Spatial-Temporal Variations Of The Upper Tropospheric And Lower Stratospheric Water Vapor Over The Tibetan Plateau And Its Surroundings

In this study,we combine the satellite datasets with the reanalysis datasets to analyze the variations of the long-term trend of water vapor in the upper troposphere and lower stratosphere(UTLS)over the Tibetan Plateau(TP)in summer,and reveal the underlying mechanisms.Futher,based on the Empirical Oorthogonal Function(EOF)analysis,we discuss the spatial and temporal distribution characteristics of the upper tropospheric water vapor(UTWV)over the TP in summer,and the potential physical mechanisms of the spatiotemporal variations of the UTWV are also explored.Finally,the characteristics of the UTWV in the west of the TP are analyzed,and possible physical mechanisms are discussed.The main conclusions are as follows:(1)First,using water vapor data from Microwave Limb Sounder(MLS)as a reference,we compare the applicability of water vapor data from ERA-Interim and ERA5 reanalysis in the UTLS over??the TP in summer.It is found that ERA5 reanalysis can reasonably reflect the distribution of water vapor in the UTLS over the TP.The results based on ERA5 reanalysis show that the lower stratospheric water vapor over the TP shows a significant negative trend from 1979to 2001.This is due to the stronger dehydration caused by the lower tropopause temperature over the TP and the enhancement of the residual circulation,which leads to a decrease in water vapor entering the lower stratosphere.During 2002-2016,the lower stratospheric water vapor shows an obviously positive trend,which is caused by the increase in the tropopause temperature over the TP and the weakening of the residual circulation.However,the UTWV is a continuing positive trend over the TP from 1979 to 2016 in summer.The strengthening of the intensity of the South Asian High and the South Asian summer monsoon is the main reason for the positive trend of UTWV over the TP.(2)Based on the EOF analysis,the spatial and temporal characteristics of the UTWV over the TP during 1979-2018 in summer are further analyzed.The results show that the distribution of the UTWV over the TP is mainly distributed as"regional uniform","east-west dipole"and"north-south dipole".The cumulative variance of the first three modes is 79.1%,which can well represent the spatial distribution of UTWV over the TP.The center of the high values of the first eigenvector locates the southwestern of the TP and shows a positive trend.This is mainly related to the strengthening of convective activities,the South Asian summer monsoon and the South Asian high as well as the Indian low pressure.These factors contribute 67.9%to the variations of the first eigenvector of UTWV.The second eigenvector reflects that UTWV has a slightly positive trend in the east of the TP and a slightly negative trend in the west.This is mainly related to the weakening(intensification)of the westerly wind in the eastern(western)of the TP,the strengthening of the Tibetan Plateau summer monsoon and the eastward of the South Asian High.These three factors contribute 66%to the variations of the second eigenvector of the UTWV.The third eigenvector shows that there is a significant positive trend of UTWV in the northern part of the TP and a negative trend in the southern part.The positive trend in the north TP is mainly due to the abnormal anticyclonic circulation over the northern TP,the decrease of static stability and the tropopause folds located at the northwestern TP,which causes the upward movement over the northern TP and further leads to the lower-layer water vapor transported to the upper-layer.This results in the increase in UTWV over the northern TP.These three factors contribute 64%to the variations of the third eigenvector of the UTWV.(3)Finally,we use water vapor data from AIRS and MLS observation combining with ERA5reanalysis over the same period,to analyze the characteristics of UTWV in the west of the TP and to further analyze its possible mechanisms.The results show that there is an obvious low-value area of water vapor in the 30°N-40°N range of the upper troposphere over the west of the TP.The low-value area develops in May,gradually expands westward to around 30°E in June and July,then contracts in August and contracts eastward to the northwestern TP in September.From May to September,high frequencies of shallow tropopause folds are located at the northwestern part of the TP and around 30°E,which is consistent with the distribution of low water vapor centers located the west of the TP at the upper troposphere.When the tropopause fold occurs,it can cause the tropopause to drop and the low water vapor,high O₃and high PV air in the stratosphere can invade into the troposphere.Therefore,the tropopause fold is one of the mechanisms for the low-value area of water vapor in the west of the TP.