Evolution of withdrawal features of the southwest monsoon over India

Abstract

ABSTRACTThe withdrawal features of the Indian summer monsoon (ISM) are investigated with the large‐scale circulations and significant terms of kinetic energy (KE), heat and moisture budget. The daily reanalysis datasets of National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) for the period of 1981–2005 are used. The results reveal that the strength of low‐level westerlies over the Arabian Sea, Indian landmass and Bay of Bengal decreases during the transition of withdrawal phase of the summer monsoon. On the other hand, the strength of upper level tropical easterly jet weakens from pre‐retreat to post‐retreat. The intensity and position of rising and sinking flow of vertical velocity shifted towards south from pre‐retreat to post‐retreat period. The diagnostic analysis indicates that the zones of Somali coast and southwest Bay of Bengal show stronger generation of KE during pre‐retreat period. Further, the flux convergence of heat observed over central Indian Ocean and the flux divergence of heat over east African region during post‐retreat period. The flux convergence of heat and moisture are stronger over the equatorial Indian Ocean during post‐retreat phase as compared to pre‐retreat. The convective activity and moisture flux convergence are stronger over the Indian Ocean during post‐retreat period, which is due to the movement of Inter Tropical Convergence Zone towards south during the transition from summer to winter monsoon over India. Thus, the dynamic and thermodynamic behaviour of monsoon has great impact on the transition during retreat phase of the ISM.