Influence of Indian Ocean SST anomalies on the Indian monsoon during Eastern Pacific warm events

Ensemble GCM simulations are used to understand the mechanisms by which the warming in the Indian Ocean that often accompanies ENSO events can weaken the Indian monsoon. With only Pacific SSTA forcing, the GCM captures a weakening of the monsoon over much of India, but the signal is not highly statistically significant. Adding SSTAs in the northern Indian Ocean, especially in the Bay of Bengal and the Arabian Sea, strengthens the drying signal over India and leads to a realistic precipitation increase over the Bay of Bengal.

The warm Indian Ocean SSTAs modify the ENSO monsoon signal by first causing an increase in the low-level water vapor mixing ratio. This increased moisture enhances precipitation over the Bay of Bengal and the Arabian Sea without modifying the flow. Low-level wind convergence then increases due to the intensification of the latent heating and vertical velocities at middle levels, causing enhanced divergence over Indian by continuity. As a result, the Somali jet is perturbed in a realistic way, and the monsoon intensity over India weakens.