88th Annual Meeting (20-24 January 2008)

Monday, 21 January 2008
Role of large-scale environment in the interannual variability of Australian region tropical cyclones
Exhibit Hall B (Ernest N. Morial Convention Center)
Hamish Andrew Ramsay, CIMMS/Univ. of Oklahoma, Norman, OK; and L. M. Leslie, P. J. Lamb, and M. Leplastrier
Large-scale ocean atmosphere interactions play an important role in determining the interannual and multi-decadal variability of tropical cyclones in the Australian region. These large-scale environmental factors include sea surface temperature (SST), deep tropospheric vertical wind shear, and low-level relative vorticity. A principal component analysis of the SST data reveals two main modes of Pacific Ocean variability that match closely with basin-wide patterns of correlation between SST and TC frequency. Previously established statistical relationships between local and non-local SST, and Australian TC characteristics, are revisited using a 36-year data set from 1970/71-2005-06. Large, relatively stable correlations are found between seasonal TC number and Ni�o 3.4/ Ni�o 4 SST, with August-October Ni�o 4 SST producing the highest correlation (-0.73). In contrast, much weaker correlations (+0.37) are found in the SST region to the north of Australia where many TCs develop. These correlations reduce to almost zero after the effect of Ni�o 3.4 SST is removed by partial correlation analysis. When only data up to 1985 are considered, the positive correlation between local SST and TC frequency is substantially higher (+0.88). The annual frequency of TCs is strongly influenced by low-level relative vorticity and deep tropospheric wind shear in the Australian region that are modified by the slowly-varying tropical Pacific SSTs. Finally, while the overall trend in TC frequency is downward for 1970-2005, a pronounced upward trend is evident between 1986-2005, emphasizing caution when selecting start and end points of time series for analyzing relationships between predictors and TC activity.

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