind field to determine the onset of the

monsoon. Contrary to earlier observational studies, Murakami and Sumi demonstrated

that low level westerlies were first established over the western South Pacific

(160"E-lZOo\X/,0o-10oS) in response ro the intensification of the trade winds over the

North Pacific and crossequatorial northerlies near 170oE. Consequently, they suggest that

the monsoon become. n"Uy eastablished over tropical Australia with the westward

propagation of low-level westerlies into the Indonesian-Arafura Sea region' Based on

,ut.ttii. pictures Davidson et at. (1983) defined the onset with an active large-scale blow-

up of tropical convection occurring over a large area. The blow-up is required to cover 10o

latitude by *or. than 30o longitude and to persist for several days. Holland and Nicholls

(lgg5) also defined the onset dates by the first appearance of 850 mb westerly wind spells

at Darwin. As an example, Figure 6 shows monsoon onset, active and break periods based

on 850 mb winds over Darwin.Holland and Nicholls (1985) observed a strong positive correlation between the onset

dates and the eastern equatorial Pacific sea surface temperature (SST) averaged over the

area 10oN-10os, lgo.-go"urr. Based on this relationship, they pointed out that the early

onsets of the Australian summer monsoon tend to precede the subsequent El Nifro events'

Generally, El Nifio events tend to indicate that the subsequent monsoon onset will be late'

These observations are consistent with the findings of Rasmusson and carpenter (1982)'

Changes in the sign of correlation coefficient patterns from positive to negative beNveen

the onset dates and the SST on the biennial timescales, suggest that this relationship may

arise, at least partly, due to the influence of the strong quasi-biennial oscillation (QBO) in

the pressure and SST variations in the Indonesia-north Australia regions (Hopwood,

1968; Nicholls, 1978) and generally over the tropics (Trenberth' 1975; 1980a; Roplewski

et al., 1988).

R. Suppiah 291

MONTH

Figure 6 Time sequence of daily mean zonal winds at Darwin for the L978/79 summer and wintermonsoon seasons. The light curve is a Iine joining all data points and the slightly smoothed heavy curve iscomprised of a set of cubic splines. The summer monsoon onset and finish, and active and inactiveperiods area indicated.Source: Holland (l 986).

During the onset of the monsoon, increased crossequatorial flow from the NorthernHemisphere winter monsoon reaches a broad region extending from 100"E to 170orJrbetween 0o and 10oS. As a consequence, the maximum zone of cloud bands with activeconvective activify is often located at the intersection of two Hadley Cells, from theNorthern and Southern Hemispheres (Davidson er al., 1983; 1984). The SouthernHemisphere Hadley Cell also becomes more intense during and after the monsoon onset(Berson, 1 96 I ; Murakami and Sumi, 1982a; I982b; Davidson et al., 1983). These fearuresare associated with strong low-level convergence and upper-level divergence over themonsoon region. However, Radok (1971) stated that the strength of the upper westerliesin the Southern Hemisphere represents a secondary control on the monsoonal flow.

Abrupt changes also occur at upper levels, especially at the 200 mb level during themonsoon onset (Troup, 1961; 1967; Berson and Troup, 1961; Radok, 1971; Healy,1972;Murakami and Sumi, 1982a;1982b; Davidson et aL,1983i McBride, 1987; Gunn et al.,1989; Hendon et al., 1989; Hendon and Liebmann, 1990a). Streng*rening of easterlywinds is observed in the area extending from Malaysia to Papua New Guinea. A weakenedSouthern Hemisphere subtropical jet moves poleward by more than l0o, from 25oS to40'S. Another significant characteristic is the southward shift of the 200 mb ridge over theIndian Ocean one or two weeks prior to the onset. The seasonal shift of the ridge fromnorth to south of Cocos Island (l2"ll'Sr 96o50'E) leads to the replacement of the upperlevel westerlies by easterlies. Radok and Grant (1957) repofted that though the shift occurscoincidentally with the southward displacement of the Southem Hemisphere jet, a time lagexists between the major flow pattems. Berson and Troup (1961) argued that the low-levelwesterlies of the summer monsoon are dynamically linked with the upper-level tropical

a

i ;z t sO 2N U> z< ;

! >- o> O- ^o 4

B

U

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tsO

U

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1978/79 SEASON85Omb LEVEL

Ia

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O N D J F M A M J J A S

292 The Australian summer monsoon: a revlew

easrerlies which exist simultaneously. However, Frost and Stephenson (L964) stated thatthe surface westerlies and upper easterlies are parts of a simple zonal cell which has theascending branch over Indonesia and a descending branch over, or near, Gan. The chartsproduced by Ramage and Raman (1972) demonstrate that the upper-air flow patterns ofboth hemispheres during their summer circulations play an important role in inter-hemispheric flow patterns.

A number of recent studies 0Villiams, 1979; 1981; Murakami and Sumi, 1982b;Davidson et al.,1983;Johnson and Houze, 1987; McBride, 1987; Webster, 1987b) haverevealed that the onset of the Southern Hemisphere summer monsoon appears to occur asan integral part of planetary-scale circulation changes over the Northern and SouthernHemispheres. Compared with the Northern Hemispheric summer monsoon circulationfeatures, w_eaker anticyclonic circulation at upper levels during the Australian monsoonseason is most likely a consequence of the absence of a mechanical barrier of highmounrains plus the smaller latitudinal and longitudinal extent of the Australian continent.In contrast, Asia, which occupies a larger area and has high mountains, can produce stronganticyclonic activity over the Tibetan Plateau.

It is worth noting that the beginning of the monsoonal rain in the northwestern part ofAustralia does not necessarily coincide with the onset, or the burst, of the summermonsoon as defined by the winds. The monsoon rains are usually preceded by transitionalrainfall (Nicholls et al., 1982), which accounts for 30o/o of the total wet season rainfallaround Darwin. This pattern is very similar to the Indian summer monsoon onsetcondition, particularly over southern India and Sri Lanka. In these areas, a considerableamount of rainfall is observed to be associated with a series of synoptic scale disturbancesprior to the onset of the Indian summer monsoon (Rao, 1976; Joseph, 1988). The time lagfor the Indian monsoon varies from four to six weeks between rainfall and Indian monsoononset, but for the Australian monsoon the time lag is not regular and has a stronginterannual variability. The above mentioned facts suggest that the onset of the westerlywinds at the 850 mb level seems to be a reasonable criterion to declare the onset of theAustralian monsoon.

An increase in easterlies over Darwin during mid-March is defined as the withdrawal ofthe monsoon (Holland et al.,1984; Holland and Nicholls, 1985; Holland, 1986). Duringthe withdrawal of the monsoon, weakening of equatorial upper easterlies and strenglhen-ing of subtropical upper westerlies are observed. The strengthened Southem Hemispheresubtropical jet shifts equatorward by about 10' (Muffatti, 1964). Simultaneously theanticyclonic circulation at the 200 mb level above the equatorial heat source moves fromI0'S to 15'N (Murakami and Nakazawa, 1985); and eventually propagates northward andreaches the Tibetan Plateau by Northem Hemisphere midsummer. Such seasonaldisplacemenrs are well documented in the half-yearly oscillations deduced from windfields (van I-oon, 1967). Based on the Winter Monsoon Experiment flWMONEX)observations, Holland et al. (1984) pointed out that upper-level circulation changes lag thelow-level circulation changes by two weeks. Of course, this time lag varies on theinterannual timescale as noticed by Holland (1986). At lower levels, southeast trade windsgradually occupy tropical Australia. The above discussion reveals that the onset and thewithdrawal of the Australian summer monsoon occur as part of global-scale circulationchanges, but also have strong influences on regional-scale rainfall intensity anddistribution.

Figure 7 depicts the interannual variability of the extent of the summer monsoon atDarwin together with onset and withdrawal dates from 1952 to 1983, taken from Holland

R. Suppiah 293

t\il I N M E A N I " l A X g D

( T N S E T D A T T ' l O V 2 3 DEC 2 J A N 2 7 r 5

F I N I S H D A I F . I A N 1 M A R 7 A P R 6 r 8

L E N G I I I 7 4 r r g 2 5

J A N F E B I , I A F

D A T E

Figure 7 Summer monsoon periods at Darwin, 1952-83 (stippled) together with onset, finish andlength statistics. The standard deviation (SD) and the length are in days, the FGGEMMONEX season ishighlightened, and the 1982/83 season has been included.Source: Holland (1986).

(1986). The mean onset date is December 24rbut the date can range from November 23to January 27. The mean withdrawal date is March 7, but it can vary from January I toApril 6. It is also evident that the length of the monsoon season shows considerablevariability on interannual time scales.

Vl Active, moderate and break phases of the monsoon

The main features of the planetary and regional scales of the Australian summer monsoonmay be identified with the following circulation characteristics, which were extensivelystudied during the UfMONEX in 1978-79 in the Australian region:

1) The lower tropospheric 'true monsoonal' westerlies predominantly come from theNorthern Hemisphere.

zoa

a

5 5 , ' 5 6

6 0 / 6 l

6 5 / 6 6

7 0 1 7 1

7 5 / 7 6

8 0 / 8 1

294 The Austral ian summer monsoon: a review

2) l-nw level westerlies in the northwestern part of Western Australia are southeasterliesdeflected toward the main heat low over the Pilbara area.

3) The flow with a westerly component has greater thickness from the surface to 400 mb

level over the region between 0o and 15'S and has an eastward extension from 100'E.4) There is ascending motion with strong convergence in the lower trophosphere south of

the equator.5) There is a clear exchange of air between the upper level easterlies of the tropics and

upper level subtropical westerlies of both hemispheres east of 100"E.

The above circulation patrerns show that the Northern Hemisphere Hadley Cell extendsinto the Sou*rern Hemisphere.

