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

Abstract

The Indian summer Monsoon (ISM) is evolving over time, with a westward shift in precipitation, creating a dipole-like structure with an increase (decrease) in precipitation over the Northwest (Northeast) regions, which has been proposed as a westward expansion of the boreal summer ITCZ over recent years. Meanwhile, the interpretations of monsoons have also shifted from traditional land-sea contrast-based arguments to an ITCZ-driven explanation. We here propose two pathways that build upon the existing framework for this westward expansion of rainfall, both mediated by the Indo-Pacific Warm Pool (IPWP) and aided by a westward shift in the Walker cell. The first is an enhanced basin-wide moisture transport crossing through Pacific to Indian ocean, manifesting in an enhanced Low-Level Jet, and the second is a Gill-like response to diabatic heating over the Maritime Continent (MC), resulting in anomalous south-easterlies traversing the northern parts of the Indian subcontinent. The 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 revealed a strikingly dominant east- west pattern intervened by westward expansion in IPWP, as later found to be associated with latent heating patterns driven by increased precipitation over the MC and western parts of the Indian subcontinent. The results document new perspectives on recent significant shifts in the ISM, potentially affecting a large fraction of the populous Indian subcontinent.