8C.3
Intense tropical cyclogenesis over the northwest Australian Region in 1998/1999: causal factors and case studies
Milton S. Speer, BOM, Sydney, Australia; and L. M. Leslie and R. F. Abbey
The 1998/99 tropical cyclone (TC) season over northwest Australia was notable for an above average number of TCs (seven compared with average of five) and, especially, the number of severe TCs that made landfall (three category 5 TCs). The unusually active 1998/99 TC season is attributed to a combination of favorable broad-scale features that formed over the south east Indian Ocean and the Australian region about 5 months prior to the usual start time of the TC season. These precursors included: below normal SLP values; abnormally warm SST and ocean sub-surface temperatures; above average relative humidity in the low- to mid-troposphere levels; anomalous low-level cyclonic vorticity; and weak vertical wind shears in the genesis region under study (between 100 S to 200 S and from 1050 E to 1350 E). These favorable conditions first appeared in early August 1998. The presence of such anomalous conditions well ahead of the TC season therefore provided a predictability lead-time of over 6 months ahead of the TC season. Although the season as a whole was an active one, strong intra-seasonal variability also was evident. There were two named TCs in December 1998 that formed within three days of each other. Only one formed in January 1999 and none in February. By contrast, in March and April, 1999, TC activity was enhanced once again with four named TCs, three of which attained category 5 status. The roles of the above-mentioned precursors in favoring tropical cyclogenesis during the 1998/99 season will be discussed in detail and several TC case studies (Thelma, Vance and Gwenda) will be presented.
Session 8C, Tropical Cyclogenesis (Parallel with Sessions 8A, 8B, and 8D)
Wednesday, 1 May 2002, 8:30 AM-10:30 AM
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