Moist heatwaves in the tropics and subtropics pose substantial risks to society, yet the dynamics governing their intensity are not fully understood. The onset of deep convection arising from hot, moist near-surface air has been thought to limit the magnitude of moist heatwaves. Here we use reanalysis data, output from the Coupled Model Intercomparison Project Phase 6 and model entrainment perturbation experiments to show that entrainment of unsaturated air in the lower-free troposphere (roughly 1–3 km above the surface) limits deep convection, thereby allowing much higher near-surface moist heat. Regions with large-scale subsidence and a dry lower-free troposphere, such as coastal areas adjacent to hot and arid land, are thus particularly susceptible to moist heatwaves. Even in convective regions such as the northern Indian Plain, Southeast Asia and interior South America, the lower-free tropospheric dryness strongly affects the maximum surface wet-bulb temperature. As the climate warms, the dryness (relative to saturation) of the lower-free tropospheric air increases and this allows for a larger increase of extreme moist heat, further elevating the likelihood of moist heatwaves.

热带和亚热带的潮湿热浪给社会带来巨大风险，但控制其强度的动态尚不完全清楚。人们认为，由炎热潮湿的近地表空气引起的深层对流的发生限制了潮湿热浪的强度。在这里，我们使用再分析数据、耦合模型比对项目第 6 阶段的输出和模型夹带扰动实验来表明，低自由对流层（地表上方大约 1-3 公里）中不饱和空气的夹带限制了深层对流，从而允许更多的空气进入。较高的近地表湿热。因此，大规模沉降和干燥的低层对流层的地区，例如毗邻炎热和干旱土地的沿海地区，特别容易受到潮湿热浪的影响。即使在印度平原北部、东南亚和南美洲内陆等对流区域，较低的自由对流层干燥度也会强烈影响最大表面湿球温度。随着气候变暖，低自由对流层空气的干燥度（相对于饱和度）增加，这使得极端湿热的增加更大，进一步增加了湿热浪的可能性。