Changing characteristics of Indian Summer Monsoon Rainfall–Dynamic and Energetic Perspectives

The Indian summer Monsoon (ISM) is steadily evolving over time, with a westward shift in precipitation, creating a dipole-like structure with increased (decreased) precipitation over the western (eastern) regions of the subcontinent. We here propose mechanistic pathways that build upon the expansion/intensification of the Indo-Pacific Warm Pool (IPWP) and an associated westward shift in the Walker Cell (WC) to explain this emerging westward trend in ISM Rainfall (ISMR). The first pathway is characterised by an increase in basin-wide moisture transport across the Pacific to the Indian Ocean, leading to an enhanced Low-Level Jet (LLJ) associated with ISM. Secondly, there are anomalous southeasterlies driven by a Matsuno-Gill-like response to diabatic heating over the Maritime Continent (MC), reducing moisture transport across the eastern parts of the Indian continent, thus shifting the moisture convergence over India towards the west. This stationary Rossby-wave response emanating from the diabatic heating over MC is further confirmed using the Linear Baroclinic Model. An analysis using the MSE-based energetics framework also revealed a strikingly dominant east-west pattern, which is found to be associated with latent heating patterns driven by increased precipitation over the MC and western parts of the Indian subcontinent. The results offer new perspectives into the recent structural changes within the ISM, with profound implications for a large fraction of the Indian Population.