Spatiotemporal Patterns and Synoptics of Extreme Wet‐Bulb Temperature in the Contiguous United States

Abstract

Extremes of wet‐bulb temperature (WBT)—jointly reflecting temperature and specific humidity—have seen relatively little study in terms of climatology, despite their demonstrated relevance for health and economic impacts. In this study, we uncover and characterize distinct spatiotemporal patterns of WBT extremes in the contiguous United States for the 1981–2015 period, focusing on identifying and making a first pass at understanding regional differences. We find that anomalies of specific humidity are of greater importance than those of temperature in controlling extreme WBT in most of the contiguous U.S., particularly for southern and arid regions. Composites of extreme‐WBT days for each region reveal coherent sea‐surface temperature anomalies and midlevel and upper ‐level geopotential‐height anomalies that differ considerably between regions, particularly in terms of the resulting low‐level temperature and moisture fields. These findings suggest that the primary factors controlling the timing and intensity of WBT extremes, while ultimately forced by synoptic‐scale weather patterns, vary spatially according to both local geography and baseline climate. We demonstrate this conclusion by showing how regional features such as late‐summer WBT extremes in the Southwest and southern Great Plains derive primarily from spatial and temporal variations in moist low‐level flows.

Nonlinear increases in extreme temperatures paradoxically dampen increases in extreme humid-heat

“…In addition, relative EF change across the T W distribution is generally more positive on days above the 50th T W percentile (figures 3(b); S2(b)). Together, these results suggest that the highest T x days will become relatively drier while the highest T W days will become relatively wetter, and highlight the fact that the hottest days often are not the same as those with the highest T W values [30].…”

“…Recent research has shown that anomalously high specific humidity, rather than temperature, is often the dominant driver of present-day extreme humidheat events [30], while a dry land surface often accompanies the extreme temperature events projected in climate models [31,32]. Because T W is nonlinearly dependent on both temperature and humidity, it is not evident how the competing effects of temperature (and its associated surface drying) will combine with specific humidity to alter future risks of extreme humid-heat.…”

How Important Is Humidity in Heat Stress?

“…Two important questions are what kind of weather patterns lead to dangerous levels of heat stress and how such weather patterns might be affected by global warming. Recently, Raymond et al () examined the first of these questions for the continental United States. Their main finding was that heat stress extremes more often coincide with extreme values of humidity rather than temperature—in other words, that humidity variations are more important than those of temperature in creating extreme heat stress.…”

“…A measure of heat stress being used more frequently to index heat stress (e.g., Raymond et al, ) is the wet‐bulb temperature ( T W ). A number of heat stress metrics have been devised that account for the relevant environmental factors according to different models of a human and different situations (e.g., working outdoors vs. sitting indoors).…”

Increased frequency of and population exposure to extreme heat index days in the United States during the 21st century

“…Groups such as children (Xu et al 2012), the elderly (Anderson and Bell 2011), and individuals with low socioeconomic status (Harlan et al 2013, Schmeltz et al 2015 are particularly susceptible to heat-related illness. Recent studies have highlighted the importance of humidity in the occurrence of days with extreme heat stress (Raymond et al 2017), but most projections of extreme heat in the US have relied heavily on temperature-based projections (e.g. Peterson et al 2013a, 2013b, Habeeb et al 2015, Vose et al 2017.…”