This is because of apparent SUN movement and consequential heating of land or ocean. During 21,22 MARCH the SUN will be exactly over equator. There after it moves to North. Most of the places in NORTHERN hemisphere is land mass and it get heated easily. During this time since the SUN is in Northern hemisphere the southern hemisphere is relatively COLD. The major portion of southern hemisphere is OCEAN water. the wind associated with sub tropical HIGH regions in southern hemisphere blow from SOUTH to NORTH. As it traverse a vast portion of ocean the wind is moist and when it crosses the equator due to Coriolis force the wind turned from south to South West and hit Kanyakumari & Kerala Coast in main land India and Andaman coast This is SOUTH WEST MONSOON. SWM is considered to be the principal rainy season of Indian sub continent. The location of ‘MASCARENE HIGH” in Indian Ocean [IO], that is close to WESTERN locations in IO or to the EASTERN LOCATION in IO determines the spatial distribution of monsoon rainfall in India (even drought or flood )This is NOT EL NINO or ENSO.Similarly the SUN will be exactly ove equvator on 21/22 September and afterwards it moves to Southern Hemisphere. The land mass in NH gets cooled and the SIBERIAN HIGH get activated. Now winds from NORTH blow towards SOUTH. This is North East monsoon. The Coramandal coast (Kanyakumari to Nellore and beyond] in AP & TN and Srilanka gets rainfall. Formation of low pressure system in BoB brings Tropical cyclones and associated weather during this period to East coast of India