Ocean and Atmospheric Characteristics Associated with the Cyclogenesis and Rapid Intensification of NIO Super Cyclonic Storms

Abstract

An attempt is made to investigate the various atmospheric and oceanic conditions that contributed to the genesis and rapid intensification (RI) of the super cyclonic storms (SUCS) formed over the North Indian Ocean (NIO) basin during 1982-2020. Weak to moderate vertical wind shear values are found in all the cases supporting genesis. The genesis potential parameter was > 30 in four cases, whereas the other two cases, viz., Gonu and Odisha SUCS experienced values ≤ 30. Prior to genesis, Equatorial Rosssby (ER) waves followed by Madden Julian Oscillation (MJO) were dominant, whereas in case of Amphan, all the three convectively coupled equatorial waves viz., MJO, ER, and Kelvin wave were present. The Ocean conditions were more conducive for tropical cyclone (TC) genesis than the atmospheric conditions. Both sea surface temperature and tropical cyclone heat potential (TCHP) supported the cyclogenesis. In most cases, the setting up of the pre-genesis scenario was heavily influenced by the ocean characteristics, whereas the atmospheric conditions were supportive enough. The environmental conditions that prevailed before the starting of RI process, showed the presence of thick warm waters, a sufficient supply of moisture at the middle of the troposphere, and moderate wind shear in all cases. Mid-tropospheric relative humidity, sea surface temperature and low-level relative vorticity, all had a significant role in the RI process of all SUCS storms across the NIO basin. During the RI days, TCHP ≥ 60 kJ.cm-2 was observed for Amphan and Gonu, with thick barrier layers for all cases. Gonu encountered a warm core eddy along its track, which provided extra fuel for the RI process. All six TCs are slow to moderate moving ones, which enabled them to spend significant time over the Ocean surface and interact with the warm waters to get positive feedback for the RI process.