MAUSAM . 52. 2 (Apri l 2001.,333·350

551.553.2 1 : 551.5 15.1 (94)

The Australian monsoon, Part 2: Depressions and cyclones

KSHUDIRAM SABA

27. B-Road. Mllllllrmri Bugh. New fJelhi-11006 5, India

and

SURANJANA SAJIA

Environmen tal MOlle/in g Center. NCEPIN WS/ NOAA , U.S.A.

(Received 8 July /999. Modiji"d / 0 Augusr 2000)

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AIl STRAC T . A stud) of the NCEI'/NCAR Reanalysis fOf the Australian region during the so uthern summerreveal s tha i 100:501 of lilt: depressions and cyclo nes mer the region form and devel op in a stationary wave that developsalong (he continent's northern coastline during this period due 10 land -sea thermal co ntrast. The structure and propertiesof the stationary wave are brou ght our in detail and in terna l and externa l forcings thai lead tc its developme nt imodepressions and cyclones arc discussed . Environ mental factors Ihal appear 10 influence the movemen t and recurvarurc ofcyclones over the region arc discussed with two case studies .

Kl·)· words - Australian mon soon disturbances. Depression s and cyclones in Australia . Tropica l cyc tones inAustrali an region .

1. In troducti on

The mean monsoon circu lation over Australi a andsurrounding oceans during the southern summ er,described in Part 1 of this study (Sa ha and Saha , 1999). iso ften perturbed by the formation of depressions andcyclones. As is well-known, these disturbances aresynoptic-scale 10\1,,· pressure systems which have cyclon iccirculations around the center with maximum tangent ialvclocuy near the cen ter of at leas t 17 mls in depressionsand 24 mls or more in cyc lones. as. per existi ng officialtermi nology in India and Australia (e.g.. Rae . 1976;Bureau of Meteorology . 1956. 1978). Because of theirgrea t importance in the con tex t of the cont inent's weathe rand climate. human safety and potential for des tructio n oflife and property. they have received spec ial atten tion ofmeteorologis ts from very early times.

Several studies (Bureau of Meteorology. 1956. 1978;Gray . 1968; Falls. 1970; Davidson et ill.. 1983. 1984;Mcllride. 1981; McBride and Zchr. 1981 ; McBride and

(33 3)

Keenan. 1982; Helland. 1983; Holland. 1984 a.b .c :Holland and Merrill. 1984; Love and Garden. 1984; Love.1985a.b; McBr ide and Holland.1989) have addressed theproblems of their genesis. deve lopment and movement. Asa result of these studie s. it is now know n ' hat:

(i) Most of the disturbances originate in themonsoon troug h or shear zones over warmocean surface close to the coast of northernAustralia includi ng the Gu lf of Carpentaria anda wide strip of the southwest Paci fic ocean.extending eastsoutheastward from the CoralSea region , which is popu larly kno wn <IS theSouth Pacific Con vergenc e Zo ne (SPCZ) .

(ii) In terms of cyc lone days. the highest frequencyof occurrence of tropical storms is in the Gulfof Carpentaria with seco ndary ma xima in theCora l Sea region and the areas off the coas t ofnorth western Australia where (he oceansurfaces arc very warm( Holland. 1984a).