The Characteristic Of Low-level Cross-Equatorial Flow Over The South China Sea And Its Association With Precipitation Anomaly In China And Pacific Sea Surface Temperature Anomaly In Summer Half Year

According to the ERA-interim reanalyzed gridding data provided by European Centre for Medium-Range Weather Forecasts(ECMWF),the monthly mean precipitation data of 160 stations in China by the National Climate Center and the sea surface turbulent heat flux data by the Woods Hole Oceanographic Institution,we analyze the temporal and spatial variations of the Cross-Equatorial Flow(CEF)over the South China Sea(SCS)in the summer half year.On this basis,its association with precipitation anomaly in China in summer half year is then studied.Furthermore,we also discuss the relationship of the SCS low-level CEF(SCEF)and eastern tropical Pacific Sea Surface Temperature Anomaly(SSTA).The results show that:(1)Summer monthly SCEF has a significant inter-annual variation.The core large value areas,periodic changes,mean values and mean square error of SCEF changes in different months vary.During the midsummer(July and August),the SCEF intensity is stronger than that in the early summer(May and June),and the inter-annual variation is even more significant.However,the inter-decadal variations of SCEF intensity are not significant in different months.(2)There is an apparent difference between the SCEF intensity index and the precipitation anomaly in China in summer half year.The SCEF intensity changes in early summer are negatively correlated with the precipitation anomaly in most part of East China and Central China.However,the intensity of SCEF in the midsummer is significantly positively correlated with the precipitation anomaly in South China,and is significantly negatively correlated with the precipitation anomaly in central China.In midsummer,the relationship between SCEF and the precipitation anomaly in China is more closely related,and the SCEF has a significant positive(negative)correlation with the SCS summer monsoon index(East Asian subtropical summer monsoon index).(3)It was shown that the index of the intensity of the SCEF has a significant positive(negative)correlation with the SCS summer monsoon index(East Asian subtropical summer monsoon index).When the SCEF in midsummer is stronger(weaker),the intensity of Asia low pressure and Inter-tropical Convergence Zone(ITCZ)in the west Pacific increases(decreases),the west Pacific subtropical high becomes weaker(stronger)and moves eastward(westward),cyclonic(anti-cyclonic)circulation anomalies emerge in the northern area of SCS and Northwest Pacific,resulting in the strengthening(weakening)of the SCS summer monsoon and the weakening(strengthening)of the East Asian subtropical summer monsoon.Moreover,when the SCEF is stronger,there is northeast(southerly)wind anomaly in the South China(North China)induced by the weakening of the East Asian subtropical summer monsoon.As a result,water vapor anomaly is converged(diverged)in the South China(Central China),leading to more(less)precipitation in the South China(Central China);and vice versa.(4)The SSTA in the eastern and central tropical Pacific has important influence on the change of SCEF in the summer,and there is a significantly positive correlation between them.When the SSTA of the eastern and central tropical Pacific gets wanner in the summer,the Walker circulation anomaly weakens,and anomalous subsidence occurs in the western tropical Pacific.Sea level pressure in the western tropical Pacific and northern Australia rise abnormally.At the same time,the East Asian Hadley circulation anomaly weakens,the anomalous sea-level pressure gets lower in the northwest Pacific,which is conducive to generating a north-south pressure gradient and then guides SCEF enhancement,and vice versa.