Changes in large-scale circulation over the Indo-Pacific region and its association with 2018 Kerala extreme rainfall event

Abstract

We investigate the characteristics and causes of extreme rainfall event occurred in Kerala (south western regions of the peninsular India) in August 2018. The changes in large-scale circulations over the Indo-Pacific domain and their association with regional circulation features, which made the extreme rainfall in Kerala are analysed. During this extreme event, Kerala experienced eleven days of incessant rainfall from 8 to 18 August 2018. It is observed that these extreme rainfall days were accompanied with two extreme spells of rainfall during 8–10 and 14–17 August 2018 that made severe floods in Kerala. Kerala experienced rainfall of more than 100 mm day−1, particularly in the south Kerala during these events. The triad (average of three days) analysis of vertical velocity at 500 hPa and moisture convergence at the surface level reveal the dynamics and thermal structure of the regional deep convective activity. A remarkable eastward shift of cross equatorial flow over the Indian Ocean produced an intense offshore vortex, which induced more moisture towards the southern regions especially over Kerala where the extreme event was occurred. The west Pacific subtropical high (WPSH) is one of the large-scale circulations over the Indo-Pacific region, which shifted northward and the western flank of WPSH extended westward resulting a blocking high that was created over East Asia during the period. The eastward shift of cross-equatorial flow over the Bay of Bengal, outflow from the East Asian blocking high were mainly maintaining the low-pressure system over the Bay of Bengal during the extreme event. Therefore, the key cause of the extreme flood over Kerala in August 2018 is the deep moisture convergence through the offshore vortex, intense intrusion from subtropics and out flow from low-pressure system over the Bay of Bengal.