Why is ENSO influencing Indian northeast monsoon in the recent decades?

摘要

This study examines decadal changes of the El Ni?o/Southern Oscillation (ENSO) influence on the interannual variability (IAV) of Indian northeast monsoon rainfall (NEMR). In recent decades, the ENSO influence has strengthened over the NEMR variability. During El Ni?o years, the subsidence anomaly over the warm-pool region of Indonesia was much stronger in the recent decades as compared to the earlier decades. The zonally elongated warm sea surface temperatures (SSTs) of the warm-pool region along the tropical Indo-Pacific Ocean were much warmer than the earlier decades, which had intensified the deep convections over the Indian Ocean. The northwestward propagating trailing Rossby-gyres, formed in the western portion of the equatorial deep convection and stratiform heating, intensified as tropical storms and cyclones over north Indian Ocean. These storms and cyclones strike southern peninsular India and hence flood northeast monsoon (NEM) years. During the recent decades in the La Ni?a years, the contracted warm SSTs over the warm-pool region were much warmer than the earlier decades which intensified the deep convection and stratiform heating over there. The tropical storms and cyclones that formed due to Rossby-gyre dynamics had to travel a long distance towards the northwestward direction over the Bay of Bengal and misses southern peninsular India, and hence, the drought NEM years.