Recent Changes in U.S. Regional Heat Wave Characteristics in Observations and Reanalyses

Humidity is a key determinant of heat wave impacts, but studies investigating changes in extreme heat events have not differentiated between events characterized by high temperatures and those characterized by simultaneously elevated temperature and humidity. The authors present a framework, using air temperature ( T ) and equivalent temperature ( T E ; a measure combining temperature and specific humidity), to examine changes in local percentile-based extreme heat events characterized by high temperature ( T only) and those with high temperature and humidity ( T -and- T E events). Application to one observational dataset (PRISM), four reanalysis products (1981–2015), and seven U.S. regions reveals widespread changes in heat wave characteristics over the 35-yr period. Agreement among the datasets employed on several heat wave metrics suggests that many of the findings are robust. With the exception of the northern plains region, all regions experienced increases in both T -only and T -and- T E heat wave day (HWD) frequency in each of the reanalyses. In the northern plains, all datasets have negative trends in T -only HWD frequency and positive trends in T -and- T E HWD frequency. Trends in HWD frequency were generally accompanied by changes in the spatial footprint in heat wave conditions. Temperature has increased significantly during T -only HWDs in the western regions, while increases in T E during T -and- T E HWDs have occurred in the central United States and Northeast region. These findings suggest that equivalent temperature provides an alternative perspective on the evolution of regional heat wave climatology. Studies considering changes in regional heat wave impacts should carefully consider the role of atmospheric moisture.