Reorganization of Heatwave Day Regimes across India under Recent and Near Future Warming

Heatwaves are among the most rapidly intensifying climate extremes over India, yet their evolving spatial characteristics under recent and near future climate change remain inadequately quantified. This study examines the spatio-temporal variability of Heatwave Days (HWDs) across India using daily maximum temperature from the India Meteorological Department (IMD) gridded dataset for the historical period 1975-2024 and extends the analysis to the near future (2025-2044) using CMIP6 climate projections. Heatwave days are identified at each grid point using a calendar day based percentile approach, where daily maximum temperature exceeding the local 95th percentile threshold for the same calendar day, computed over a fixed reference period of 1981-2010, is classified as a heatwave day. Grid wise cumulative and decadal HWDs are analysed to assess long-term exposure and spatial redistribution. To objectively identify dominant heatwave regimes, Self-Organizing Maps (SOMs) are employed using multiple HWD metrics, enabling classification of regions with distinct heatwave characteristics and temporal evolution. Observational results indicate a clear reorganization of heatwave patterns over India. During the late 20th century (1975-1994), HWD accumulation is largely limited to north-western and parts of central India, typically ranging between 26 to 50 days per decade, with most eastern and peninsular regions experiencing fewer than 25 HWDs. From the mid-1990s onward, a pronounced intensification and spatial expansion is evident. By 2005-2014, large parts of central and eastern India exhibit decadal HWDs in the range of 51 to 100 days. The most recent decade (2015-2024) shows widespread moderate to high HWDs accumulation across the country, with several regions of central, eastern, and peninsular India experiencing 101 to 150 HWDs, and localized hotspots exceeding 150 days per decade. Future HWDs for 2025-2044 are derived from daily maximum temperature projections of the MPI-ESM1-2-HR model under the SSP2-4.5 scenario. The near-future decadal projections (2025-2034 and 2035-2044) indicate a continued intensification and spatial expansion of HWDs, with extensive areas of north-western, central, and peninsular India experiencing 151 to 250 HWDs per decade, and emerging hotspots exceeding 250 to 350 days, particularly over parts of north-western and southern India. Eastern India also shows a marked transition toward higher HWDs classes, indicating increasing regional vulnerability. Overall, the combined observational and CMIP6 based analysis demonstrates a transition toward widespread and persistent heatwave exposure across India in both recent decades and the near future. The integration of a grid specific, calendar day based percentile definition with SOM based classification provides a robust framework for identifying evolving heatwave regimes and supports improved heat risk assessment, climate adaptation planning, and early warning strategies under continued warming.