MAUSA M . 52. 2 ( Apri l 2001.,333·350 551.553.2 1 : 551.5 15. 1 (94) The Au stralian monsoon, Part 2: Depressions and cyclones KSHUDIRAM SABA 27. B-Ro ad. Mllllllrmri Bugh. New fJelhi-110065, India and SURANJANA SAJIA Environmental MOlle/in g Center. NCE PIN WS/ NOAA , U.S.A. (Recei ved 8 Ju ly /999. Modiji"d / 0 Au gu sr 2000 ) m<' - <:fuu!l aJ ",,,,,,'1 ip Wl'l. ip ip '.fI : flrnB'lUJ 'l;; 'lJ<l1 <iffil1 iii 3!<l<:TiI 3i'R il1l><ITO 3IJPlllft ii <R<l 3i'R 1M 'l;; tl'<l'<I'R <tt mtft>l iii ilm"T 3!<Ifu iii <:im 'lNlftq <tl \ffitl <IiTll WIT i\;- m Ol-mOl M I Wl -'l?l -Ii 3IJPlllft iR'T <tl 3i'R i\;- <ffi ij f<ffilR = lJ<lI ii'lT 3!i<ift<I; 'ffi'l iii <ffi ij >ft qqj <tl lf$ ttmiI> ilm"T 3!<l<:TiI 3i'R il1l><ITO <R<l I ;;m iii <tl iffi'! 3i'R llft't<l<R lfll1fIt<l m <tl it i3"fiI> fll'l'l ii <it 3IUI1A1 <tl 'llr.= = TTlII I AIl STRACT. A stud) of the NCE I'/NCAR Reanalysis fOf the Australian region during the southern summer reveals thai 100 :501 of lilt: depressions and cyclones me r the region form and develop in a stationary wave that develops along (he continent's northern coastline during this period due 10 land -sea thermal contrast. The structure and properties of the stationary wave are brou ght our in detail and internal and externa l forcings thai lead tc its development imo depressions and cyclones arc discussed. Environmental factors Ihal appear 10 influence the movemen t and re cu rvarurc of cyclones over the region arc discussed with two case studies. Kl·)· word s- Australian mon s oon disturbances. Depressions and cyclon es in Australia. Tropica l cyc tones in Australian region. 1. Intr odu cti on The mean mon soon circu lation o ver Australi a and surrounding oceans during the southern summ er, described in Part 1 of this study (Saha and Saha , 1999). is o fte n pertu rbed by the formation of depressions and cyclones. As is well-known, these disturbances are synoptic-scale 10 \1,,· pressure systems which have cycl on ic circulations around the center with maximum tangential vclocuy near the cen ter of at least 17 mls in depressions and 24 mls or more in cyclones. as. per existi ng official termi nology in India and Australia (e.g.. Rae. 1976; Bureau of Meteorology. 1956. 1978). Because of their great importance in the context of the cont ine nt's weathe r and climate. human safety and potential for destruction of life and property. they have received special attention of meteorologis ts from very early times. Several studies (Bureau of Meteorology. 1956. 1978; Gray . 1968; Falls. 1970; Davidson et ill .. 1983. 1984 ; Mcllr ide. 1981; McBride and Zchr. 1981; McBride and (333) Keenan. 1982; Helland. 1983; Holland. 1984 a.b .c : Holland and Merrill. 1984; Love and Garden. 1984; Love. 1985a.b; McBride and Holland.1989) have addressed the problems of their genesis. deve lopment and movement. As a result of these studie s. it is now know n ' hat: (i) Most of the disturbances originate in the mon soon troug h or shear zones over wa rm ocean surface close to the coast of northern Australia includi ng the Gu lf of Carpentaria and a wide strip of the southwest Pacific ocean. extending eastsoutheastward from the Coral Sea region, which is popu larly kno wn <IS the South Pacific Con vergence Zo ne (SPCZ) . (ii ) In terms of cyc l one days. the highest frequency of occurrence of tropical storms is in the Gulf of Carpentaria with seco nda ry maxima in the Cora l Sea region and the areas off the coast of north western Australia where (he ocean surf aces arc very warm( Holland. 1984a).