Increasing incidence of Arabian Sea cyclones during the monsoon onset phase: Its impact on the robustness and advancement of Indian summer monsoon

Abstract During recent decades, the Arabian Sea witnessed frequent occurrences of cyclones during the onset phase of the Indian Summer Monsoon. This study examined the impact of these cyclones on the establishment and northward propagation of the monsoon system. The main dynamical components of the monsoon semi-permanent systems, including wind at 850 hPa and 150 hPa, are analysed separately for Arabian Sea Cyclone Years (ACY) and Non-Cyclone Years (NCY). Pentad (5-day average) composites for ACY and NCY are prepared with respect to pentads centred on monsoon onset date over Kerala (MOK). Six pentad composites (2 before; 1 during and 3 after the onset pentad) are prepared to examine the evolution of the monsoon system over the Indian region. The analysis shows that LLJ and TEJ have been altered by the cyclones formed within ±8 days of monsoon onset over Kerala. For ACY, the LLJ and TEJ patterns require 10 days to stabilise into (acquire) its normal pattern (spatial and intensity) as NCY. Pentad (similar) analysis of SST also shows a consistent pattern to that of LLJ. As the speed of LLJ increases, the SST decreases along the path of LLJ. Total Precipitable Water shows a significant change over the Arabian Sea rather than the Bay of Bengal on the day of onset and thereafter. The changes in the upper tropospheric meridional temperature gradient are responsible for TEJ alteration. Cyclone-induced perturbations are found to disturb the thermodynamical and dynamical state of the background monsoon circulation. The high value of vertical wind shear of zonal wind over the Indian Monsoon area during NCY is ideal for monsoon and arises due to strong LLJ and TEJ. The north-south temperature difference is less for ACY, which leads to a slight delay in establishing TEJ after the onset of monsoon. Kinetic energy at 925 hPa level is higher during NCY than ACY and is supporting the rainfall variations.