The cross-equatorial flow(CEF) plays an important role in the momentum, mass and water vapor exchange between the northern and southern hemispheres. It is one of important factors that reflect and affect the weather and climate anomalies in the northern and southern hemispheres, and especially has close connections with Asian monsoon, summer precipitation and typhoons. In recent years, the extensive research on the CEF has been conducted by domestic and foreign researchers. Previous studies have focused on the impact of CEF over the South China Sea(SCS) on Asian summer monsoon, especially on East Asian summer monsoon and South China Sea summer monsoon. However, there are few studies on the characteristics of SCS low-level CEF(SCEF) and the connection between SCEF variation and precipitation anomalies in the Asian-Australian monsoon region.
Therefore, according to the latest released Interim reanalysis data by ECMWF and the precipitation rate data by GPCP during 1979-2014, this paper analyzes the spatial structure and temporal variation of summer SCEF by using the correlation analysis and synthesis analysis methods. It discusses the relationship between SCEF and precipitation anomalies in the Asian-Australian monsoon region, and further presents the possible physical process of influence of SCEF variation on precipitation anomalies in the Asian-Australian monsoon region by analyzing characteristics of atmospheric circulation, water vapor transport and local Hadley circulation. Results show that the intensity of SCEF has a significant interannual variation in summer, with a 3-4 yr period. The intensity of SCEF in summer shows significant negative(positive) correlations with precipitation anomalies in the tropical eastern Indian Ocean and the maritime continent(the tropical western Pacific). In addition, the intensity of SCEF has a clearly negative correlation with precipitation anomalies in central China in summer. When the intensity of SCEF is stronger in summer, there are westerly wind anomalies from the Arabia Sea to the tropical western Pacific and easterly wind anomalies in the tropical eastern India Ocean at 850 hPa, leading to convergence(divergence) of water vapor in the tropical western Pacific(the tropical eastern India Ocean and the maritime continent). As a result, there are more(less) precipitation in the tropical western Pacific(the tropical eastern India Ocean and the maritime continent), and vice versa. When the intensity of SCEF is stronger(weaker) in summer, there is a cyclone(anticyclone) wind anomaly field in the southeast coast of China, which is unfavourable to(favourable to) water vapor transport from the tropical ocean to the central part of China, leading to less(more) precipitation in central China. When the intensity of SCEF is stronger in summer, the local Hadley circulation in East Asia is abnormally weaker. The anomalous SCEF converges with the cold air from the northern hemisphere around 20°N, and rises. There is an anomalous subsidence near the equator and 30-40°N area, making the precipitation in the tropical maritime continent and central China decrease;and vice versa.