Possible causes for the more precipitation over North China in autumn 2021

In the autumn of 2021, the North China experienced the highest historical precipitation of the same period since 1961. Using daily precipitation data from national meteorological observation stations, ERA5 reanalysis data, and NOAA SST data, the characteristics and impact factors of more precipitation were analyzed. The results show that the North China region experienced a total of 5 heavy rainfall processes in the autumn of 2021, with the total precipitation being the highest in history since 1961, reaching 284 mm, which is 169.2% higher than the average climate (105.5 mm). In the autumn of 2021, the largest source of water vapor in the North China was in the eastern seas of China, with a contribution of 41% to water vapor transport, far exceeding the climate average of 21%. The eastward airflow guided by the abnormally strong Northeast Asian high pressure in the autumn of 2021 resulted in significantly stronger water vapor transport from the East China Sea, Yellow Sea, and Northwest Pacific. The circulation over the Eurasian high latitude region presented a pattern of one trough and one ridge, while the North China region was located behind the low trough, with active cold air from the north continuously affecting North China. The weak La Niña event caused abnormally low ground pressure in North China, and the winter monsoon circulation system was not yet established. On the contrary, the summer monsoon circulation system remained stable, leading to abnormal southeast winds bringing more water vapor transport and ultimately resulting in abnormally high autumn precipitation in North China.