Mechanisms Responsible for Variability of Tropical Cyclone Activity in the South Indian Ocean

Tropical cyclones (TCs) in the South Indian Ocean (SIO) concentrate over the eastern and western ends, which affect the Australian region and coastal African regions respectively. It has been found previously that interannual to interdecadal variability of TC activity in the SIO is influenced by the anomalous patterns of sea surface temperature (SST) and vertical wind shear (VWS) in the basin (Cheung et al. 2015). Co-variability analysis showed that these SST and VWS patterns are modulated by the development of the subtropical dipole events in the SIO. The subtropical dipole event has similar zonal dipole pattern as the well-known Indian Ocean dipole but occurring at higher latitudes. However, there are different proposed mechanisms in regard to how the VWS pattern in the SIO can be changed. The standard model of subtropical dipole development (Behera and Yamagata 2001) is due to shift in the position of the subtropical high pressure. Accordingly, there are anomalous low-level winds during such shift in the subtropical high that modulate the VWS. On the other hand, Chang-Seng and Jury (2010) proposed that eastward propagation of wave train in the jetstream from the southeast Pacific may split into the polar and subtropical axe, with the latter modulating the upper-level winds over the SIO and thus the VWS over there. In this study, we verify these two pathways (one low-level and the other upper-level) to changes in VWS through observations and their relevance to TC activity in the basin, especially those intense storms. Whereas an interdecadal shift has been identified for intense TCs in the SIO (Cheung et al. 2015), the issue of whether there is corresponding variability in the underlying mechanism will be addressed.