Comparison of Atmospheric Circulation Anomalies between Daytime and Nighttime Extreme High Temperature in North China

Many previous studies have shown that atmospheric circulation anomalies are usually the direct cause of extreme high temperatures (EHT). However, the atmospheric circulation anomalies associated with daytime and nighttime EHTs in North China and their differences are less discussed. The present study divides the summer EHTs in North China into independent daytime EHT (ID-EHT) and independent nighttime EHT (IN-EHT) according to the 90th percentile thresholds of the daily maximum and minimum temperature from CN05.1 and compares their atmospheric circulation anomalies. Composite results show that the sinking motion anomaly over North China and the southward displacement of the Western Pacific Subtropical High (WPSH) cause less low cloud cover and water vapor, which is conducive to absorbing more solar radiation at the surface, and leads to the daytime high temperature of ID-EHT. With the disappearance of solar radiation at night, the heat is rapidly dissipated, and the high temperature cannot be maintained. A wave train from high latitudes can affect ID-EHT weather. On the contrary, the upward motion anomaly over North China cooperates with the northward displacement of the WPSH, leading to more clouds and water vapor over North China. As a result, the absorption of solar radiation in North China during the daytime is reduced, and EHT has difficulty in forming during the day. The higher humidity causes slower heat loss from daytime to nighttime, resulting in an IN-EHT. IN-EHT is more likely to be affected by a wave train such as the Silk Road pattern from the midlatitudes.