Impact of evaporation in Yangtze River Valley on heat stress in North China

In North China (NC), heat stress, which can be quantitatively characterized by wet-bulb globe temperature (WBGT), is closely related to specific humidity. This study focuses on the total days for NC averaged daily maximum WBGT exceeding 26°C (WGBT26) per summer (June to August) from 1979 to 2017. Rather than local precipitation or evaporation in NC, the NC WBGT26 is significantly related to the nonlocal evaporation around the Yangtze River Valley (YR). The abnormal positive evaporated water vapor in YR, associated with abnormal water vapor flux from south to north at 925 hPa, is continuously transported to NC in the above-normal WBGT26 years. Such an abnormal “evaporation and transportation” process can significantly increase the water vapor in NC and therefore enhance WBGT26. The evaporation in YR peaks in mid to late July and is closely associated with the occurrence days for daily maximum WBGT exceeding 26°C and maximum daily mean specific humidity at 925 hPa in summer. The main driver for the strongest YR evaporation anomaly from July 15^th to August 15^th is the simultaneous surface air temperature rather than the simultaneous or earlier soil moisture, precipitation, and vapor pressure deficit (VPD) anomaly. This condition is due to the continuously abundant soil moisture in the YR from April to September. The results of this study provide new ideas for studying heat stress in NC, indicating that nonlocal land-atmosphere interactions are crucial.