An Observational Study on the Interdecadal variability of Intense Tropical Cyclones in the Southern Hemisphere

The best tracks of tropical cyclones (TCs) in the Southern Hemisphere from the 1976/77 season are examined due to more reliable annual number of TCs and intensity estimates by satellite images compared with earlier decades. The results show that there was a period of inactive intense TC (categories 4 and 5 according to the Australian Bureau of Meteorology definition) occurrences during 1976/77-1987/88 (Period 1) followed by an active period from 1988/89 until the latest available records in 2009/10 (Period 2). Most of the intense TCs in the second period were located in southeastern Indian Ocean and northwestern Australian region. Statistical analyses are then performed on the TC-related environmental parameters in relation to this interdecadal variability of intense TCs. T-mode principal component analysis (PCA) of the sea surface temperature (SST) in the Indian Ocean, which focuses on the spatial covariance patterns, followed by an iterated k-means clustering technique identifies 12 major patterns of SST anomaly (SSTA) in the basin. It is found that there is a shift in the dominating patterns of SSTA from Period 1 to Period 2. Some major patterns during Period 2 indeed match with the locations of intense TC occurrences. On the other hand, PCA of vertical wind shear in the Indian Ocean reveals a mode of variability showing differential shear magnitudes during Period 1 and Period 2. This study aims at determining whether the development intense TCs in the South Indian Ocean is modulated by dynamical or thermodynamic factors. Given that the atmospheric circulation may be affected by SSTA patterns, the relationships between SSTA and vertical wind shear and their effects on the development of intense TCs are being investigated. Moreover, potential links of the observed variability of intense TCs in the South Indian Ocean to the tropical climate modes such as the Indian Ocean Dipole will be identified. Besides a basin-wide mode, the IOD is known to have relationships with the El Niño Southern Oscillation. Whether these relationships have effects to the TC-related environment on decadal and interdecadal time scales is of particular interest. These analysis results will be presented during the conference.