Intense tropical cyclogenesis over the northwest Australian region in 1998/1999: Causal factors |
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| Authors: | L M Leslie R F Abbey Jr M S Speer T C L Skinner |
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| Institution: | (1) Centre for Environmental Modelling and Prediction School of Mathematics, University of New South Wales, Sydney, Australia, AU;(2) Office of Naval Research, Arlington, VA,;(3) Bureau of Meteorology, Darlinghurst, Sydney, Australia, AU;(4) National Meteorological and Oceanographic Center Bureau of Meteorology, Melbourne, Australia, AU |
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| Abstract: | Summary The 1998/99 tropical cyclone (TC) season over northwest Australia was notable for an above average number of TCs (seven compared
to five on average) and a number of unusually intense TCs making landfall (three category 5 TCs). The active 1998/99 TC season
is attributed here to a combination of a number of broad-scale features over the south east Indian Ocean and the Australian
region, with identifiable precursors favoring tropical cyclogenesis. These precursors include: below normal MSLP, abnormally
warm ocean temperatures, above average relative humidity in the low- to mid-tropospheric levels and weak wind shears in the
genesis region under study, that is, between 10° S to 20° S and 105° E to 135° E. These favorable conditions first appeared
as early as August 1998. The appearance of favorable conditions so far ahead of the TC season indicates that they are the
likely cause of the enhanced TC activity rather than simply an effect. Although the season as a whole was an active one, strong
intra-seasonal variability was evident in that there were two named TCs in December 1998, forming 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 importance of the above-mentioned precursors
in favoring tropical cyclogenesis during the 1998/99 season is discussed in terms of seasonal time scales of the preceding
spring and down to synoptic and mesoscale time scales ranging from several days to 48 hours or less.
Received October 5, 2001 Revised December 28, 2001 |
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