Characteristics and Causes for Summer Climate Changes in Southwest China under the New Climate State

Using meteorological data from 393 observation stations in Yunnan, Guizhou, Sichuan, and Chongqing in Southwest China spanning from 1981 to 2020 in summer, alongside NCEP/NCAR reanalysis data in the same period, this study examines the distinct characteristics and variances in the climatological values of summer temperatures and precipitation in the Southwest China under different climatic states. Additionally, it delves into the analysis of causes for these variations from the perspective of atmospheric circulation. The results show that the summer temperature and precipitation values in Southwest China vary under the old and new climatic conditions. In addition, the summer precipitation in Southwest China is slightly reduced under the new climate state than that of the old one, and the temperature displays the characteristics of consistent warming. A decrease in precipitation in the eastern part of the Sichuan basin and an increase in summer precipitation in northern Yunnan, western Sichuan, and Guizhou are related to the west–northward shift of subtropical high pressure in the western Pacific Ocean. Meanwhile, the relative inactivity of high blocking systems at the middle and high latitudes and the weakening of the Baikal Trough play a positive role in the warm and dry changes experienced in most of the southwest region. Under the new climate regime, Guizhou shows weak warm–wet changes, Yunnan shows strong warm–dry changes, and Sichuan and Chongqing show strong warm–dry changes in summer. These changes are closely related to variations in water vapor level in the western Pacific and Indian Ocean regions.