Spatial-Temporal Characteristics and Causes of Summer Heat Waves in Southwest China

Southwestern China(SWC) has experienced more frequent heat waves in recent summers. In this study,we discussed the temporal and spatial variation characteristics of summer heat waves over Southwest China and its possible causes on the interannual time scale,based on the daily observational summer maximum temperature data at 98 stations and ERA-interim reanalysis dataset from 1979 to 2018. The results indicate that the spatial distribution of climatic frequency of summer heat waves(HWF) is generally characterized by less in northwestern and more in southeastern SWC. The first mode features HWF increases over most of SWC except northwestern Yunnan province,which can reflect the main distribution characteristic of the HWF,and the second mode exhibits less HWF in the north but more HWF in the south part. It is suggested that the increasing SWC summer heat waves are correlated with the obvious abnormal high pressure from the east of Tibet Plateau to the Korean Peninsula at middle troposphere. Under the control of this high pressure anomaly,the western Pacific subtropical high moved westward,which is conducive to the decrease of precipitation in the SWC,the decrease of cloud cover,and the increase of solar shortwave radiation reaching the surface. At the same time,the position of the East Asian Westerly Jet moves northward,which can prevent cold air from mid-high latitudes invading the southwest region,causing frequent droughts in the SWC. Finally,the unusually dry and hot surface conditions,combined with the favorable atmospheric circulation conditions,lead to the occurrence of summer heat waves over SWC.