On the dynamics of cyclogenesis, rapid intensification and recurvature of the very severe cyclonic storm, Ockhi

Abstract

A very severe cyclonic storm (VSCS), ‘Ockhi’ started its journey from southwest Bay of Bengal (BoB) over Comorin area (7.5°N–77.5°E) as a low-pressure area on 28 November, 2017. Initially it moved north-westwards across Lakshadweep Islands without hitting Tamil Nadu or Kerala coast, then moved north-eastward over the Arabian Sea and dissipated upon hitting south Gujarat coast on 06 December, 2017. Rapid intensification (RI), clockwise recurvature and a longer track were the major features of the VSCS, Ockhi. In the present study, an attempt has been made to understand the mechanism of cyclogenesis, RI and recurvature of the Ockhi using satellite and reanalysis datasets. Initially, role of easterly waves (EW) and Madden Jullian Oscillations (MJO) on the cyclogenesis of tropical cyclone Ockhi is addressed. Our analysis suggests that the EW (MJO) played a seminal (insignificant) role in preconditioning the atmosphere for the cyclogenesis of the Ockhi. Our detailed analysis using various oceanic parameters indicate that, the passage of the cyclonic storm over the regions of high thermal energy, especially warmer ocean mean temperature (OMT) at 100 m depth, was instrumental in its rapid intensification. Further, we addressed the recurving feature of the VSCS Ockhi using steering flow analysis. It is found that strong north-eastward steering winds, embedded in subtropical westerlies with deep southward extent, favoured the recurving of the Ockhi towards north-eastward by suppressing the conventional westward (north-westward) track movement.