The Abrupt Increase in Pre-Monsoon Heatwaves Since the Late 1990s over Northwestern South Asia

This study analyzes the characteristics of pre-monsoon (March - April - May) seasonal heatwaves across two periods: P1 (1976–1997) and P2 (1998–2024) in the Northwestern South Asia (NWSA) region. We observe a statistically significant abrupt shift (Pettitt Test, p < 0.05) in 1997 for the time series of area-averaged maximum temperature (Tmax), number of heatwave events, heatwave frequency (HWF), and cumulative heatwave intensity (HWI) over NWSA. The area-averaged Tmax, HWF, and HWI increased by 1.4°C, 5.9 days, and 9.9°C in P2 compared to P1 over NWSA. In P2, the mean spatial distributions of Tmax, HWF, and HWI increased by ~1-3°C, ~3–12 days, and ~12-24°C over the NWSA region, respectively. The regime shifts in Tmax and heatwave characteristics were associated with an abrupt increase in geopotential height at 200 hPa/500 hPa, outgoing long-wave radiation, and soil temperature level 1, along with an abrupt decrease in total precipitation, total cloud cover, and soil moisture over NWSA in 1997/1998. An increase in atmospheric thickness, reduced subtropical westerlies, and increased subsidence over NWSA in P2 exacerbate heatwaves over NWSA. This research provides mechanistic insights into the increasing heatwave, which will be useful for regional heatwave prediction and for formulating adaptive strategies for future heatwave risks.