Convective Entrainment Effect Intensifies Surface Extreme Moist Heat Waves

Moist heat waves in the tropics pose high risks to a large population. It has been hypothesized that the onset of deep convection and precipitation arising from hot and humid near-surface air limits the magnitude of moist heat waves. Using daily data from the ERA5 reanalysis and CMIP6 climate model outputs, we instead find a critical role for the lower free troposphere (LFT) via a previously unrecognized mechanism: the entrainment of subsaturated air in the LFT severely limits the onset of deep convection, allowing for the occurrence of extreme moist heat waves. The LFT entrainment effect not only indicates an enhancement of the convective limit for extreme moist heat in a regime with active convection and precipitation, but importantly, it reveals a different dry inhibition regime with subsiding motion that hosts a large number of high moist heat cases. We further investigate this entrainment effect using a set of non-entraining convection experiments in CAM5 where the entrainment rate is set to vary from zero to a standard value. Experiments with a higher entrainment rate yield more intense moist heat waves. As climate warms, LFT dry inhibition increases; this leads to a greater entrainment effect and allows for a higher exceedance of extreme moist heat compared to the non-entraining case