Spatial Shift and Intensification of Compound Drought and Heatwaves (CDHW) towards Southern and Eastern Tropical Regions over India

Abstract. Quantifying the regional intensification of Compound Droughts and Heatwaves (CDHW) is necessary in the context of changing precipitation regimes. We investigated different characteristics of CDHW events over India, with an intent to unravel the regional shifts and intensification of these events, especially under the context of changed precipitation spells over the region. We deployed the Standardized Net-Precipitation and Distribution Index (SNEPI) to account for the impact of intramonthly precipitation spell variability for drought quantification and further to examine the spatiotemporal evolution of CDHWs from 1951 to 2016. There was a dramatic 86 % rise in the frequency of individual heatwaves and a significant intensification of individual droughts in historically humid zones over the past 6 decades. Whereas, the proportion of India affected by the compound (CDHW) occurrences expanded from 10 to 35 %, with hotspots transitioning from historically arid northwestern regions to historically humid eastern and southern regions. The Moderate, Severe, Elevated and Intense CDHW events spread by 3 to 5 times spatially, while the worst Extreme category experienced a 25-fold spatial increase. 46.4 % of Moderate and 43.3 % of Elevated events escalated to higher severities, suggesting a cascading amplification of extremes. Despite intervening wet periods, 10–16 % of CDHWs recurred, indicating that mitigation was unsuccessful, especially over the traditionally wet and resilient regions. The study emphasizes the necessity of reevaluating climate adaptation strategies across both traditionally arid and humid zones under evolving rainfall distributions as a result of a 4–10-fold increase in exposed urban populations across emergent hotspots (Kerala and Assam) in southern and eastern frontiers.