To reveal the spatio-temporal differences and atmospheric circulation causes of summer meteorological drought in the Sichuan-Chongqing region, based on daily observational data and meteorological drought composite index (MCI) of 188 meteorological stations in Sichuan and Chongqing during 1981-2023, the spatio-temporal evolution characteristics of summer drought in Sichuan and Chongqing are analyzed. In addition, monthly reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and climate system monitoring indices are employed to investigate the drought formation mechanisms from the perspectives of circulation patterns, water vapor transport, westerly jet streams, and vertical motion. The results show that droughts in the Sichuan-Chongqing region exhibit strong synchronicity and significant regional differences. Drought in Sichuan mainly concentrates in June and August, while that in Chongqing is more severe from July to August. The drought centers are primarily located in the Sichuan Basin, with a certain degree of synchrony between eastern Sichuan and Chongqing, whereas the drought in the western Sichuan Plateau is relatively weak. During typical drought years, the South Asian High moves northward and strengthens, the Western Pacific Subtropical High is abnormally located further north, water vapor transport at 700 hPa is weakened, and the region is controlled by subsidence motion, resulting in significantly less precipitation. Distinct atmospheric circulation anomalies in different types of drought years are the main cause of the spatial variability of drought in the region.

It is of great significance to study the relationship between summer atmospheric heat source over the Qinghai-Xizang Plateau and its surrounding area and the number of high temperature days in the Sichuan-Chongqing Basin for summer high temperature prediction and high temperature and drought disaster prevention. Based on the daily maximum temperature data of 125 meteorological stations in the Sichuan Chongqing Basin and monthly NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) reanalysis dataset, the characteristics of summer high temperature days in the Sichuan-Chongqing Basin and relationship between the inter-annual variation of high temperature days there and atmospheric heat source over the Qinghai-Xizang Plateau and its surrounding area are discussed. The results show that the first mode of EOF decomposition of high temperature days presented a consistent type in the whole region, which can reflect the main distribution characteristic of summer high temperature days. The summer high temperature days in the basin significantly increased from 1979 to 2022, showing obvious inter-annual and inter-decadal variation characteristics. There is a close relationship between the atmospheric heat source over the eastern part and its eastern side of Qinghai-Xizang Plateau and the inter-annual variation of summer high temperature days in the basin. When the heat source over the key region of the plateau is relatively weak (strong), the corresponding high temperature days in the basin are significantly more (less). On the inter-annual scale, when the atmospheric heat source over the eastern part and its eastern side of Qinghai-Xizang Plateau is weaker than normal, the position of South Asian high shifts northeastward, and the western Pacific subtropical high shifts northwestward, The water vapor transport from the South China Sea, western Pacific, and Bay of Bengal to the basin is significantly weakened, combined with significant subsidence anomalies over the basin, resulting in less precipitation and high temperature days there. At the same time, the total cloud cover in the basin is less than normal, and the short-wave solar radiation flux reaching the ground increases significantly, resulting in a rise in ground temperature and an increase in the number of high-temperature days. When the atmospheric heat source over the eastern part of the plateau and its eastern side is relatively strong, the circulation pattern is beneficial for reducing the number of high temperature days in the basin.

The typical fog and haze episode in the Sichuan basin from December 16, 2014 to January 27, 2015 was selected according to the method of determination of fog and haze days at a single station and the regional fog and haze process. Based on air quality index (AQI), mass concentration of pollutant, meteorological element characteristics and atmospheric circulation, the characteristics and mechanism of the generation, evolution and transformation of this continuous fog and haze weather were studied. The results are as follows: (1) The fog and  haze process was characterized by strong intensity, long duration and wide range. (2) Both mass concentration of pollutant and AQI were highly consistent with the fog and haze weather process, and the main pollutant in this fog and haze process was PM2.5, followed by PM10. (3) It can be seen from the process of formation, evolution and transition of a longlasting severe fog and haze episode that there were stages of pollutant accumulation with different intensities, reaching the peak value and rapidly weakening. Therefore, the intensity of fog and haze weather process was closely related to weather situation and vertical structure of boundary layer. The average temperature was 1.24 ℃ higher than the normal years, the rainfall was 34.77% lower than the same period, relative humidity was 2.67% lower than the same period, sunshine hours was 10.33 hours more than the same period, and wind speed was close to or slight higher than that of the normal years. The stable atmospheric circulation feature provided favorable environmental field for fog and hazy weather and heavy air pollution. Strong temperature inversion layer, sinking motion in the boundary layer and convergence in weak wind fields on the ground prevented water and pollutants from diffusing to the upper air.