Understanding Future changes in Semi-Permanent Systems and associated Rainfall during the Indian Summer Monsoon

This study employs Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations to assess how large-scale semi-permanent systems of Indian Summer Monsoon (ISM) change in future under varying greenhouse gas emission scenarios. Eight CMIP6 models are analyzed for three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) across two future periods: near future (2031-2060) and far future (2071-2100). Model evaluation shows that MCM-UA-1-0 and MIROC-ES2L capture ISMR variability more realistically, whereas ACCESS-CM2 and CanESM5-CanOE exhibit dry biases. Projections indicate an overall intensification of ISMR with increasing emissions, most pronounced under SSP5-8.5. Dynamic responses reveal a strengthening and equatorward shift of the Subtropical Westerly Jet (SWJ), a weakening and southward displacement of the Tropical Easterly Jet (TEJ), and a poleward shift of the Low-Level Jet (LLJ) from the near- to far-future period. Thus, the meridional wind shear weakens while zonal shear strengthens, modifying monsoon dynamics in higher emission scenarios. Teleconnection analysis indicates a persistently negative ENSO-ISMR relationship, while DMI-ISMR and NAO-ISMR linkages intensify under higher emission scenarios. In accordance with these changes, the Central and South Peninsular India would be experiencing more rainfall, particularly in September, but a noticeable decrease is noted in Northeast India rainfall. These findings highlight the future changes in synoptic conditions and rainfall of the ISM over homogeneous regions.