A fully developed monsoon circulation system has at least three synoptic scale pattems,

namely activer moderate and break phases (see Figure 6) whose occulTences are associatedwith intraseasonal and interannual variations of the main components of the entire

monsoon system. The relative phases of these three synoptic features determine the overall

spatial and temporal patterns of rainfall during the monsoon season, which in turn control

the agricultural production of tropical Australia. The active phase is essentially a more

energetic form of the moderate phase, with a pronounced poleward location of the

monsoon trough or 'monsoon shear line', strong low-level westerlies with enhanced

sparially organized convection and heavy rainfall, and strong upper-level easterlies' These

synoptic features are frequently associated with intense tropical depressions or tropical

cyclones. The break or dormant phase of the monsoon is associated with a shallow

equatorward monsoon shear line, weak low-level westedies or easterlies and weak upper

easterliesr and little or no rain. The moderate phase is defined as the phase with an average

condition in the synoptic components of the monsoon. The flow patterns associated with a

moderate monsoon can be found in the average flow charts produced by Ramage and

Raman (1972).Recently, McBride (1986) classified the monsoon circulation into three phases which

consist of a break, active type I and active type 2, rather than the two-phase system of

active and break. His classification scheme is reproduced in Figure 8, in which the breakphase is demarcated by the absence of convection. The active phase I is marked by

ionvection without tropical depressions and cyclones. The active phase 2 is classified as

one in which there is a sequence of convective features, such as tropical depressions and

cyclones, embedded along the monsoon shear line. The active phase I in Figure 8 is very

similar to the moderate phase described in this text. In this section, the circulation

characteristics for the active and break periods are discussed in detail, as they are ofgreat

interest from an agroclimatological point of view.A simple index that captures the onset, active, break or inactive, and withdrawal of the

monsoon is the 850 mb level wind components at Darwin. Figure 6 depicts schematicallya time series of daily mean zonal winds at Darwin for the 1978179 summer and winter

monsoon seasons covering the WMONEX period. McBride (1987) showed an oppositephase in wind components at the 150 mb level for the summer seasonr an indication of

westerlies (easterlies) at the lower (upper) levels during the monsoon season. These

findings together with other observational studies (Berson and Troup, 1961; Murakami

and Sumi, 1981; 1982a; 1982b; Sumi and Murakami, 1981; McBride, 1983; Allan' 1983;

Davidson et al., 1983;1984; Johnson and Houze, 1987) clearly indicate that strong low-

level westerlies and strong upper-level easterlies are the main wind components for an

active monsoon, and that weak low-level westerlies or easterlies and weak upper-level

R. Suppiah 295

BREAK MONSOON-\7TACTIVE MONSOONTYPE '

uolrs@|. coilvEcnoil

Figure 8 Proposed separation of monsoon activity into a three phase (two active, one break) system.The dashed line shows the location of the monsoon trough. The shaded area depicts the cumulonimbusactivity.Source: McBride (l986).

easterlies are the wind components indicative of a break or an inactive monsoon.Another important aspect is the organized convection during the monsoon season over

tropical Australia. In an active phase of the monsoon, a zone of spatially organizedconvection lies over the area between the equator and l5oS, l00o and 140"E. Convectiveactivity is particularly striking along the western boundary of the strong westerly windregime, where the land-sea temperature gxadient is large. As previously pointed out, thewesterly flow in the northwestern part of Vestern Australia is not derived from cross-equatorial flow but rather is associated with Southern Hemisphere trade winds curvingaround the main heat low (Davidson et aI., 1983). However, during a break period, thezone of active convective activity is located further eastward, east of 150'E. Theidentification of the primary location of the active convective activity zone associated withlow-level westerlies and its phase propagation are controversial. Based on the 850 mb windcomponents Murakami and Sumi (1982b) demonstrated that the low-level westerlies werefirst established over the South Pacific region from 140'E to 170"E between the equatorand 10'S. They also noticed strong convective activity over the regions of pronouncedequatorial westerlies east of New Guinea. Based on these observations, they claimed that

296 The Austral ian summer monsoon: a review

the zone of strong convective activity with strong westerly regime is established first overthe westem South Pacific (160'E-170o\U7 0o-10oS) and then propagates westward. In thesame study, Murakami and Sumi stated that this eastward propagation may not beindicative of all years and that these phenomena may differ on interannual timescales. Thiswestward propagation was not supported by Davidson et ql. (1983), though they noticed alarge area ofenhanced convection over the region 0o-10oS, 155'E-180'in response to theincreased trade winds and crossequatorial flows. However, McBride (1983) concludedfrom a study using the satellite imagery that these synoptic scale convective systems canmove either eastward or westward. These synoptic scale weather systems, of course, do notrepresent the characteristics of the large-scale monsoonal circulation features that emergefrom land-sea temperature contrast.

Rainfall patterns which represent convective zones also reveal contrasting features intheir distribution between active and break phases of the monsoon. Tanaka (1981)observed two rainfall maximum zones during a strong monsoon season, one that coversnorth Australia and the other that occupies the area from 0o-15oN, 110'-170'E. Hereported that these two zones apparently coincide with two intertropical convergencezones. A zone of minimum rainfall was also observed over the region east of New Guinea.The opposite pattern of rainfall distribution has been noticed for a weak monsoon.Interestingly, the locations of these rainfall zones agree well with the areas of strongwesterlies and maximum cloud zones described in this paper.

The latitudinal/longitudinal displacements and intensity of the monsoon shear lines onthe intraseasonal and interannual timescales also play an important role in the active/breakcycle of the summer monsoon (Murakami and Sumi, 1982b; Davidson et al., L983).InpafticularJ an intense (weak) monsoon shear line is associated with strong (weak) monsooncirculation and the presence (absence) of tropical cyclones and heavy (less) rainfall.During an active phase, the monsoon shear line is embedded with tropical depressions andcyclones (Holland, 1978) and is located poleward of its mean position, while it is locatedmore equatorward and eastward during break periods (Bond, 1954). A schematicrepresentation of the interannual variations of the monsoon shear line with its dis-continuities during the preonset and onset periods is reproduced from Davidson et al.(1983) in Figure 9. In their study, the influence of the sequence of high- and low-pressuresystem movements and their development in the Southern Hemisphere midlatitudes on ayear to year basis were emphasized. During an active phase, which is apparently similar tothe onset phase, the following features can be seen:

1) Southward location of a continuous monsoon shear line.2) More organized eastward drift in subtropical ridge with cols and frontal cloud bands

forming over southeastern Australia, and extending in a southeast-northwestdirection.

3) A strong anticyclonic circulation dominating over southwestern and southcentralAustralia.

4) A strong convective activity region, which comprises Indonesia-North Australianareas, is located north of the shear line and a rainfall minimum area is located east ofNew Guinea.

During a dormant or break phase, the opposite conditions to those of an active phase areobserved. As Falls (I970a; 1970b) stated, marked changes in the low-level temperaturefield can alter the location of the shear line. This occurs, particularly, when energetic cold

R. Suppiah 297

PRE - ONSET

n

t \ - I

-tm Mos@n shear ire

- SubtqiadriCiF

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Figure 9 Schematic representation of the low-level flow prior to and at the onset of monsoon

convection for six Southern Hemisphere summers,Source: Davidson et al (1983).

20

o

20

20

0

20

40

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298 The Australian summer monsoon: a review

fronts penetrate equatorward towards the latitudes of the shear line, which in turn weakensthe anticyclonic maximum above it.

Upper air circulation above the region, which covers the monsoonal climate over thetropical and subtropical regions of Australia, display dramatic changes between the activeand break monsoons. In particular, an abrupt poleward shift of the weak subtropical jet

stream and strong upper-level equatorial easterlies above the lower tropospheric monsoonwesterly regime are associated with an active phase (Troup, 1961; Murakami and Sumi,1982b; Davidson et al., 1983;1984; Hendon et a1.,1989; Hendon and Liebmann, 1990a).The opposite processes can be observed for a break monsoon. Another finding is that atupper levels strong equatorial easterlies are surrounded by weak upper-level westerlies ineach hemisphere. Murakami and Sumi (1982b) found that weakening of the iet near 25'Sis followed by strengthening of upper westerlies near 25'N. They pointed out that u and aperturbations at upper levels near 20'S tend to propagate eastward with a phase speed ofapproximately 15 ms t during a preonset period, a period that closely resembles a breakphase. But during an active phase or after the onset of the monsoon the phase propagationwas not observed near 20oS.

Another important question that emerged from the study by Davidson et al. (1983) isthe role of frontogenesis and its relationship to cloud bands in southeastern Australiaduring the onset of the monsoon. This topic needs further research.

Vll Surges and monsoon activity

Surges during the Australian monsoon approach the region from both the Northern andSouthem Hemispheres. They play an important role in the modulation of the monsooncirculation, particularly during the active and break cycle and the formation of tropicaldepressions and cyclones over the north Australian region. These surges can be dividedinto two categories, northeasterly and west coastal surges. Northeasterly surges come fromthe Northern Hemisphere crossing the South China Sea. The west Australian coast surgesreach the main heat low from truncated southeasterlies. First, a description is given ofnortheast surges, for which there is ample evidence, and then the characteristics of westcoast surges are explained using the limited sources of information available.

During the Southern Hemisphere summer monsoon season the interhemisphericexchange of mass, heat and moisture is mainly attributed to surges and their relatedcrossequatorial flows over the 'maritime continent'. Cold surges which mainly developover east Asia, propagate equatorward crossing the South China Sea and neighbouringareas. At the time of the development of cold surges, the Siberian high-pressure system is apersistent feature over east Asia. Radiative cooling and persistent cold air advection in thisregion maintain a layer of very cold air over the frozen land (Cheang, 1987). The TibetanPlateau acts as a mechanical barrier, blocking the southward movement of cold surges andbuilds up a surface high on its western flank (Murakami, 1987). Due to these factors, coldsurges generally cross over the area west of the South China Sea. These surges may triggerSouthern Hemisphere summer monsoon circulation features. \Tilliams (1979) reportedstrong crossequatorial flows associated with cold surges. Furthermore, he pointed out thatthese surges disappeared before reaching the Solomon Islands. Ramage (1971) andMurakami (1979) have stated that these cold surges are a part of the southward expansionof the intensified east Asian Hadley cell. Based on satellite pictures, Lau (1982)

demonstrated that eastward and westward propagating cloud clusters in the equatorial and

R. Suppiah 299

Southern Hemisphere tropics can be generated as a result of cold surgesr and that they canbe identified with Kelvin and Rossby wave modes explained on rhe basis of midlatitudeforcing during the \fMONEX period (Lim and Chang 1981). It is generally believedfrom previous studies (Ramage, l97l; cheang, 1977; L97g; l9g7; Lim and euah, 197g;lTill iams, 1979; chang et al., 1979; chiyu, L9z9; chang and Lau, l9g0; Ldve , l9g5a;1985b; Allan, 1983; rilTebster, lgglb; l9g7b; euin and rilfang, l9g6) that the southwardpenetration of these cold surges can cause the following prominent effects of synoptic-scaleconvective systems over Australasia: l) an increase in deep convective

""tiuity o1..,

southeast Asia and northem Australia; 2) generate an active cloud zone over the .maritimecontinent'; 3) that they determine the onset of the Australian summer monsoon and itsmodulation during the active/break cycle; and 4) influence the formation and intensity oftropical cyclones in the Southern Hemisphere, particularly over the north Australianregion. !(/ebster (1987b) demonstrated that the Nonhern Hemisphere surges, whichinfluence southern monsoon activity, have coherent variations associated with ihe intenseSiberian High. However, an understanding of these effects and their related mechanismswhich are responsible for the enhancement of equatorial convection by cold surges, islimited, though some evidence for heavy rainfall and flooding over Maiaysia and northBorneo were providedby Ramage (1971), Cheang (Ig77) ana Cfriyo (Ig79).This is alsotrue with regard to their influence on the Australian monsoon and iropical cyclones.

Recent observational (Murakami, 1987; cheang, l9g7) and numerical (ziu and,yang,1990) studies have pointed out the role of the mechanical (blocking) effect of the TibetanPlateau on the formation of cold surges over southeast Asia. The Tibetan plateaucontributes to the formation of cyclonic vortices and troughs with a length of about500 km on the eastern side of the plateau. Such cyclonic vortices and troughs seem rogenerate cold surges over southem China, one of the key regions responsible for cold airoutbreaks. However, an understanding of the influence of the Tibetan plateau on thedevelopment of cold surges in southeast Asia is not complete and needs furtherinvestigation.

