Shifting Patterns of Summer Humid Heatwaves Highlight Growing Threat over South Asia and Middle East–North Africa

Saptashree Deb, Sparsha Rajesh Sherke, Poulomi Ganguli

Humid heatwaves (HHWs), marked by elevated heat−high moisture compounding, are increasingly concerning for sustainable development in South-Asia (SAS) and the Middle East–North Africa (MENA). We examine the spatiotemporal compounding of HHWs during summer (March−August) across 268 urban and peri-urban sites within the IPCC–reference regions in the SAS and MENA using in-situ observations. We investigate roles of two heat–humidity indices—wet-bulb temperature (Txw) and extended heat-index (Hx) to analyze changes in the distribution properties of HHW attributes over two distinct timeframes: 1980–2000 (TW1) and 2001–2024 (TW2). Over 80% of sites show rising trends in daily median magnitudes for both indices in TW2, while ≤25% exhibit increases in median HHW severity. In contrast, skewness increases across 65% of sites, and inter-arrival time shortens across approximately 60–70% of sites, indicating that median changes alone may not capture increasing frequency and temporal clustering. HHW spatial coverage expands by about 10% per decade during 1980–2024. Post-2000, HHW days across SAS and MENA coincide with stronger mid-tropospheric ridging/blocking, enhanced northwesterly warm-air advection, and elevated atmospheric moisture. Regionally, SAS HHWs are characterized by reduced outgoing longwave radiation, increased cloud cover, enhanced sensible heat flux, and weak latent heat flux anomalies. In contrast, MENA shows a markedly different regime, dominated by clear-sky, subsidence-driven desert environments, together with moisture-influenced coastal zones. MENA coastal regions show weaker sensible heat flux but stronger latent heat flux, which can enhance urban-coastal humid heat and inland thermal stress through advection.