Meteorological drivers of humid heat extremes across the global (sub)tropics

Humid heat is a serious risk to human health, reducing the body’s ability to cool through sweating. The intensity, frequency and impact of humid heat extremes will increase under climate change, particularly in tropical and sub-tropical ‘hot spots’, such as equatorial Africa and the Indian subcontinent, which are highly populated, and already very hot and humid. Research on the meteorological drivers of humid heat extremes is immature compared to that for dry heatwaves. Here an overview of the latest results from the Humid heat extremes in the Global (Sub)Tropics (H2X) project will be presented. We have shown that rainfall is a key ingredient for humid heat, and its role varies depending on the type of land-atmosphere coupling regime. In moisture-limited environments, mostly in the semi-arid sub-tropics, humid heat extremes occur during or immediately after rainfall through increased evaporation into a shallower boundary layer. In energy-limited environments, mostly in the moist tropics, humid heat extremes occur during the easing of rainfall through increased solar heating. Our most recent work focuses on atmospheric waves as a source of predictability. Equatorial Kelvin waves modulate humid heat, where the convergent phase of the wave (in the 850hPa winds) brings rainfall, followed by increased solar heating in the divergent phase. A similar process occurs over the Sahel region of west Africa within African Easterly Waves. We have also performed a set of idealised experiments with the Met Office Unified Model to quantify the role of surface moisture sources such as lakes, wetlands, and patches of wet soil from rainfall on evaporation, mesoscale circulations, and humid heat. Our results are valuable for identifying processes that must be well represented in weather and climate models for accurate weather forecasts and climate projections, and for informing early warning system development.