Increases in the strength of crossequatorial flow during the time of cold surges have beenwell documented between west Malaysia and Borneo. In this region the maximum windspeed coinciding with cold surges occurs near the 600 m level (910 mb), just above the topof the mixed layer. This phenomenon was reported during ttre WAIIONEX periodSVilliams, 1979;'wiiohamidjojo, lggl), though the winrer monsoon was weaker thannormal during this experiment. ufilliams (1979) stated that cold surges observed earlier inthe South China Sea brought increased convection over Borneo and iventually propagatedeastward with the equatorial westerlies to the north of Australia. He further rrlti".a a slowpressure rise over western Indonesia which initiated an east-west equatorial pressuregradient. Under this condition, the convective systems seem to develop in the southernequatorial region and move eastward at a speed of l0 m s-t, from central Indonesia to thesolomon Islands. Such propagation was larer reported by Love (19g5a; lgg5b).

The effect of cold surges on Australian monsoon circulation features is a controversialsubject that has received renewed attention in the recent past (Keenan and Brody, lggg).However, based on wind patterns, Davidson et al. (1983) have presented some evidencefor a possible relation between cold surges and the Australian monsoon onset. Figures l0and I I taken from Davidson et al. (1983), clearly depict that a cold surge was in

-progress

on 22 December. Two days later, the surge winds reached the Southern Hemisphere andAustralian monsoon conditions set in. This is very similar to a crossequatorial driftsituation explained by Ramage (1971). It seems that the influence of cold *r-rrg., on the

300 The Australian summer monsoon: a revlew

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Figure 10 Synoptic situations showing the progess of cold surges on 22Decembet' 1978'

Source: Davidson et al. (1983).

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R. Supp iah 301

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Figure 11 Synoptic situations showing the progress of cold surges on 24 December, L978.Source: Davidson et al. (1983).

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3O2 The Australian summer monsoon: a review

early part of the Australian monsoon is more striking than in later stages. Previousobservational studies flVebster, 198lb; \Tilliams, 1979; 1981) and flow patterns in Figuresl0 and 11 give a strong impression that the Australian monsoon circulation and coldsurges may not be independent phenomena. However, based on their surge index (5'by

10o area-average 950 mb northerly wind centred at 10o, Il2.5o), Keenan and Brody(l9SS) stared thar surges in the northeast flow through the South China Sea did result in aweak modulation of near-equatorial cloudiness, but did not necessarily result in a majormodulation of the cloudiness activity in the monsoonal flow near Australia. Furthermore,they also pointed out that South China Sea cold surges that meet the MalaysianMeteorological Service surge definition, did not result in a major modulation of theAustralian monsoon region.

There is a limited amount of evidence available on west coast surges that influencemonsoon activiryr.particularly during the onset period. Davidson et al. (1983), Davidson(1984) and McBride (1987), have demonstrated a modulation of the Australian monsooncirculation due to west coast surges. They considered that these surges are the result ofgeostrophic winds that are caused by the east-west pressure gradient off the west coast ofAustralia. However, Keenan and Brody (1988) stated that variations in the west coast iet,which is associated with the surges, do not appear to be linked with any major modulationin convective activity within the Australian summer monsoon region.

From the above discussion, it appears that some evidence exists with regard to theinfluence of South China Sea cold surges on the Australian monsoon activity, although it isnot conclusive. This confusion may arise because of the definition of cold surges, and theselected area and parameters, period of observation and lack of a complete climatologicalpattern of cold surges from long-term data. Moreover, the interannual and intraseasonalvariabilities in the monsoonal activity could also be dominant factors that have led topulzzling conclusions regarding the relationship between cold surges and Australian

monsoon activity.

Vlll lntraseasonal scale (40-50 day) oscillations in the Australian monsoon circulations

The intraseasonal or subseasonal oscillation of about 40-day period was first observed inthe tropical wind field by Madden and Julian (L97L; 1972). Madden and Juliandemonstrated the presence of east-west overturnings in the equatorial longitude-heightplane. They also reported a slow eastward propagation of this wave. This oscillation issomerimes called a 40-50 day or 30-50 day oscillation in the text as they are based onvarious authors' definitions and results. In the Indian monsoon region, particularly insummer, the 40-50 day oscillation has been detected in the northward propagation ofcloudiness (Yasunari, 1979; Sikka and Gadgil, 1980), in the active/break cycle of windcomponents (Alexander et al., 1978; Krishnamurti and Subrahmanyam, 1982), in thetemporal variations of outgoing long-wave radiation (OLR) (Murakami et al.' 1986) and inrainfall (Singh and Kripalani, 1985). Keshavamurty et al. (1986) stated that the steadyequatorial heat source is the primary factor driving the intraseasonal oscillation and itsphase propagations.

The intraseasonal oscillation and its eastward propagation during the Southern Hemi-sphere summer have been observed in OLR and in wind composites (I-au et al., 1983:'Karoly, 1989; Lau and Peng, 1990; Murakami et al., 1986). McBride (1987) reported a40-50 day signal in the acrive and break cycle of the Australian summer monsoon by

R. Suppiah 303

applying a second-order Butterworth filter to *re 850 and 100 mb wind levels at Darwin.He observed the following results:

1) For all years the upper level wind variatisn has a large amplitude in *re 40-50 dayrange.

2) T\e low level zonal wind has a consistent large-amplitude 40-50 day oscillation onlyduring the time of the summer monsoon.

3) During the summer monsoon the upper and low level oscillations have a consistentphase lag of 180'.

4) The peaks and troughs in the 40-50 day oscillation correspond to the active and breakperiods.

A comparison by Puri (19S8) of general circulation model (GCM) results and actual meansea-level pressure and zonal wind components at 850 and 200 mb levels also revealed anout of phase relationship and a steady eastward propagarion of precipitating systems. Bymodulating the SST anomalies in the model, Puri demonstrated that the osciilations ar thistimescale play an important role in tropical-midlatitude interactions resulting fromconvective activity in the tropics. Recently, Kuhnel (1989) examined cloudiness daia from1979 to 1983, and confirmed the eastward propagation of cloudiness from the Indianocean toward the central Pacific. Similar to McBride (1987), and based on cloudinessvariation Kuhnel (1989) stated that the 40-50 day oscillation is strongest during thesummer monsoon season. Such seasonal dependency is not only confined to the Australianregion, but appears generally over the tropics (Anderson et al.,l9g4; Madden, l9g6).

It is noteworthy that the eastward propagation in cloudiness and in OLR anomaliesassociated with the 40-50 day oscillation indicates irregular patterns on inrerannual timescales. Such irregular patterns are, at least, evidenr between El Nifio and anti-El Nifioyears and can be observed in Figure I of Murakami (1986) and in Figure 3 of Kuhnel(1989). The contrasting features in the zonal propagation of wind, cloudiness, rainfall andOLR feature's between the El Nifto and anti-El Nifio years suggesr a sffong link betweenthe 40-50 day oscillation and the interannual ENSO cycle, whiih varies roughly betweentwo and 10 years (Trenberth, 1976a;1976b). A possible link between the 40-50 dayoscillation and the ENSO cycle was discussed by Lau and Chan (1986). They believedthat the onset of El Nifio may be triggered as a result of 40-50 day waves beingamplifiedepisodically through coupled ocean-atmosphere interaction. They showed thai the onsetof the 1982183 El Niflo event was apparently associated with the slow eastward migrationof the entire convection pattern and the retreat of El Nifro was linked to the return-to thenormal of the 40-50 day mode. The reverse pattern in 1974-75 was related to an anti-ElNifro year. More recent studies of Australian monsoon onset (Murakami et al., 19g6;Hendon et aI., 1989; Liebmann et aI.,l9g9; Hendon and Liebmann, 1990a) that defineonset dates by westerlies at the 850 mb level and heavy rain spells, have revealed a smongrelationship between onset date and the 30-50 day oscillation. In particular, the monsoononset during late December/early January is associated with the positive plur. while theretreat during March is related to the negarive phase of the 40-50 dav oscillation(Murakami et al.,1986)

The intraseasonal oscillation (30-50 day) in the area-averaged rainfall over northernAustralia (north of l5"S) and in zonal wind at 850 mb over Darwin has been recentlyreported by Hendon and Liebmann (1990b). They also reported a strong phase coherencerelationship between these parameters. However, it is obvious from theii spectral analysis

304 The Austral ian summer monsoon: a review

of rainfall data that there is not a significant peak in the 30-50 day time period. However,their spectral analysis showed a peak around the 20 day period. This discrepancy mighthave been caused by, at least, averaging rainfall over a large area. It should be noted thatnorrhem Australia covers a large longitudinal band extending roughly from 115'E to150'E. Not only that, the 30-50 day oscillation and its phase propagations exhibitconsiderable differences between El Niffo and anti-El Nifio years (Suppiah, l99l). Inparticular, more regular cyclic variations and eastward propagations of rainfall have been

noticed during anti-El Niflo years, but during El Nifio years the oscillation has a longertimescale and weak eastward propagation. The 30-50 day oscillation in rainfall is phase-

locked to the seasonal cycle of tropical Australia.From the above discussion, it is clear that the active/break phases of the monsoon are

associated with planetary-scale circulation phenomena, the 40-50 day oscillation and

ENSO events. These phenomena are driven by the active convective zones of the tropicalregion. On the other hand, they also indicate that droughts/floods occurring over northernAustralia are a part of global-scale circulation changes ranging in period from month tocentury scales. The strong relationship between the 40-50 day oscillation and the ENSO

cycle described in recent yearsJ suggests that a complete understanding of the link betweenthese oscillations is needed to explore the active/break cycle of the summer monsooncirculation and its related influence on widespread floods and droughts over Australia.Furthermore, it is noteworthy that a systematic study on the latitudinal and longitudinalpropagation, or the presence, of the 40-50 day oscillation has not been carried out for the

temporal rainfall pattern in the Australian summer monsoon region.

lX Interannual variations of monsoon circulation, rainfall and the Southern Oscillation

The Southern Oscillation (SO) is a planetary-scale phenomenon indicating a negative

relationship in pressure variations between the eastern Indian/western Pacific Oceans and

the southeastern Pacific Ocean. Pressure differences between Sydney, Australia and

Buenos Aires, Argentina were first noticed by Hildebrandsson (1897). A few years later,

Lockyer and Lockyer (1904) confirmed this relationship and demonstrated that thisphenomenon fluctuates with a periodicity of 3.8 years. Systematic studies on the influence

of tf,. SO on various meteorological phenomena were started with the works of G.T.

