Cluster Analysis of Southern Hemisphere Tropical Cyclone Tracks

A probabilistic clustering method is used to describe various aspects of tropical cyclone tracks in the Southern Hemisphere, for the period 1969-2008. A total of 10 clusters are obtained, being comprised of five in the Pacific Ocean, four in the Indian Ocean, and one to the north of Australia. Clusters are explored with respect to their large-scale environmental controls, including sea surface temperature, El Niño-Southern Oscillation (ENSO), low-level vertical vorticity, deep-layer vertical wind shear, outgoing longwave radition, and the Madden-Julian Oscillation (MJO). Composite maps, constructed 2 days prior to genesis for each cluster, indicate that many of these can be seen as significant precursors to tropical cyclone formation (relative to the quiescent days with the same seasonality). In addition to large-scale controls, clusters are also evaluated in terms of their genesis location, seasonality, mean peak intensity, track duration, landfall location, and intensity at landfall. Preliminary results indicate that both ENSO and MJO play important roles in discriminating between clusters. La Niña events are associated with enhanced tropical cyclone activity in the eastern Indian Ocean (2 clusters) and Coral Sea (1 cluster), while El Niño events result in more tropical cyclones forming in two south-central Pacific clusters. The ENSO-modulating effect on genesis appears to be caused primarily by changes in low-level zonal flow near the equator, and associated relative vorticity in the main development regions.