Impacts of the North Atlantic Subtropical High on Interannual Variation of Summertime Heat Stress over the Conterminous United States

Heat index (HI) provides a proven indicator of heat stress concerns and discomfort for the general public. The index takes into account of the integrated effects of temperature and humidity on heat events, both factors regulated by large-scale circulation patterns. This study examines the impacts of the North Atlantic Subtropical High (NASH) on HI over the conterminous United States (CONUS). The analysis suggests that the HI is primarily controlled by surface air temperature over the CONUS; but is negatively correlated with relative humidity in the western and Central U.S. north of 40oN. In addition, winds contribute to the variation of HI in the Midwest and the southeastern U.S. By regulating these meteorological parameters, the movement of the NASH western ridge significantly impacts HI over the U.S., especially the Southeast. When the NASH western ridge is located northwest (NW) of its climatological mean position, abnormally high temperatures are observed due to fewer clouds and a precipitation deficit, leading to positive HI anomalies over the southeastern US. In contrast, when the western ridge is located in the southwest (SW), temperature decreases and HI anomaly becomes negative over the Southeast, even though relative humidity increases east of 100oW. NASH has weaker impact on the HI when it is far from the North American continent, especially during southeast (SE) ridge years. In the future, CMIP5 models project an increase in HI over the entire CONUS, while NASH-induced HI will be weakened during the NW, SE and NE ridge years but strengthened when its ridge moves to the SW quadrant. These results suggest that the future increase in heat stress is likely caused by the climatological warming and NASH intensification.

Duke Scholars

Author Wenhong Li Earth and Climate Sciences