The La Niña event was so related to China's climate that it had a significant impact on the high temperature and drought inChongqing. In recent years, the high temperature and drought in Chongqing had repeatedly reached new highs. Studying the characteris⁃tics of high temperature and drought in the region and its response to the La Niña event can improve the prediction and early warningcapabilities of high temperature and drought in Chongqing. Based on the MCI (meteorological drought composite index), high tempera⁃ture days, NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis of circu⁃lation field data and Hadley SST (sea surface temperature) data from 1981 to 2022, the temporal and spatial distribution characteristicsof meteorological drought stations and high temperature days during summer in Chongqing were analyzed. Furthermore, the impact ofLa Niña event on summer drought stations and high temperature days in Chongqing and its circulation anomaly characteristics were ex⁃plored. The results showed that summer was the season with the highest frequency and longest duration of drought in Chongqing.Drought and high temperatures were mainly distributed in the northeast and along the Yangtze River. High temperature and meteoro⁃logical drought promoted so each other that prolonged high temperature was conducive to the development of severe drought, while pro⁃longed drought was also conducive to the occurrence of extreme maximum temperatures. In the early winter and spring, the SST in theequatorial middle east Pacific was in La Niña state, which was conducive to the occurrence of high temperature and drought in Chongq⁃ing during summer. The weak La Niña event was conducive to the occurrence of summer high temperatures in Chongqing, while themoderate intensity La Niña event was conducive to the development of drought. Both the central and eastern types of La Niña eventwere conducive to the occurrence of summer high temperatures and drought in Chongqing. The reason was that, influenced by the LaNiña event, the south Asian high pressure was larger, stronger, and northward. Affected by this, the western Pacific subtropical highwas larger, stronger, and northward. Furthermore, high-altitude jets located in the northern region of China. These factors were impor⁃tant circulation reasons for high temperature and drought in Chongqing during summer. The middle and lower layers of the tropospherein Chongqing region were controlled by weak southwest winds. Combined with the two ridges and one trough pattern of the Eurasianmid high latitude circulation, forming a climate environment that was not conducive to the southward movement of cold air. As a result,Sichuan Basin, including Chongqing had been controlled by sinking airflow for a long time, which was the main reason for frequent hightemperatures and droughts in Chongqing.

Multi-time scale characteristics of ventilation quantity, mixed layer height, atmospheric stability and mean wind speed in mixed layer in Chongqing main city zone were quantitatively analyzed using the recent 30-year data observed at Shapingba meteorological station in Chonqing. The ventilation quantity in Chongqing main city zone exhibited evident decadal variations, and it lay in a high level and showed a descending trend before 2003, while it lay in a low level after 2003 and showed an ascending trend in recent 10 years. The decadal changes of ventilation quantity in four seasons were consistent with that of the annual mean. The ventilation quantity had an oscillation period from 2 to 3 years. In different seasons, the inter-annual oscillation intensity varied with the largest oscillation amplitude in summer and a relatively smaller one in autumn and winter. The ventilation quantity was significantly correlated with mixed layer height and mean wind speed in mixed layer. The seasonal difference of ventilation quantity was obvious with the largest value in summer and the smallest value in winter. The diurnal variation of ventilation quantity was basically consistent with mixing height and mean wind speed in mixed layer, which reached its maximum in the afternoon and could last until the evening.

 The temporal evolution characteristics of cold and warm variation in winter of Chongqing and the abnormal circulation in cold and warm winter years were analyzed by using monthly average temperature data in winter from 34 national weather stations in Chongqing, NCEP/NCAR reanalysis data, atmospheric circulation index and sea temperature data from 1961 to 2018. The results show that the average temperature in winter  in Chongqing had shown a warming trend in recent 58 years, which passed the significance test at 0.05,  and the average winter temperature in recent 58 years had increased  0.7 ℃, and the abrupt change of warming began in 1993. There were obvious differences between the circulation patterns in cold winter and warm winter years in Chongqing. When the tropical sea temperature field in summer and autumn was colder (warmer), the Siberian high pressure on sea level pressure field in winter was stronger (weaker) because of the atmosphere-ocean coupling effect. Meanwhile, the 500 hPa height anomalies was obviously higher (lower) in the north and lower (higher) in the south over the Eurasian continent, which made the Ural mountain high pressure and the east Asian trough stronger (weaker). At the same time, Western Pacific subtropical high and the plateau cold high were weaker (stronger) and Indo-Myanmar trough was stronger (weaker), which resulted in stronger (weaker) winter monsoon in East Asia, it was (was not) beneficial for cold air moving southward to affect Chongqing area. The stronger (weaker) Indo-Myanmar trough made the stronger (weaker) southerly wind in front of the trough, which was (wasn’t) conducive to warm and humid air transportation in front of the trough and precipitation in Chongqing area. This circulation configuration made Chongqing tend to be cold (warm) winter. The tropical Indian Ocean, the northwest Pacific Ocean and the equatorial central Pacific Ocean in the southern hemisphere were the key sea temperature zones affecting the cold and warm winter in Chongqing and had certain early indication for prediction of winter temperature  in Chongqing.