\(/alker and his colleagues in the early part of this century (\7alker, 1923; 1924; 1928;

Walker and Bliss, 1932; 1937). The SO has been linked to the oceanic event called El

Niffo, a striking phenomenon involving large-scale interannual variations of SSTs, sea

level, currents, thermocline and wind and rainfall over the tropical Pacific Ocean,parricularly in the east (Bierknes, 1969; r$7yrtki, 1975;Philander, 1983; Philander and

Rasmusson, 1985). El Niflo events are associated with warrn SST anomalies in the eastern

tropical Pacific, which are related to weak trade winds, and therefore to a weak pressure

gr"dient berween the Indonesian/north Australian low and the South Pacific subtropical

ttigtr. ttre influence of the SO on the interannual variability in the surface and 500 mb level

circulation in the Southern Hemisphere has been shown by Trenberth (1980b; 1981) and

van Loon (1984). ENSO signals have been of particular interest in the seasonal forecasting

of Australian rainfall since the beginning of this century.Exploration of the relationships between the SO and Australian rainfall, and their

potentiality for seasonal forecasting, began with the early works of Quayle (1918; 1929)

who used barwin pressure as an index for seasonal rainfall prediction in northern Victoria.

R. Suppiah 305

Following Quayle's work, Treloar (1934) suggested the possibiliry of foreshadowingmonsoonal rainfall in northern Australia with the same index. Between the 1930s and1960s, research on the SO and its influences on Australian weather were generallyneglected with a few exceptions (Berlage, 1957). However, in the 1960s a number ofstudies were carried out (Priestley,1962; Priestley and Troup, 1966; Troup, 1965) thatled to a considerable interest in the influence of the SO on Australian weather. Therelationship between Troup's Southem Oscillation Index (SOI, normalized, Tahiti-Darwin pressure difference, negative for El Nifio years) and the area-averaged annualrainfall of 107 Australian rainfall districts for 30 years (1941-70) was studied by Pittock(1975). Pittock's first spatial pattern of the principal component analysis, which accountedfor 36%o of the total variance with large loadings over eastern Australia, indicated a srrongrelationship to the SOI.

Recently, a number of studies were conducted on ENSO signals in the Australianrainfall and their impact on crops (Allan, l9g3; l9g5; tggg; l9g9; 1990; Allan andHeathcore, 1987; Allan and Pariwono, 1990; Allan et al., 1990, McBride and Nicholls,1983; McBride, 1987; Nicholls, 1981; 1983; 1984a. 1985b; 1985c; Nicholls and\floodcock, 1981; Nicholls er al., 1982; Pittock, 1984; streten, 1981; r9g3; Taylor andTulloch, 1985). These studies have generally demonstrated that El Nifio (anti-El Nifio)years are associated with Australian droughts (floods). In particular, anti-El Nifio (ElNiflo) are associated with positive (negative) SST anomalies over the eastern Indian Oceanand western Pacific and wet (drought) years of the Australian summer monsoon season.Figure 12 (adapted from McBride and Nicholls, 1983) shows seasonal relationshipsbetween the SOI and rainfall. It is clear that a strong relationship exists between the SOand late autumn to spring rainfall over Australia. Howeverr one disturbing result of thispicture is a rather poor correlation between the SOI and the summer rainfall in lone-termrecords over the monsoon dominated area, although individual El Nifio and anti-ElNifroyears show below (above) normal rainfall in the monsoonal region. The stability ofrelationships between summer rainfall and the SOI is weak though the SOIs show aconsiderable degree of persistence from spring to summer (McBdde and Nicholls, 1983;l7right et al., 1988). Nevenheless, a significant relationship exists only along thenorthwestem and northern coasts of Australia.

Nicholls (1981) has suggested a strong persistence in both SST and atmosphericpressure anomalies between February and October but weak persistence after October. Ina subsequent study Nicholls (1984c) obtained a negative relationship between norrhernAustralian SST and summer rainfall. Nicholls (1984c) and Allan and Pariwano (1990)have suggested that this negative relationship is due to the onset of the Australianmonsoonal conditions and a change in ocean-atmosphere coupling between spring andsummer. Tanaka (1981) has commented that frequent cold surges in the East and SouthChina Seas are responsible for increased cloudiness and rainfall, and decreased tem-perature over northem Australia. Recently, Guenni et al. (1990) have pointed out that astrong negative correction between cloudiness and maximum temperature at Katherine(l4o 28'S, 132" L8'E) is the result of synoptic systems during the summer season.Presumably, an increase in evaporation over land is a dominant factor that results in anegative relationship.

A strong relationship between upper tropospheric zonal wind indices over the Australiansummer monsoon region and the SO phenomenon was reported by Troup (1967) andTanaka (1981). They found that strong (weak) upper easterlies are associated with strong(weak) monsoon circulation over nofthem Australia. Surprisingly, Tanaka's composite

306 The Austral ian summer monsoon: a revlew

DEC - FE8 MAR - MAY

JUN . AUG S E P T . N O V

Figure 12 Simultaneous correlations between Darwin pressure and district rainfall for the four seasons,

December-February, March-May, June-August, September-November. Data from 1932-74'

Source: McBride and Nicholls (1989)'

analysis of rainfall for strong and weak winds at 150 mb over Singapore also supported the

above stated relationship.-Recent studies flWilliams, 1987; Drosdowsky, 1988) have

lationships between the SOI and geopotentialaddition to this view, the first eigenvector

y McBride and Nicholls (1983) revealed a

ainfall variations and the SOI. The above

a relationship between large-scale circulation

features of the Australian monsoon and the ENSO phenomenon.

There are several well-known synoptic-scale features that may be responsible for the

poor correlation, particularly during th" an-tttat monsoon season. Dominant features are

the slow-moving monsoon depressions or tropical cyclones that give a considerable

amount of rainfall during the monsoon season. Although the rainfall associated with these

synoptic systems is und-oubtedly related to the intensity of the depressions or cyclones,

variations in their intensity and irequency of occurrence are not easily understandable'

Understanding of locai controls on the interannual variability of the ENSO phenom-

enon needs to be analysed in detail. Deser and \ilallace (1987) have shown that swings in

Darwin pressufe and El Nifro events are loosely coupled in the year to year variations'

They also pointed out that sffong local controls on the climate of the eastern equatorial

Pacific that someti-.r rr"rrr..ndIne influence of the SO. This process may also be active

in the coastal area of Darwin where the interannual variations in SST anomalies do not

necessarily represent the large-scale climatic changes in the eastern Indian and westem

R. Suppiah 3O7

Figure 13 Average number ofannual thunder days.Source: Bureau of Meteorology (1989).

Pacific, but could have a strong influence on local rainfall. An inspection of Figure l2 andFigure l3 (Bureau of Meteorology, 1988a) reveals that the areas having large numbers ofthunderstorms, show significant correlation between summer rainfall and the SOI.Interestingly, these areas indicate a greater number of thunderstorms during the summerseason. The above patterns suggest that the contribution of rainfall due to localthunderstorm activity to total summer rainfall plays an important role in the northern partof tropical Australia.

Recent studies have questioned the stability of relationships between the SOI and thesummer rainfall during the last century (Allan, 1985; McBride and Nicholls, 1983;Pittock, 1984). The instability in correlation pattems is not only apparenr in the Australianregion, but is a common feature in the global teleconnection pattern. Basically the SOphenomenon undergoes monthly to decadal scale variations in long-term fluctuations aspointed out by Berlage (1957.1966). Certainly, the rainfall and temperarure variarionsindicate long-term variations (Russell, 1981; Pittock, 1975; Tucker and Stokes, 1980;Coughlan, 1975). Based on rainfall records, Pittock (1975) demonsrrated a dry phasebetween 1913 and 1945 and a wet phase from 1946 to 1978 over the Australian rigion.Variations and unstable patterns of correlations have been detected in SST anomalies,continental temperature and pressure indices (Elliott and Angell, 1988; \Tright et al.,1988). It seems that these trends may be the reason for poor relationships between rainfalland the SOI in long-term data sets. The correlations of pressure and SO indices with SSThave been greatest since the second world war, but were also relatively high prior to thefirst world war. It has also been shown that, correlations among the variables remain highwhen moderate or strong El Nifio were most frequent in the posr-I949 period (Oort andPan, 1985). The question arises as to whether the weak correlation pattern between thetwo world wars reflects a real change in the global atmospheric circulation or simplyreflects data problems. Further, it is important to analyse the relationship between the SOIand rainfall after removing their long-term trends.

Recent studies of century-scale SST changes and sea-level pressure (Paltridge and\7oodruff, l98l; Folland et a1.,1984; Barnett, 1985) and hemispheric surface remperarurefluctuations (Jones, 1990; Jones et al.,1986a; 1986b) have indicated a fairly clear trend ofincreasing temperature with time during the past 100 years over land and ocean regions.

308 The Austral ian summer monsoon: a review

C U M U L A T I V E P R E S S U R E V A R I A T I O N S A T D A R W I N1 8 8 2 - 1 9 9 0

co

LrJulaaL!uo_LrJ

):)

lO

4 0

2 0

0

- l v

- 4 0

- 6 01 8 8 2 1 8 9 : : 9 0 0 1 9 1 0 1 9 2 0 1 9 3 0 1 9 4 0 1 9 5 0 1 9 6 0 1 9 7 0 1 9 8 0 1 9 9 0

Y E A R S

Figure 14 Cumulative pressure variations at Darwin 1882-1990.

Such century-scale changes in temperature may alter the north-south and east-westtemperature gradients. These play a major role in controlling the ocean-atmospheresystem that is so important in the ENSO phenomenon. A relative increase in SST over the

eastern Indian/western Pacific Oceans to South Pacific Ocean could increase the pressuregradient between these regions. This process may cause a more intense \Talker Circula-tion, affecting both short- and long-term fluctuations. Pressure variations at Darwin(Figure 14), which are generally used to measure the intensity of the SO, show decadal-scale temporal variations. This pattern suggests that more intense anti-El Nifio events have

occurred in recent years, but from our observations it is clear that we have also experiencedmore intense El Niflos in recent years. These findings reveal that more intense El Niflos

and anti-El Nifios have occurred under a strengthening east-west zonal circulation; as they

are closely related to short-term and,long-term oscillations (QBO and ENSO cycle) in

SST, wind and rainfall anomalies in the tropics.

X Tropical cyclones and the monsoon

Tropical cyclones and slow-moving depressions play an important role in northernAustralia, as they produce significant amounts of rainfall during the monsoon season(Brunt, 1966;Linacre and Hobbs, 1977; Milton, 1980; Bonell and Gilmour, 1980) and

R. Suppiah 309

result in calamities for people, crops, wildlife and natural vegetation. Such impacts havebeen reported in previous studies (Oliver, 1974; Southern, 1979 and others). An excellentreview of disasters caused by severe tropical cyclones in the Northern Territory and achronology of tropical cyclone events in that area was given by Murphy (1984). Based ondata from July 1909 to June 1980, monthly and seasonal statistics on tropical cycloneformation, trajectories and frequencies in the Australian region were given by Lourensz(1981) and the tropical cyclone data quality has been discussed by Holland (1981).However, as has been discussed by Lourensz (1981), prior to weather satellite information(before 1959)' it is hard to say that all depressions and storms over oceans and remoteareas of the land have been routinely recorded.

