Possible Causes of Persistently Extreme-Hot-Days-Related Circulation Anomalies in Yunnan from April to June 2019

A series of persistently extreme hot days, which broke the historical record for length, was experienced by Yunnan province from April to June 2019 and induced a severe local drought. Based on the daily in-situ surface air temperature in China, JRA-55 reanalysis datasets, and outputs from the Community Earth System Model Large Ensemble (CESM-LE) Project, we investigate the general circulation regime related to the hot weather in Yunnan and discuss the possible causes of these persistently hot days in 2019. Our results indicate that lower- and upper-level strong anticyclonic circulation anomalies directly result in the local hot weather, which induces adiabatic warming in a descending motion and enhances solar radiation heating at the surface. The anticyclonic anomaly is a node of high and mid-latitude eastward-propagating wave trains originating from the North Atlantic. The 2019 April–June mean temperature and associated anticyclonic anomalies were the highest recorded since 1961. The anthropogenic contribution to the magnitude of this 2019 temperature anomaly was approximately 37.51% and the occurrence probability of extreme-high-temperature events analogous to or higher than those of 2019 is 56.32%, with an important impact of internal variability. The negative phase of the Arctic Oscillation (AO) and warm phase of El Niño and the Southern Oscillation (ENSO) are important external forcing factors responsible for the persistent anticyclonic anomalies over Yunnan from April to June 2019. The negative phase of the AO causes a southward shift of the positive geopotential height anomalies over the Arctic at the longitudes of Eastern Europe, and favors a high-latitude wave train and an anticyclonic anomaly over Yunnan. The warm phase of ENSO enhances the western North Pacific anticyclone and favors its westward shift, which leads to the persistence of the anomalous anticyclonic circulation over Yunnan. Under the recent global warming conditions, the negative phase of the AO and the warm phase of ENSO jointly reinforce the intensity and duration of the anomalous anticyclonic circulation over Yunnan, which result in frequent and persistent occurrences of the record-breaking extremely high temperatures and the severe drought event in 2019.