The Response Of East Asian Summer Monsoon Atmospheric Moisture Transport To Global Warming And Its Mechanism

The strength,path,divergence and convergence of summer(June-July-August)atmospheric moisture transport over East Asia(EA)can directly affect the intensity and range of precipitation and play an important role in regional water balance.In this study,the variation characteristics of summer atmospheric moisture transport over EA and its response to global warming from 1979 to 2019 were investigated based on ERA-Interim reanalysis data.First,the atmospheric moisture transport were decomposed into thermodynamic component(TH)and dynamic component(MCD),which are induced by the change in specific humidity and wind velocity,respectively.Using multivariate empirical orthogonal function(MV-EOF)analyses,temporal and spatial variation of the TH and MCD were studied,we found that the first four MV-EOF modes describe 61.5%of the total TH variance.The time series of principal component of the second and fourth mode of TH both mainly contained strong interannual variations,which accounts for 14.6%and 8.1%of the total variance,respectively.Both the two modes show significant correlation coefficients with MCD.Interdecadal variation of EASM atmospheric moisture transports was mainly reflected in the first and third modes of TH,which are closely related to climate change and global warming.Therefore,the causes and formation mechanism of the first and third modes under the background of global warming were emphatically analyzed.The main conclusions are summarized as follows:(1)The time series of principal component of the first(TH_PC1)and third(TH_PC3)mode of TH both exhibited considerable interdecadal variation.The first mode of TH explained 27.4%of its total variance,TH_PC1 decreased from 1979 to 1992 and increased significantly(p<0.001)after that.The third mode of TH explained 11.4%of its total variance,TH PC3 showed a decreasing trend(p<0.01)from 1979 to 2000 and exhibited an increasing trend(p<0.001)afterward.(2)For the first mode of TH,southwesterly moisture transport is located along the East China Sea to southern Japan.The intensified moisture transport resulted in a strong moisture convergence there.TH_PC1 showed significant positive correlation coefficients with precipitable water over most EA regions.On the one hand,it was caused by the abnormally high local evaporation,but more was caused by the anomalous atmospheric moisture transport over the Pacific Ocean.The anomalous high SST and temperature in EA lead to the increase of atmospheric water holding capacity,which provides a thermal condition for the anomalous excess of water vapor over EA in summer.The anomalous SST in the Pacific and Indian Ocean promoted the atmospheric circulation to guide the transport of water vapor to EA,which provided a dynamic condition for the anomalous excess of water vapor and the first mode of TH.(3)The third mode of TH had a weak westward atmospheric moisture transport over Northeast Asia and a strong eastward atmospheric moisture transport over Indo-China Peninsula,which converged with the water vapor from the western Pacific Ocean in the area around South China and 30°N.The third mode corresponds to the negative anomaly of atmospheric precipitable water in the north and positive anomaly in the south of EA.The anomalous SST and temperature in EA are high in the south and low in the north,which provide thermal conditions for the anomalies of water vapor and precipitable water.MCD can transport water vapor from the southern Indian Ocean to the south of EA.The SST anomalies in the Pacific and Indian oceans lead to a weak power of water vapor transport northward over EA,and the mid-high latitude circulation also has a inhibiting effect on water vapor transport northward over EA.It is not conducive to the northward transport of water vapor,which provided a dynamic condition for the anomalous excess of water vapor and the third mode of TH.(4)The first mode of TH is mainly related to the Pacific SST anomaly.The negative SST anomaly over the tropical Middle East Pacific is close to the negative phase of IPO/PDO.The abnormal SLP gradient over the eastern and western Pacific drives atmospheric circulation,which leads to the strengthening of equatorial trade winds and the strong subtropical high over the western Pacific,which is conducive to the transport of water vapor from the Pacific Ocean to EA.The third mode of TH corresponds to the positive phase of IPO/PDO.The anomalous SST gradient in the Pacific Ocean weakens the equatorial trade winds and weakens the subtropical high in the western Pacific Ocean.At the same time,there are cyclonic circulations in Mongolia and Aleutan Islands,which are not conducive to the northward flow of water vapor.The tropical Indian Ocean has an anomalous SST distribution of high in the northwest and low in the southeast.Under the action of the feedback mechanism of WES,an anomalous cyclonic circulation is formed,which is conducive to the water vapor from the Indian Ocean to enter EA through Indonesia and Indo-China Peninsula,resulting in more water vapor in the southern part of EA.