The influence of Asian summer monsoon synergy on the Meiyu onset date in the region of Yangtze-Huaihe basin in China

The time of Meiyu onset directly affects the amount of rainfall during Meiyu period, and its accurate prediction is of great significance to meteorological services such as agriculture, transportation and tourism. In this paper, the date of Meiyu onset by China Meteorological Administration in 2017 and NCEP / NCAR reanalysis data are used, the climatic characteristics of the beginning of the rainy season and the influence of Asian summer monsoon on the Meiyu onset are analyzed in the region of Yangtze-Huaihe basin from 1981—2020. The results show that: (1) The Meiyu onset date in the region of Yangtze-Huaihe basin has obvious interannual variation characteristics. The average time of Meiyu onset in the region of Yangtze-Huaihe basin is June 21, the standard deviation is 11 days, with a difference of 39 days between the earliest and latest Meiyu onset. (2) The Meiyu onset date is significantly negatively correlated with the South Asian summer monsoon (SASM) and positively correlated with the East Asian summer monsoon (EASM). In strong SASM years, the South Asian high is eastward, mid-high latitude upper jet stream is southward, and the region of Yangtze-Huaihe basin is water vapor convergence areas, which is conducive to the early start of Meiyu. In strong EASM years, the subtropical high in the Western Pacific is stronger northward, the southerly airflow is strong, the water vapor converged in South China and Northeast China, and divergent in the region of Yangtze-Huaihe basin, which is not conducive to the generation of Meiyu. (3) Since the Asian summer monsoon has the characteristics of co-occurrence, in the strong SASM-weak EASM year, the average Meiyu onset date is 4 days earlier, and in the opposite co-occurrence year, the Meiyu onset date is 11.6 days later. In strong SASM-weak EASM years, the Meiyu area is located on the right side of the upper jet exit, the Iran high is located to the east, and the upper air is affected by the eastward movement of the small trough. The low-level South Asian summer monsoon is developing vigorously, and the water vapor is converging in the Jianghuai region, which is conducive to the early Meiyu onset. In weak SASM-strong EASM years, the upper-level jet axis in the middle and high latitudes is northward, the the region of Yangtze-Huaihe basin is controlled by the northerly airflow in front of the ridge, the low-level Asian summer monsoon convergence zone is eastward, and the region of Yangtze-Huaihe basin is only a water vapor channel, which is not conducive to the Meiyu in the region of Yangtze-Huaihe basin. Therefore, the strength and coordinated development of the Asian summer monsoon have certain guiding significance for the early and late Meiyu onset of the Yangtze-Huaihe basin.