Locations and statistics of more recent tropical cyclone origin for the Australian regionwere given by McBride and Keenan (1982) who examined the data from 1959 to 1979.Their average number of cyclones for the 20 years was 9.7 plus 1.3 regenerations totalling11 per year. This is a figure very close to the 10.3 given by Simpson and Riehl (1981), whocalculated the data from 1958 to 1978. Holland et aL (1988) give an average figure of eightto 10 cyclones per year.

The occurrence frequency of tropical cyclones in the Australian region is highly seasonaland it is confined to summer monsoon months (McBride and Keenan, 1982; Holland,L984a;1984b; 1984c). This season extends from November to May, with rhe maximumfrequency of occurrences in the months of January to March, Tropical cyclones anddepressions form in three distinct areas; the western (105'to L25"8), the central and ornorthern (125'to 145"E) and the eastem region (145'to 165'E). All these regions showmaximum frequency of occurrences during the monsoon season.

Tropical cyclones form mostly between l0o and 20o of latitude, equatorward of the 27oCSST isotherm (McBride and Keenan, 1982; McBride, 1986; Holland et al., 1988). Basedon the cyclone development statistics between 1974-75 and 1978-79, McBride andKeenan pointed out that 84%o of the precyclone cloud clusters first appear at the gradientlevel (850 mb) of the monsoon shear line, whereas 97o/o arc on the shear line at the point ofdevelopment. Although individual cyclone development and Australian regional cyclonegenesis locations coincide with the average location of the monsoon shear line, the monthlyclimatological positions of the monsoon shear line in the three regions and the locations ofcyclone origins (see Figures 4 and 6 of McBride and Keenan, 1982) show considerablediscrepancies in their intraseasonal and inter-regional variations. In particular, the pointsof origin in the western and central regions are found north of the monsoon shear lineexcept during February and March, while in the eastem region the points are located southof the monsoon shear line, an indication of southward penetration of warm water. Thisdiscrepancy may arise from day to day variations in the location of the monsoon shear line,because the monsoon trough is a favourable birth place for tropical cyclones, as has beenexplained for the Indian summer monsoon (Ramage, 1971; Rao, 1976; Krishnamurtiet al., 1975; Sikka, 1977). Further, a study on the day to day variability of the location ofthe monsoon shear line and tropical cyclone genesis is a topic of much research, as tropicalcyclogenesis and its regional and interannual variations are closely related to ENSO events(Dong, 1988; Hastings, 1990; Nicholls, 1979;1984b; 1985a).

Previous studies (Murakami and Sumi, 1982b; Davidson et al.,1983; McBride, 1983;1986; Holland, 1981; 1984a; 1984b; 1984c) have shown that active phases of theAustralian monsoon circulation are associated with intense tropical cyclones, particuladyin the western and northern regions. The formation of tropical cyclones at the beginningand during the monsoon season has been suggested as a triggering mechanism for

310 The Austral ian summer monsoon: a review

EoztrJfoUGL

UzrrJ(r(rUO

Ocf Dec

Figure 15 Seasonal distributions of hurricanes for the Australian region. Except for the extremities' the

curves have been smoothed by a l5-day running mean.

Source: Holland (l 984c).

monsoon onset. This is evident in Figure 15, which shows the seasonal distribution of

intense tropical cyclones (hurricanes) over the Australian region (Holland, 1984b)' Here

the period irom late-January to mid-February is marked with less hurricanes over nofihern

"rrd *.*t.- regionsJ a piriod that generally coincides with the break period of the

Australian monsoon. Duiing ttris period the monsoon shear line also moves southward.

The occurrence of both active monsoo.t circulation and tropical cyclones is associated with

an equarorward protrusion of an upper-level midlatitude westerly trough. Similarly, they

are influenced by cold surges from the Northern Hemisphere and so the Northern

Hemisphere circulation features become favourable prior to tropical cyclone genesis

(Davidson and Holland' 1987).

Xl Conclusions

The formation of heat lows over the Pilbara and Cloncurry regions, and their sffong

persistence preceding the monsoon season, plays an important role in the subsequent

inonsoonal circulation. A steady development of heat lows over desert regions associated

with maximum heating could bring stronger monsoonal circulation features and aice oersa.

The strong., -orrooi usually brings heavy rainfall to tropical Australia. The existence of

two heat lows has been studied by using observational data and numerical simulations,

which are based on the heat balance scheme. These studies, however, indicated that these

heat lows have great complexity in their vertical and horizontal scales, because they are

apparently related to radiational and dynamical processes. They also show strong day to

aay "ariaUitity

in both their location and intensity. A study of day to day variability of the

loiation of heat lows and variations in their intensity in relation to heat balance

components is required to further understand the characteristics of these heat lows,

because any changes in heat balance components under the enhanced greenhouse effect

It,,IIIt

t(

I,

R . S u p p i a h 3 1 1

could certainly alter the monsoonal circulation features and rainfall over this region.Recent observational and diagnostic studies of the Australian summer monsoon have

substantially increased our knowledge of the main components of the monsoonalcirculation. Based on upper wind and OLR data, the onset, withdrawal and active/breakcycle of the monsoon have been studied. Intensive studies were conducted based on$TMONEX and the Australian Monsoon Experiment (AMEX) data. These srudies werelimited to eight or l0 years and produced contrasting results with regard to the onset ofactive convective activity, equatorial westerlies and their phase propagations. In particular,eastward and westward propagations were noticed on OLR, cloudiness and wind.However, the meridional propagation in these fields were not noticed.

The influence of surges approaching the northern Australian region both from the SouthChina Sea and the west coast has gained special attention in recent years. Studies havebeen carried out on the modulation of the Australian summer monsoon circulationfeatures by cold surges, particularly from the Northem Hemisphere, and have often beenambiguous, not always giving clear conclusions. These conflicting results might have beenproduced as a result of two factors 1): a lack of a long-term climatological study on therelationship between cold surges and the Australian mofrsoon activity; and 2) the surgeindex derived for the South China Sea region does not necessarily meet the requirementsof the north Australian region.

Intraseasonal variation and eastward propagation in OLR, zonal wind components andin rainfall seem to be dominant during the Southern Hemisphere summer rather than inthe winter. This 40-50 oscillation has a smong link with the active/break cycle of themonsoon and the ENSO phenomenon. These features are observed on OLR and windanomalies, but they have not been systematically noticed so far in the summer rainfallpattern over tropical Australia.

The relationship between the summer monsoon rainfall and the SOI is weak, althoughindividual El Niflo (anti-El Niflo) years coincide with droughts (floods) over tropicalAustralia. However, a strong correlation has been noticed between wind componentsduring the summer monsoon season and the SOI. Onset dates of the Australian summermonsoon correlate well with the eastern equatorial Pacific SST in the QBO timescale. Inparticular' the Australian monsoon onset date serves as a useful predictor of thesubsequent ENSO event. However, the contribution of local SST changes to large-scalecirculation changes in the ENSO phenomenon and its relation to summer rainfall patternhas not been well understood. Further, a study is required on the relationship between theSOI and the summer rainfall after removing their long-term rrends.

Slow-moving monsoon depressions and tropical cyclones occur as a part of the monsooncirculation and give considerable amounts of rainfall. Their occurrence frequency is linkedto the active/break cycle of the monsoon in the intraseasonal timescale. In particular, astronger monsoon circulation is associated with a larger number of monsoon depressionsand tropical cyclones. On the other hand, their frequency is also associated with ENSOevents on interannual timescales. Their frequency and intensity are not well understood,because they are associated with various atmospheric and oceanic processes. The influenceof crossequatorial surges on the formation of tropical depressions and cyclones has beensuggested but results are not conclusive.

The summer monsoon circulation features are closely linked to various cycles thatinclude the 30-50 day oscillation, QBO, ENSO and others. Therefore, studies on theinfluences of these cycles on various monsoonal systems would be useful for providingfurther information on the dynamics of the monsoonal circulation features. Such

312 The Austral ian summer monsoon: a review

information is essential for studying the monsoon activity under enhanced gleenhouse

conditions, which could alter heai balance components over the entire globe'

Acknowledgements

I would like to thank A.B. Pittock, R.J.Allan, P.H'Whefton, J.L. Evans, B'F' Ryan and

J.L. McBride for their helpful comments. Trevor casey kindly provided pressure data for

Figure 14. Sean Higgins and Louise carr prepared the ficges. This work has been funded

by the federal Depariments of Arts, Sport, Environment, Tourism and Territories and by

the \Testern Australia and Northem Territory governments.

References

Adams, M. 1986: A theoretical study of the inland

trough of northeastern Australia, Australian Meteoro-

logical Magazine 34' 85-92.

Alexander, G., Keshavamurty, R.N., De, U'S',

Chellappa, S.K., Das, S.K. and Pillai, P.V. 1978:

Fluctuations of monsoon activity. Indian Joumal of

Meteorologt, Hydrologt and Geophysics 29 ' 7 6-87 '

Allan, R.J. 1983: Monsoon and teleconnection vari-

ability over Australasia during the southem hemi-

sphere summers of 1973-77 . Monthly Weather

Reaiew 711,11342.-- 1985' The Australian summer monsoon, telecon-

nections, and flooding in the Lake Eyre basin. Sozrft

Austrahan Geographical Papers No' 2, Royal Geo-

graphical Society of Australasia (South Australian

Branch).- 1988' El Nifro Southem Oscillations influences in

the Australasian region. Progress in Physical Geo-

graphy 12,313-48.- I989' ENSO and climatic fluctuations in Australia'

Proceedings of the Third Intemational Conference on

Southem Hemisphere Meteorologt and Oceanography,

Buenos Aires,37l-72.- 1990' Australasia. In Glantz, M., Katz, R. and

Nicholls, N., editors, ENSO teleconnecions linktng

world-wide climatic anomalies. Scientffic basis and

societal impacts, Cambridge: Cambridge University

Press,73-120.

Allan, R.J., Beck, K. and Mitchell, W.M. 1990: Sea

level and rainfall correlations in Australia: tropical

links. Joumal of Ckmate 3, 838-46'

Allan, R.J. and Heathcote, R.L. 1987: The 1982-83

drought in Australia. In Glantz, M.M. Katz, R' and

Krenz, M., editors, The societalimpacts associateduith

the 1982-83 world-widc climate anomalies, Geneva:

United Nations Publications, l9-23.

Allan, R.J. and Pariwano, t.I. 1990: Ocean-atmos-

phere interactions in low-latitude Australasia- Inter-

national Joumal of Climamlogt 10, 145-78.

Anderson, J.R., Stevens, D.E. and fulian' P'R'

1984: Temporal variations ofthe tropical 40-50 day

oscillation; Montht3t Weather Reaipw 712' 2432-38'

Batnett, T.B. 1985: Variations in near-global sea level

pressute. Journal of the Atmospheic Sciences 42,

478 -501 .Berlage' H.P. 1957: Flucruations of the general armos-

pheic circulation of more than one year; their nature and

prognostic oaJue. Mededelingen en verhandelingen,

Koninkliik Nederlands Meteorologisch Instituut'

N o . 6 9 .- 1966: The Southem Oscillation and world aeather'

Mededelingen en verhandelingen, Koninklijk Neder-

lands Meteorologisch Instituut, No. 88.

