Effect of Arabian Sea Cyclones during the Onset Phase of Indian Summer Monsoon

Abstract

The Indian Summer Monsoon (ISM) is one of the popular and strongest monsoon systems newlinein the world due to its inherent variability in the quantum of rainfall, spatial extent and newlineduration of four months (June to September). The Monsoon Onset over Kerala (MOK) newlineis considered the most awaited weather singularity in the Indian Subcontinent and marks newlinethe beginning of the rainy season over India.The onset process starts in May and is newlinecharacterised by many dynamical and thermodynamical changes over South Asia and newlinethe Indian Ocean region. The pre-monsoon thermal field over the Indian landmass has an newlineimportant bearing on the land-sea heating contrast in the region, consequently influencing newlinethe establishment of monsoon over India. The Outgoing Long-wave Radiation (OLR) newlinepattern, depth of westerlies over the Lakshadweep area and rainfall over the Southwest newlinecoast are taken into account for the declaration of MOK. The normal date of MOK is newlinearound 1st June with a standard deviation of 5 6 days. The strengthening of Low-Level newlineJet (LLJ) around 850 hPa and the establishment of Tropical Easterly Jet (TEJ) at 150 hPa newlineare the most noticeable features during MOK. Many studies reveal that LLJ and TEJ newlinehave a good correlation with peninsular rainfall activity during the onset and thereafter. newlineClimate change is already affecting the seasonality and characteristics of the ISM as newlineevident from the spatial and temporal redistribution of rainfall with the frequent incidence newlineof extreme rainfall events. Major cyclonic seasons over the North Indian Ocean (NIO) newlinebasin are during the annual transition periods before and after the ISM. Throughout newlinethe pre-monsoon time (April June), the NIO basin has warm surface temperatures newlineresulting in frequent cyclonic activity followed by the annual shift of the Inter Tropical newlineConvergence Zone (ITCZ) to the northern hemisphere. During recent decades, the rate newlineof warming in the Arabian Sea (ARB) is more prominent and the occurrence of Tropical newlineCyclone (TC)s are becoming more frequent during the monsoon onset phase. Thi