The Australian summer monsoon: a Tropical cyclones and the monsoon; and Conclusions. ll Monsoon climate

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

    The Australian summer monsoon: a revtew R. Suppiah Climate lmpact Croup, CSIRO, Division of Atmospheric Research, Private Bag No. 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 period from December to March. During this period, interannual and intraseasonal vadations of rainfall associated with global-scale circulation anomalies strongly effect human life and economic activities in this region. Any changes 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 serious impacts to human life in the monsoon-dominated region. To provide a basis for looking at chinges in monsoonal characteristics under enhanced greenhouse condition, iha arlrr.rrt understanding of the Australian summer monsoonal circulation characteristics is reviewed here. Detailed information is givei on the formadon and the steady development of che Pilbara heat low over the nprhwestern pan of Australia and the importance ofthe location of the monsoon shear line, active and break cycles ofthe monsoon, influence of South Ciina Sea cold 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 interannual time scales and the link between monsoonal activity and tropical cyclones. The problems related to the above mentioned 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, intraseasonal oscillation, 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 the seasonality of winds (Ramage, l97l). However, the term monsoon is presently used to distinguish various climatic, agricultural and land use phenomena. Based. on Ramage's criteria that rely on atmospheric circulation differences between January and July, the area north of 25oS over Australia could be delineated as monsoonal. This monsoonal area consists of more than 400 000 square kms covering the nofihern halves of $Testern Australia and Queensland and more than 90% of the Northern Territory. As the summer monsoon rainfall strongly determines the agricultural and industrial r"ctor. of this area, any strengthening or weakening of the Australian summer monsoon associated with changes in general circulation would cause drastic changes in crop production and cattle numbers, particularly in the inland regions. These impacts could be exacerbated by extreme 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 review of 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 the monsoonJ influences of the South China Sea cold surges on monsoon activity, and intraseasonal rainfall variability during the monsoon season. The article is divided into 10 sections. They are: Monsoon climate in northern Australia; The Australian monsoon definition; Summer monsoon heat lows and the monsoon shear line; Australian monsoon onset and withdrawal; Active, moderate and break phases of the monsoon; Surges and monsoon 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 in soils and vegetation, a certain unity is observed due to the influence of the monsoon. In particular, this region is characterizedby a pattern of uniform high temperature and solar radiation and strong seasonal rainfall incidence (Fitzpatrick and Nix, 1970; Nix and Kalma, 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 desert climate) in the interior, and as Af (equatorial wet climate), Aut and Czu (temperate rainy climate) on the east coast (Strahler, 197 5). The seasonal march of rainfall in tropical and subtropical latitudes in Australia can be explained by seasonal movements of the equatorial trough (Hobbs, 1973;1975; Riehl, 1979). The seasonal, poleward shift of the subtropical high pressure belt from about 30oS in winter to about 40'S in summer and the southward migration of the equatorial trough following the sun, bring pronounced changes in atmospheric circulations and rainfall characteristics over north Australia. This region experiences a short wet season in the north and east, that is modulated by the proximity of the active equatorial trough (Nicholls et aI., L982i Kininmonth, 1983). It also experiences a prolonged dry season during the rest of the year under the influence of the subtropical ridge and southeast trades. In the Southern Hemisphere summer, the monsoonal-type weather, which is associated with violent thunderstorms, tropical depressions and severe cyclones, gives much rainfall to the northern half of Australia when the equatorial trough lies at its extreme southern limits.

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

    The distribution of mean annual rainfall over tropical Australia is illustrated in Figure la. The heavy rainfall regions are concentrated along the coastal area, with rainfall decreasing markedly towards the inland regions. The influence of topography upon rainfall distribution is clearly seen, particularly in the eastern part of north Australia. During the summer, moisture-laden, on-shore winds cross the warm Coral Sea, giving a substantial increase in rainfall along the eastem flanks of the Great Dividing Range and the east coast of Queensland. On the other hand, the western slopes of the Great Dividing Range also receive 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 cover the nonhwest, north and northeast coasts. On the northwest and norih coasts the annual rainfall exceeds 1600 mm (Snake Bay - 1638 mm; Darwin - 1659 mm). There are rwo maximum rainfall regions on the northeast coast of Queensland. One with above 3800 mm is located around Innisfail (3813mm) and another with above l700mm is around Proserpine (L794 mm). In general, rainfall decreases rapidly toward the inland and reaches its minimum over the Simpson Desert, in South Australia with less than 150 mm. (See Figure I a for the location of stations.)

    Figure lb depicts mean annual rainy days over tropical Australia. Mean rainy days also reveal a clear contrast in their spatial pattern which is strongly influenced by topographical features. 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 in rainy days is observed from the coast towards inland on the eastern haf of northern Australia.

    The rainfall of tropical Australia is highly seasonal and heavily concenrrated in the period between December and March, the summer wet season. The distribution of median 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

    in