Higher coastal heat stress maxima in storm-resolving GCMs

Heat stress and extreme humid heat pose an escalating threat to many regions worldwide, particularly along coastal regions in the Middle East, where conditions can occasionally reach life-threatening levels.  Projections indicate that humid heat extremes will become more frequent and intense as global temperatures rise. We analyze humid heat extremes in the two first-ever fully-coupled, multi-decadal, high-resolution (~10 km) Earth System Models (ICON and ECMWF-IFS) projections performed within the H2020 Next Generation Earth Modelling Systems (nextGEMS) project.  

We here demonstrate that extreme humid heat events tend to be substantially underestimated at the resolution of CMIP6 models, especially in coastal hotspot regions where localized dynamics play a significant role. We focus on the Red Sea, Persian Gulf and Mediterranean coasts, which are hotspots for humid heat and areas characterized by dense populations and critical relevance for economic activity and assess the added value 10-km global coupled models in simulating extreme Wet Bulb Temperature (TW), a critical metric that combines air temperature and humidity. 

We demonstrate that at the higher resolution of the nextGEMS Storm-Resolving Models TW maxima are more than 2–3°C higher than at the coarser resolution commonly used within the CMIP6 models. Furthermore, the coarser resolutions often fail at capturing localized extremes and the effects of topography, particularly in coastal areas. Additionally, the findings reveal that onshore wind convergence plays a pivotal role in amplifying TW maxima by enhancing moisture accumulation and limiting atmospheric mixing. These results underscore the indispensable role of Storm-Resolving models in accurately assessing extremes and providing actionable insights for adaptation strategies in regions with significant human and economic vulnerabilities.