Interdecadal shift of intense tropical cyclone activity in the Southern Hemisphere

The variability of intense tropical cyclone (TC) in the Southern Hemisphere and its relation to large-scale environmental parameters are examined through temporal and spatial principal component analysis (PCA). The results show that there was a shift from low intense TC activity during 1976/1977-1987/1988 (period 1) to high activity during 1988/1989-2007/2008 (period 2). Accumulated cyclone energy maps indicate that this shift is related to more occurrences of intense TCs in the southwestern Indian Ocean (SWIO) and northwestern Australian region during period 2. The spatial-mode PCA identifies distinct sea surface temperature anomaly (SSTA) patterns dominating in periods 1 and 2. The four dominating patterns in period 1 have low or negative SSTA in the TC development regions (especially over SWIO), whereas the five dominating ones in period 2 have much higher SSTA in the same regions. The temporal variability of the vertical wind shear (VWS) over the Indian Ocean (IO) is examined through the temporal-mode PCA. The first mode is El Nino Southern Oscillation (ENSO)-related, but there is no clear interdecadal variability identified. The second mode shows high VWS during the 1970s to early 1980s that is similar in length to period 1, followed by low VWS afterwards during period 2. This change in VWS thus may be responsible for the shift in intense TC activity over the IO. Linear correlative analysis shows that the mode of variability in VWS is significantly related to subtropical dipole events in the IO. Potential linkages of these findings with ENSO and Interdecadal Pacific Oscillation are discussed.