Pacific Decadal Oscillation Causes Fewer Low-Latitudinal Cyclones in the North Indian Ocean

Tropical cyclones are far fewer near the equator but can intensify rapidly, leaving little time for preparation. We investigate the number of low-latitudinal (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon season (October to December) during 1951-2010. We show a marked 43% decline in the frequency of such cyclones in recent epoch (1981– 2010) compared to earlier (1951– 1980). This decline in tropical cyclone frequency can be attributed to the weakened low-level vorticity modulated by the Pacific Decadal Oscillation (PDO) and increased vertical wind shear. Moreover, the influence of PDO is found to be largely independent of ENSO phases. In the presence of warming sea surface temperature in the equatorial north Indian Ocean and a favorable phase of the PDO, both the intensity and frequency of such cyclones are expected to increase.