A Study On The Interannual And Interdecadal Variability Of The South China Sea Summer Monsoon Retreat And Its Physical Mechanism

The present study investigates and analyzes South China Sea summer monsoon withdrawal(SCSSMW)systematically based on multiple reanalysis data and observation data and combining with numerical experiments.Firstly,the climatological characteristics of SCSSMW are analyzed,and then the characteristics,influencing factors and physical mechanism of its interannual variability and interdecadal variability are studied.Main conclusions are as follows:(1)The climatological characteristics of SCSSMW are mainly as follows: The SCSSMW lasts for a long time,which is characterized by a gradual process from north to south,and the westerly wind changes to easterly wind in the lower troposphere,and the intensity of convective and precipitation decreases obviously.The continental surface cold high strengthens and invades southward,and westward development of the Western Pacific subtropical high causes the zonal wind in the lower troposphere over the South China Sea to shift from westerly to easterly,and the trans-equatorial flow weakens and the equatorial convergence zone moves southward.The subtropical westerly jet moves southward,and the seasonal South Asia High weakens and moves southeastward,and the prevailing easterly over the South China Sea weakens obviously in the upper troposphere.The vertical upward movement center of the atmosphere moves from the northern part to the southern part of South China Sea and forming an anomalous local meridional circulation.(2)The interannual variability of SCSSMW is particularly significant.Compared with the earlier SCSSMW,the zonal wind direction change time in the lower troposphere is about 3pentads later,and there are significant cyclonic circulation anomalies over the South China Sea and its adjacent regions,with more active convective activities and more precipitation in the later SCSSMW.The results show that the interannual variability of SCSSMW is significantly positively correlated with the meritonal displacement of the subtropical westerly jet.When the position of the subtropical westerly jet is north(south),the position of the western Pacific subtropical high is north(south),which is not(delete not)conducive to the southward movement of cold air,and the SCSSMW is later(earlier).There is a significant negative correlation with the Tibetan Plateau heat source anomaly.When the Tibetan Plateau heat source is negative(positive)anomaly,the cyclonic circulation anomaly over the South China Sea and its adjacent region could be strengthened(weakened)by causing the vertical circulation anomaly between them,and the SCSSMW is later(earlier).(3)The time of SCSSMW also has obvious interdecadal variability,and the most significant interdecadal turning point is the mid-2000 s,with from earlier retreat to later retreat.There is a significant positive correlation between AMO with the interdecadal variability of SCSSMW.The warming SST over the North Atlantic(i.e.,AMO in the positive phase)releases more heat flux from the ocean to the atmosphere,leading to significant increase of convective activity in the troposphere over the North Atlantic,which can then stimulate the abnormal wave activity over the North Atlantic through sea-air interaction and enhanced convective activity.And this further affects the formation and propagation of mid-latitude Eurasian teleconnection wave train that is closely related to the variation of atmospheric circulation over the key area of Northeast Asia,causing the positive geopotential height anomaly and obvious descending motions in the whole troposphere over the key area of Northeast Asia,and strengthening the cyclonic circulation anomaly over the South China Sea through the meridional circulation,which makes the SCSSMW later.Roughly opposite mechanism works during the negative phase of AMO,making the SCSSMW earlier.