Thermodynamic and dynamic effects of increased moisture sources over the Tropical Indian Ocean in recent decades

Abstract

In the present work, the mechanisms for the changes in moisture sources (evaporation minus precipitation; EmP) during boreal summer (May–September) are explored over the tropical Indian Ocean during 1979–2016. We apply a moisture budget analysis to quantify the thermodynamic and dynamic effects. Our results show that the EmP in the tropical central-eastern and southwestern Indian Oceans experienced significant increasing trends during boreal summer. The increased EmP in the tropical central-eastern Indian Ocean is due to the enhanced dynamic divergence (account for approximately 51%), while a stronger dynamic advection contributes more moisture supply to the southwestern Indian Ocean (account for approximately 34%). We find that during recent decades, the enhanced east–west thermal gradient in the Pacific strengthens the Walker Circulation, which leads to a westward shift in convection over the Indian Ocean warm pool, resulting in weakened convection and ascent over the tropical central-eastern Indian Ocean. The weakened convection leads to an anomalous low-level atmospheric divergent circulation, which intensifies the dynamic divergence contributing to the enhanced EmP over the tropical central-eastern Indian Ocean. Additionally, the warming climate during recent decades also increases the land–sea thermal contrast in the vicinity of the Indian Ocean, which enhances the southeastern wind in the low-level troposphere and leads to an enhanced EmP over the southwestern Indian Ocean.