Berson, F.A. 1961: Circulation and energy balance in

a tropical monsoon. Tellus 11,472-85.

Berson, F.A. and Troup, A.J. 1961: On the angular

motnentum balance in *re equatorial trough zone of

the Eastem Hemisphere. TeIIus 13' 66-78.

Bierknes, J. 1969: Atmospheric teleconnections from

the equatorial Pacific. Monthl! Weathet Rettiew 91 ,

163-72.Bond, H.G. 1954: Northem Territory drought,

lg5l-1952. Australian Meteorological Magazine 5,

5-r9.Bonell, M. and Gilmour, D.A. 1980: Variations in

short-term rainfall intensity in relation to synoptic

climatological aspect of the humid uopical northeast

Queensland coast. Singa1ore Joumal of Tropical Geo-

graphy | ' 16-30.

Brunt, A.T. 1966: Rainfall associated with tropical

cyclones in the northeast Australian tegio* Aus-

tralian Meteorological Magazine 14, 85-109.

Bureau of Meteorology 1988a Ckmatic atlas of Aus-

tralia. Department of Administrative Services'- 1988b: Climatic anerages. Departrhent of Admin-

istrative Services.

Chang, C.P., Eriksonl t.E. and Laut K'M' 1979:

Northeasterly cold surges and near-equatorial dis-

turbances over the rVinter-MONEX area during

December 7974, Paft I: Synoptic aspecrs. MonthlylVeather Redew lO7 ,812-29.

Chang, C.P. and Lau, K.M. 1980: Northeasterly coldsurges and near-equatorial disturbances over theWinter-MONEX area during December 1974, PafiII: Planetary-scale aspects. Monthly Weather Reoiew108, 298-312.

Cheang, B.K. 1977: Synoptic features and struc$resof some equatorial vortices over the South China seain the Malaysian region during the winter monsoon,December 1973. Pure and Applied Geophysics ll5,t303-34.

- 1978' Structure of a cyclonic disturbance over thesouth China sea and the Malaysian region during thewinter monsoon. Indian Joumal of Meteorologt,Hydrologt and Geopl4tsics 29, 16-25.

- 1987' Short- and I-ong-range monsoon predicrionin Southeast Asia. In Fein, J.S. and Stephens, P.L.,editors, Mozsoozs, New York: John \ffiley and Sons,579-606.

Chiyu, T. 1979: A preliminary study of low-level windsover Peninsular Malaysia during the 1976-77 nor:th-east winter monsoon. Joumal of the MeteorologicalSociety of Japan 57 , 354-57 .

Coughlan, M.l.1975: The evidence for recent climatechange in some Australian meteorological records.Conference on Climate and Climatic Change, Mel-boume, Royal Meteorological Society (AustralianBranch).

Davidson, N.E. 1984: Shon-term fluctuations in theAustralian monsoon during winter MONEX.Monthly tileather Reaiew ll2, 1697-170g.

Davidson, N.E. and Holland, G.J. 1987: A diag-nostic study of two intense monsoon depressionsover Australia. Monthly lYeather Reaiew I15,380-92.

Davidson, N.E., McBride, J.L. and McAvaney,BJ. 1983: The onset of the Australian monsoonduring winter MONEX: Synoptic aspecrs. MonthlylX/eather Reoiew 1 1 1, 496-51 6.

- 1984' Divergent circulations during the onset ofthe1978-79 Australian monsoon. Monthly Weather Re-ttiew 712, 1684-96.

Deser, C. and lilallace, J.M. l9S7: El Niffo eventsand their relation to the Southem Oscillation:L925-1986. Journal of Geophysical Research g2(Ct3),14189-96.

Dong, K. 1988: El Nino and tropical cyclone fre-quency in the Australian region and the northwestPacific. Australian Meteorological Magazine 36,2t9_26.

Drosdowsky, W. 1988: Lag relations berween the south-em oscillation and the troposphere over Australia.BMRC Research Repon No. 13.

Elliott, W.P. and Angell, J.K. 1988: Evidence for

R. Supp iah 313

changes in southem oscillation relationships duringthe last I 00 years. J ournal of Climate 1, 7 29-3'7 .

Falls, R. 1970a: Some synoptic models - north Aus-txalia. Proceeding of the Symposium on Tropical Meteot-o/ogr, Honolulu: AMS and VMO.

- 1970b' Synoptic analysis and forecasting in thetropics of Asia and the southwest Pacifi,c. Proceedingsof the Regional Treiring Seminar, Singapore: \WMO.

Fitzpatrick, E.A. and Nix, H. 1970: The climaticfactor in Australian grassland ecology. In Moore,R,M., editor, Australian grasslands, Canberra: ANUPress, 3-26.

Folland, C.K., Parker, D.E,. and Kates, F,E. 1984:rJTorld-wide marine remperature flucruations,I 856-1 98 l. Nature 3lO, 67 0-7 3.

Frost, R. and Stephenson, P.M. 1964: Mean stream-lines for standard pressure levels over the IndianOcean and adjacent land areas. Proceedings of theSymposium on Tropical Meteorolop, Rotorua: NewZealand Meteorological Service 96-1 06.

Gentilli, J. 1971a: Australian climatic factors. In Gen-tilli, J., editor, LYorld suru4t of climatologt, Amster-dam: Elsevier, 35-52.

- 1971b' Dynamics of the Australian troposphere. InGentilli, J., editor, lYorld suntEt of ckmatologt, Am-sterdam: Elsevier, 53-l 18.

- 1972' Australian climau pattems, Melboume:Nelson.

Guenni, L., Rose, C.W., Hogarth, W., Braddock,R.D. and Charles-Edwards, D. 1990: Seasonalchanges in inter-relationships between climate vari-ables. Agricuhure and Forest Meteorologt 53,45-58.

Gunn, 8.C., McBride, f.L., Holland, G.J.,Keenan, T.D., Davidson, N.E. and ffendon,H.H. 1989: The Australian summer monsoon cir-culation during AMEX Phase II. Month$t WeatherReoiew 117,2554-74.

Hadley, G. 7735: Conceming the cause of the generaltrade-winds. Philosophical Transactions of the RoyalSociety of London 29, 58-62.

Halley, E. 1686: An historical account of the tradewinds and monsoon observable in the seas beweenand near the tropics with an attempt to assign aphysical cause ofthe said winds. philosophical Trans-actions of the Royal Society of London 26,153-68.

Hastings, P.A. 1990: Southem oscillation influenceson tropical cyclone activity in the Australian/south-west Pacific region. Intemational Joumal of Climato-logt 10,291-98.

Healy, T.R. 1972: Some climatological considerationsof 200 mb troughs and associated jet streams overthe Australian rcg1on. Australian Geographical Studies10 , l 6 I - 81 .

Ilendon, H.H. and Liebmann, B. 1990a: A compo-site study of onset of the Australian summer mon-

314 The Austral ian summer monsoon: a review

soon, Journal of the Atmospheric Sciences 47'

222740.- 1990b' The intraseasonal (30-50 day) oscillation of

the Ausualian summer monsoon. Joumal of the At-

rnospheic Sciences 47 , 2909-23.

Hendon, H.H., Davidson, N.E. and Gunn, B. 1989:

Ausrralian summer monsoon onset during AMEX

1987. Monthb Weather Redpw ll7 ' 370-90.

Hildebrandsson, H.H. 1897: Quelques recherches sur

les centres d'action de I'atmosphere, Kunglica Svenska

Vetenskaps Akademiens, Handlinger 29.

Hobbs, l.E. L973: Indian Ocean intertropical atmos-

pheric circulations. Australian Geographical Studies

11,228-33.- 1975' I-ow latitude westerlies over the Indian

Ocean. Australian Geographical Studies 13, 4l-51.

Holland, G.J. 1978: Observations of a voftex em-

bedded in a monsoonal flow. Austtalian Meteoro-

logical Magazine 26' 19-24.- l98l' On the quality of the Ausualian tropical

cyclone data base. Australian Meteorolgical Magazine

29, t69-87.- 1984a' On the climatology and structure of tropical

cyclones in the Australian/southwest Pacific region: I

Data and tropical storms. Australian Meteorological

Magazine 32' l-15.- 1984b' On the climatology and structure of tropical

cyclones in the Australian/southwest Pacific region:

II: Hurricanes. Australian Meteotological Magazine

32, t7-37.- 1984c' On the climatology and structure of tropical

cyclones in the Australian/southwest Pacific region:III:

Maior hurricanes. Australian Meteorological Maga-

zine 32,3346.- 1986' Interannual variability of the Australian sum-

met monsoon at Darwin: 1952-1982. Monthly

Weather Reaiew l l 4, 59 4-60 4.

Holland, G.J. and Nicholls, N. 1985: A simple pre-

dictor of El Niffo? Tropical Ocean-Atmosphere News-

letter, March 8-9.

Holland, G.J., Keenan, T.D. and Guymer, A.E.

l9B4: The Australian monsoon ' hoceeding of the 15th

Conference on Hurricanes and Tropical Meteorologl,

Miami, Florida, AMS, 398-402'

Holland, G.f ., McBride, J.L. and Nicholls' N. 1988:

Australian region tropical cyclones and the green-

house effect. In Pearrnan, G.I.r editor, Grcenhouse:

planningfor climate change, Melboume: CSIRO, Divi-

sion of Atmospheric Research, 438-55.

Hopwood, J. 1968: A note on the quasi-biennial

oscillation at Darwin. Australian Meteorological

Magazine 16' ll4-17.

Hunt, H.A. 1908: Ckmate and meteorologt of Australia.

Australia, Bureau of Meteorology, Bdlletin No. l.- 1929' A basis for seasonal forecasting in Australia.

Quarterly Joumal of the Royal Meteorological Society

55,323,14.Hunt, H.A., Taylor, G. and Quayle,E.T. 1973; The

climate and weather of Austalia. Melbourne: Govem'

ment Printer.

fohnson, R.H. and Houze, R.A. Jr. 1987: Precipitat-

ing cloud systems of the Asian monsoon. In Chang

C.P. and Krishnamurti, T.N., editors, Monsoonmete-

orologt, New York: Oxford lJniversity Press,

298-353.

Jones, P.D. 1990: The climate of the past 1000 years.

Endeaztour L4, 129-36.

Jones, P.D., Raper, S.C.B., Bradly, R.S., Diaz'

H.F., Kelly, P.M. and Wigley, T.G.M. 1986a:

Nonhem Hemisphere Surface Air Temperature

Variations: 1851-1984. Joumal of Climate and Ap-

plied Meteorohgt 25, 16l-79.

Jones, P.D., Wigley, T.G.M. and Wright, P.B.

1986b: Global temperature variations between 1861

and 1984. Nature 322, 430-34.

Joseph, P.V. 1988: Monsoon variabiliry in relation to

equatorial trough activity over Indian and West

Pacific Oceans. Proceeding of the Intemational Sympo-

sium on Monsoon: Understanding and hediction, ln-

dian Institute of Tropical Meteorology, Pune,23-28

November, 1988.

