Controlling Factors of Global Heat-Humidity Extremes

Abstract:

Extreme simultaneous accumulations of heat and moisture in the planetary boundary layer lead to heat stress much more severe than is possible from extremes of either variable alone. The recent revealing of rapid observed trends, and of brief peak values approaching the human physiological survivability limit, incentivize the present study to determine critical geographic and meteorological factors, especially those that aid in explaining broad differences in the severity of heat-humidity extremes, such as between the tropics, subtropics, and mid-latitudes. Through an extensive observation- and reanalysis-based investigation focusing on moist static energy, we find that a steady supply of moisture (at high temperatures) and a strong capping mechanism in the low-mid troposphere (whether transient or semi-permanent) are the two essential factors, supplemented by other factors which vary regionally and seasonally. We use these results to characterize a mean 3-dimensional picture, and the variability about it, for known global hotspots, while highlighting those locations that are lacking in one or more necessary ingredients. These findings enable better physical understanding of the tight correlation observed between global-maximum moist static energy and global-maximum sea-surface temperatures, and of the geographic patterns of where increases are projected to be largest.