On the Controlling Factors for Globally Extreme Humid Heat

Humid heat presents a major societal challenge through its impacts on human health, energy demand, and economic productivity, underlined by the projected emergence of conditions beyond human tolerance. However, systematic assessment of what drives the most extreme humid heat worldwide has been lacking. Here, we investigate factors determining the location and magnitude of humid-heat extremes, framing our analysis around the four regions with the highest values: the southern Persian Gulf, north-central Pakistan, eastern South Asia, and the western Amazon. We find that strong boundary-layer moisture fluxes, together with stability that inhibits moist convection, explain well the timing and location of near-surface humid-heat extremes. These favorable conditions are achieved through regionally distinct factors, including shallow sea breezes in the Persian Gulf and large-scale subsidence in eastern South Asia. Our results demonstrate some of the principal controls on the most intense humid heat, both globally and for particular regions and heat events.