Karoly, D.J. 1989; Low frequency variations of the

southem hemisphere circulation. Proceedings of the

Third Intenzational Conference on Southem Hemisphere

Meteorologt and Oceanography, Buenos Aires,

105 -109 .Keenan, T.D. and Brody, L.R. 1988: Synoptic scale

modulation of convection during the Australian sum-

mer monsoon. Monthly lVeather Reoiew 176, 71-85.

Keshavamurty, R.N., Kasture, S.V. and Krishna-

kumar, V. 1986: 30-50 day oscillation of the mon-

soon: a new theory. Contributions m Atmosphenc

Physics 59' 443-54.

Kininmonth, W.R. 1983: Variability of rainfall over

northem Ausralia. In Street Perrot, A., Beran, M.

and Ratcliffe, R., editors, Vaiations in the global watet

budget, Dordtecht: D. Reidel, 265-7 2.

Krishnamurti, T.N. and Subrahmanyamt D. 1982:

The 30-50 day mode at 850 mb during MONEX.

Journal of the Atmospheic Sciences 39' 2OB8-95.

Krishnamurti, T.N., Kanamitsu, M., Godboler R.,

Chang, C.8., Carr, F. and Chowr J.Itr. 1975:

Study ofa monsoon depression. I Synoptic structure.

Joumal of the Meteorological Society of Japan 53,

227-40.Kuhnel, I. 1989: Spatial and temporal variations in

Australo-Indonesian region cloudiness' Intemational

Joumal of Climatologt 9, 395-405.

Lau, K.M. 1982: Equatorial responses to northeasterly

cold surges as inferred from cloud satellite imagery.

Monthly lVeather Review I10, 1306-13.

Lau, K.M. and Chan, P.H. 1986: The 40-50 day

oscillation and the El Niflo/Southern Oscillation: A

new pefspective. Bulletin oJ the Ameriran Meteoro-Ioghal Society 67, 533-34.

I-au, K.M., Chang, C-P. and Chan, P.H. 1983:Shon-term planetary-scale interactions over thetropics and midlatirudes Part II, !(/inter-MONEXpeiod. Monthljt lVeather Review 7ll,l3Z2-88.

Lau, K.M. and Peng, L. 1990: Origin of low fre-quency (intraseasonal) oscillations in the tropicalaunosphere. Part III: Monsoon dynamics. Journal oJthe Atmospheric Sciences 47, 7443-61,

Leighton, R. 1979: Anthyclonicity and cyclonicity azter-ages for 1965-1974. Ausrralian Bureau of Meteoro-logy, Technical Repon No. 35.

Liebmann, 8., Hendon, H.H. and Holland, G.J.1989: Tropospheric circulation changes associatedwittr onset of the Australian summer. monsoon. Ilo-ceedings of the Third In*rnational Conference on South-ern Hemisphere Meteorohgt and Oceanographj,Buenos Aires, 240-41.

Leslie, L.M. 1980: Numerical modelling of the sum-mer heat low over Australia. Journal of Applied Mete-orologt 19,381-87.

Lirn, H. and Chang, C.P. 1981: A theory on mid-latitude forcing of tropical motions during wintermonsoons. Joumal oJ the A*nospheric Sciences 38,2377-92.

Lim, J.T. and Quah, L.C. 1978: Cross-equatorialflow over southeast Asia during the northeast mon-soon. Indian Journal of Meteorohp, Hydrologt andGeophysics 29,5-15.

Linacre, E. and Hobbs, l. 7977: The Ausiralianclimatic enttironment. Brisbane john \Y/iley and Sons.

Lockyer, N. and Lockyer, W.J.S. 1904: Thebehaviour of the shon-period atmospheric pressurevariation over the earth's surface. hoceedings of theRoyal Society of London 73, 457-70.

Lourensz, R.S. I 981 : Tropiral cyclones in the AustralianRegion, fuly 1909 to June 1980, Bureau ofMeteorology.

Love, G. 1985a: Cross-equatorial influence of winterhemisphere subtropical cold surges. Monthly lVeatherRedew ll3,1487-98.

- 1985b' Cross-equatorial interactions during uopi-cal cyclones genesis. Month$t Weather Review ll3,1499-509.

Madden, R.A. 1986: Seasonal variation of the 40-50day oscillation in the uopics. Journal of rhe Atmovpheric S cience s 4?, 3 | 38-5 8.

Madden, R.A. andJullan, P.R. l97l: Detection of a40-50 day oscillation in the zonal wind in the uopicalPacifrc. Journal oJ the Atmospheic Sciences 28,702-708.

- 1972' Description ofglobal-scale circulation cells inthe tropics with a 40-50 day period. Journal of theAtmospheric Sciences 29, 1 109-23.

McBride, J.L. 1975: The effect of land-sea tem-perature conuast on shon-term numerical forecasts,

R. Suppiah 315

Australian Meteorological Magazine 23, 7 5-98.- 1983' Satellite observations of the Southem hemi-

sphere monsoon during winter MONEX. Tellus 35A,189-97.

- 1986: Tropical cyclones in the Southern Hemi-sphere summer monsoon, hoceedings of the secondIntemational Conference on Southan Hemisphere Mete-orologt, Dec. l-5, 1986, Wellingon, New Zealand,358-64.

- 1987t The Australian summer monsoon. In Chang,C.P. and Krishnamufti, T., editors, Monsoonmeteoro-/o9,,, Oxford: Oxford Universiry Press, 203-31.

McBriderJ.L. and Keenan, T.D. 1982: Climatologyof tropical cyclone genesis in the Australian region.Joumal of Climamlogt 2, 13-33.

McBride, J.L. and Nicholls, N. 1983: Seasonal rela-tionships between Australian rainfall and the south-em oscillation. Monthly lVeather Reaiew l l l,1998-2004.

Milton, D. 1980: The contribution of tropical cyclonesto the rainfall of uopical Vestem Australia. SingaporeJournal of Tropical Geography l, 46-54.

Moriarty, L.A. 1955: Large scale fficr of heating overAustralia, 1. CSIRO, Division of MeteorologicalPhysics, Technical Paper, 7.

Muffatti, A.H.J. f 964: Aspects of the subtropical ietstream over Australia. hoceedings of the Symposium onTropical Meteorologt, Rotorua: New Zealand Mete-orological Service 72-88.

Murakarni, T. 1979: lVinter monsoonal surges overEast and Southeast Asia. Joumal of the MeworohgicalSociety of Japan 57 , 133-58.

- 1986' 30-60 day atmospheric teleconnection pat-tems during the Southem Hemisphere sumrner. Ilo-ceedings of the Second International Conference onSouthem Hemisphere Meteorologt, December l-5,I 986, Vellington, New Zealand, 336-38.

- 1987' Effects of the Tiberan plareau. In Chang,C.P. and Krishnamurti, T., editors, Oxford: OxforcUniversity Pres s, 235 -7 0.

Murakami, T., Chen, L-X., Xie, A. and Shrestha,M.L. 1986: Eastward propagation of 30-60 dayperturbations as revealed from out going longwaveradiation data. Journal of the Atmospheic Sciences 43,961-7 t .

Murakarni, T. and Nakazawa, T. 1985: Transitionfrom the southem to northern hemisphere surrunermonsoon. Monthfu lYeather Reoiew ll3, 1470-86.

Murakami, T. and Sumi, A. l98l: Large scale as-pects of the 1978-79 winter circulation over thegreater TVMONEX region. Pan II: Long-period per-turbations. Journal of the Meteorologicel Society ofJapan 59,646-71.

- 1982a' Southern hemisphere summer monsooncirculation during the 1978-79 VMONEX. Pan I:Monthly mean wind fields. Joumal of the Meteoro-logical Soeiety ofJapan 60, 638-48.

- 1982b' Southem hemisphere summer monsoon

316 The Austral ian summer monsoon: a review

circulation during the 1978-79 U7MONEX. Part II:

Onset, active and break monsoons. Journal of the

Metarological Society of Japan 60 , 649-7 0 '

Murphy, K. 1984: Bigblow up nonh: a hismry of topical

cyclones in Austalian Nonhem Terrimry. Darwin: Uni-

versity Planning Authority.

Nicholls, N. 1978: Air-sea interaction and the quasi-

biennial oscillation. Monthljt lVeather Reoiew 106,

I 505-508.- 1979t A possible metlrod for predicting seasonal

uopical cyclone activity in the Australian region.

Monthly lVeather Review 107 ' l22l-24.- 1981' Air-sea interaction and the possibility of

long-range weather prediction in the Indonesian ar-

chipelago. Monthly lVeather Reoiew 109' 243543.- 1983' The potential for long-range prediction of

seasonal mean temperature in Australia' Australian

Meteorohgical Magazine 3L' 203-207 .- 1984a' A system for predicting the onset of the

north Australian wet-season. Joumal of Climatologt 4,

425-35.- 1984b' The southem oscillation, sea surface tem-

perature and interannual flucruations in Australian

tropical cyclone activity. Joumal of Climamlogt 4'

661-70.- 1984c' Seasonal relationships between Australian

rainfall and North Australian sea-surface-tempera-

ture, Extended abstractsr Conference on Austrahan

Rainfall Vaiabil:ity, Part 2, Arkaroola, South Aus-

tralia,7 l-73.- 1985a' Predictability of interannual variations of

Australian seasonal tropical cyclone activity. Monthly

lVeather Reoiew 113, 114449.- 1985b' Towards the prediction of maior Australian

droughts. Austalian Meteorological Magazine 33,

l6l-66.- 1985c' Impact of the southem oscillation on Aus-

tralian crops . Jo umal of Climamlogt 5 , 553-60 .

Nicholls, N., McBride, J.L. and Orrnerodr R.J.

1982: On predicting the onset of the Australian wet

season at Darwin. Monthly Weather Reoiew ll0'

14-17.Nicholls, N. and Woodcock, F. l98l: Verification of

an empirical long-range weather forecasting tech-

niqlcc. Quanerly Joumal of the Royal Meteorological

Society lO7 ' 973-76.Nix, H. 1983: Climate of tropical savannas. In Bour-

liere, F., editor, Tropical saoannas, Amsterdam: Else-

vier,37-62.

Nix, H.A. and Kalma, J.D. 1972: Climate as a

dominant control in the biogeography of northem

Australia and New Guinea. In Valker, D., editor,

Bridge and barrier, the natural and cuhural hhmry of

Tones Strait, Canberra: Australian National Uni-

versity Press, 6l-62.

Oliver, l. 1974t Tropical cyclones as natural hazards:

an evaluation of Queensland experience in its mete-

orological context. Austrakan Meteorological Maga-

zine 22,49-52.Oort, A.H. and Pan, Y.H. 1985: Diagnosis of histor-

ical ENSO events. Proceeding of the First WMO

Workshop on the Diagnosis and Prediction on Monthly

and Seasonal Atmospheric Variations over the Ghbe' 29

July-2 August, 1985, College Park, USA.

Palmen, E. 1956: Formation and development of

tropical cyclones. fuoceedings of the Tropical Cyclone

Symposium, Brisbane, Melboume: Bureau of Mete-

orology, 21311.

Paltddge, G. and Woodruffr S. 1981: Changes in

global surface temperature from 1880 to 1977 de-

rived from historical tecords of sea surface tem-

perature. Monthly lY'eather Reoiew 109' 2427-34.

Philander, S.G.H. 1983: El Niflo Southem Oscilla-

tion phenomena. Nature 302' 295-301.

Philander, S.G.H. and Rasmussonr E.M. 1985:

The southem oscillation and El Niflo. Adoances in

Geophysics 28 A' 197 -2 | 5.

Pittock, A.B. 1975: Climatic change and the pattems

of variations in Australian rainfall. Search 6,

498-504.- 1984' On the reality, stability and usefulness of

southem hemisphere teleconn ections' Australian Me-

teorological Magazine 32' 75-82.

Priestley, C.H.B. 1962: Some lag associations in

Darwin pressure and rainfall. Australian Meteoro-

Iogical Magazine 38, 3242.

Priestley, C.H.B. and Troup, A.J. 1966: Droughts

and wet periods and their association with sea surface

temperature. Australian Journal of Sciences 29,

56-57.Puri, K. 1988: On the importance of low-frequency

graviry models for the evolution of large-scale flow in

a general circulation model. Journal of the Atmospheic

Sciences 45,2523-44.

Quin, P. and Wang, J. 1986: Equator-Crossing mass

flow and its effects on the hurricanes occulrence over

Southem Hemisphere. hoceedings of the Second Inter-

national Conference on SouthenT Hemisphere, Decem-

ber 1-5, 1986, \fellington, New Zealand,33942.

Quayle, E.T. l9l8: On the possibility of forecasting the

approximate winter rainfall for nonhern Victoia. Brt-

reau of Meteorology, Australia, Bulletin, 5.- 1929' Long-range rainfall forecasting from tropical

(Darwin) air pressures. Proceedings of the Rogal Society

of Victoria 41,760-64'

Radok, U. l9? 1: The Australian region and the general

circulation of the southem hemisphere. In Gentilli'

I., editor, Vorld surttey of climatologt, Amsterdam:

Elsevier, l3-33.

Radok, U. and Grant, A.M. 1957: Variation in the

high tropospheric mean flow over Australia and New

Zealatd. Joumal of Meteorologt 74, l4l-49.Ramage, C.S. 1971: Monsoon meteorolog). New York:

Academic Press.Ramage, C.S. and Raman, C.R.V. 1972: Meteoro-

Iogical Atlas of the Intemational Indian Ocean Expedi-rion. Yolume 2: Upper ae?. lVashington DC: USGovernment Printing Office.

Ramakrishnan, K.P. and Narayanan, J. 1955: Astudy of fifty years' rainfall of Mangalore. Memoirs ofthe Indian Meteorohgical Department 30, l0I-53.

Rao, Y.P. 1976: Southwest monsoon. MeteorologicalMonograph, Synoptic Meteorology, No. l. NewDelhi: India Meteorological Department.

Rasmusson, E.M. and Carlrenter, T.H. 1982:Variations in tropical sea surface temperature andsurface wind fields associated with the southemoscillatiorVEl Nifio. Monthly Weather Redeu ll0,354-84.

Roplewski, C.F., Halpert, M.S. and Rasmusson,E.M. 1988: Interannual variability in the tropicalbek. hoceedings of the Seoenth Conference on Ocean-Atmosphere Interaction, Anaheim, Califomia, Feb-ruary l-5, 1988,70-73.

Riehl, H. 1979: Climate and weather in the topics.London and New York: Academic Press.

Russell, J.S. 1981: Geographic variation in seasonalrainfall in Australia - an analysis ofthe 80 year period1895-1974. Joumal of the Ausnalian Institute of ACn-cuhural Sciences 47, 59-66.

Simpson, R.H. and Riehl, H. 1987: The Hurricaneand its impact. Baton Rouge: Louisiana State Uni-versity Press,

Sikka, D.R. 1977: Some aspects of the life history,structure and movement of monsoon depression.hne and Applied Geophgsics I 15, I 501-29.

Sikka, D.R. and Gadgil, S. 1980: On the maximumcloud zone and the ITCZ over Indian longitudesduring the southwest monsoon. Monthly WeatherReoiew 108,1840-53.

Singh, S.V. and Kripalani, R.H. 1985: The south tonorth progression of rainfall anomalies across Indiaduring the summer monsoon season. Pure and Ap-plied Geophysics 123,625-37 -

Southern, R.L. 1979: The global socio-economicimpacts of tropical cyclones. Australian MeteorohgicalMagazine 27 , 17 5-95.

Strahler, A.N. 1975: Physical geography. New York:John Viley and Sons.

Streten, N.A. 1981: Southern Hemisphere sea surfacetemperature variability and apparenr associationswith Australian rainfall. Journal of Geopfutsical Re-search 86, 485-97.

- 1983' Extreme distributions of Australian annualrainfall in relation to sea surface temperature. Journalof Climatolog 3, 143-53.

Sumi, A. and Murakami, T. 1981: Large scale as-

R. Suppiah 317

pects of tte 1978-79 winter circulation over thegreater VMONEX region. Pan I: Monthly andseason mean fields. Journal of the Meteorological Soci-ety of Japan 59, 625-45 .

Suppiah, R. 1991: Intra-seasonal variabiliry in Aus-tralian monsoon rainfall. Paper presented at the!(/orkshop on Forecasting Intra-seasonal Variability,BMRC,20-22February 799L

Tanaka, M. l98l: Interannual fluctuations of thetropical monsoon circulation over the greaterIYMONEX arca. Joumal of the Meteorological SocietyofJapan 59,825-31.

Taylor, t.A. and Tulloch, D. 1985: Rainfall in thewet-dry tropics: extreme events at Darwin andsimilarities between years during the period18?0-1983. Austalian Journal of Ecolog 10,281-95.

Treloar, H.M. 1934: Foreshadowing monsoonal rains inNonhem Australla. Bulletin No. 18, Melboume: Bu-reau of Meteorology.

Trenberth, K.E. 1975: A quasi-biennial standingwave in the southem hemisphere and interrelationswith sea-surface temperature. Quanerty Journal of theRoyal Meteorohgical Society 101, 55-74.

- 1976a' Fluctuations and trends in indices of thesouthem hemisphere circulation. Quanerty Journal ofthe Royal Meteorological Society 1O2,65-75.

- l976bi Spatial and remporal variations of thesouthem oscillation. Quanerljt Journal of the RoyalMeteorohgical Society lO2, 639-53.

- I980a' Atmospheric quasi-biennial oscillations.Monthfu lYeather Reoiew 108, l37O-72.

- 1980b' Planetary waves at 500 mb in the SouthemHemisphere. Monthlg l{/eather Reoiew l0g1378-89.

- l98l' Interannual variability of the SouthernHemisphere 500 mb flow: regional characteristics.Monthlg Vleather Review LOg, lZ7 -36.

Troup, AJ. 1961: Variations in upper uoposphericflow associated with the onset of the Australianmonsoon. Indian Joumal of Meteorologt and Geo-physics 12,217-30.

- 1965' The southem oscillation. euanerly Journat ofthe Royal Metarological Society gl,490-506.

- 1967t Opposition of anomalies of upper tropo-spheric winds at Singapore and Canton lsland. Aus-tralian Meuorological Magazine 1 5, 32-37 .

Tucker, G.B. and Stokes, A.N. 1980: Rainfall trendsin Australia. In Pearman, G.I., editor, Carbon Dioxideand Ckmate, Canberra: Australian Academv of Sci-ence,210-14.

Van Loon, H. 1967: The half-yearly oscillations inmiddle and high southem latitudes and the coreless,xtntet. Journal of the Annospheric Sciences 24,472-86.

318 The Austral ian summer monsoon: a review

- 1984' The Southem Oscillation Part III: Associ-ations with the trades and with the trough in thewesterlies of the South Pacific Ocean. MonthlyWeather Review ll2' 947-54.

Walker, G.T. 1923: Correlation in seasonal variationsof weathet Ylll. Memoirs of the Indian MeteorologicalDepanment 24, 75-131.

- 19242 rVorld weather IX. Memoirs of the IndianMeteorohgical Departtnent 24, 27 5-332.

- 7928i rtrforld weather lIl. Memoirc of the RoyalMetnrologital Society 17, 97 -106.

Walker, G.T. and Bliss, E.W. 1932: Iflorld weatherY. Memoirs of the Royal Meteorohgicel Society 4,53-84.

-19?7' World weather Yl. Memoirc of the RoyalMeteorolngi.cal Society 4' I 19-39.

Webster, P.t. l98la: A model of the seasonally vary-ing planetary-scale monsoon. In Ughthill, J. andPearce, R.P., editorc, Monsoon dynamics, Cambridge:Cambridge University Press, I 65-92.

- 1981b' Cold surges of the winter monsoon: dy-namic structures. Inurnational Conference on SeimtificResulx of the Monsoon Experiment, Denpasar, In-donesia, rJflMO, 2-3 to 2-8.

- 1987a: The elementary monsoon. In Fein, I.S. andStephens, P,L., editors, Monsoons, New York: JohnViley & Sons, 3-32.

- 198?b' The variable and interactive rnonsoon. InFein, ].S. and Stephens, P.L., editors' Monsoons,NewYork: ]ohnT7iley & Sons, 269-330.

Williams, M. 1979: Interhemispheric interaction dur-ing winter MONEX. Australia-New Zealand GARPSymposium, December 17-19, 1979, Melboume,

Ausualian Numerical Meteorology Research Centre,Melboume, T3-78.

- l98l' Interhemispheric interaction during winterMONEX. Intemational Conference on Scicnffic Resuhsof the Monsoon Experiment, Tallassee, Florida, 10-12to 10-16.

- 1987' Relaions between the southern oscillation andthe troposphere ooer Austaka. BMRC Research Re-poft, No. 6.

Widohamidioio, S. 1981: The main synoptic featuresand the relation to the distribution of rainfall over

Java sea and its surroundings during winter MONEXpeiod. International Conference on Scienti'fic Results ofthe Monsoon Experiment, Denpasar, Indonesia,'ifMO, 5-9 to 5-12.

Wright, P.8., Wallace, J.M., Mitchell' T.P. andDeser, C. 1988: Correlation structure of the El

Nif,o/Sou*rem Oscillation phenomenon' Journal ofClimate l' 609-25.

Wyrkti, K. 1975: El Nirlo - the dynamic response ofthe equatorial Pacific Ocean to atmospheric forcing'

Joumal of Physi.cal Oceanography 5,572-84.

Yasunari, T. 1980: A quasi-stationary appearance of30- to 40-day period in the cloudiness fluctuationsduring the summer monsoon over India. Joumal ofthe Metnrological Society of Japan 59, 225-29 .

Zhu, Q. and Yang, S. 1990: A numerical study on theeffect ofthe Tibetan Plateau upon cold surges in EastAsia. hoceedings of the'Intemational Conference on EastAsia and lVestent Pacffic Meteorologt and Climate,Hong Kong, 6-8 July 1989,4-13.