Variation Of The Summer Water Vapor Transport Of The Tibetan Plateau And Its Relationship With The Indo-pacific Warm Pool Process Based On Observational Data

The changes of water vapor transport and cycle process in the Tibet Plateau（TP）have an important impact on the water vapor cycle in the surrounding areas.From the perspective of water vapor source-transport channel-water vapor sink,the Indo-Pacific warm pool（IPWP）-atmosphere-Tibet Plateau（TP）together constitute the whole process of water vapor transportation,which is represented as a hemispheric-scale sea-air-land water vapor circulation system.However,the current understanding of the relationship between the water vapor transport in the TP and the air-sea interaction of the IPWP is still insufficient,meanwhile,based on the source and transport process of water vapor,there is still a lack of research on the mechanism of changes in the hydrothermal activity of the IPWP affecting the water vapor transport of the TP.Using multi-source reanalysis data,remote sensing and observation data,based on the refined boundary,this paper firstly studies the spatio-temporal variations of summer water vapor cycle on the TP and its inside basins.Then,the multi-scale spatiotemporal variation patterns of key hydrothermal parameters of the IPWP and their effects on the water vapor cycle of the IPWP are examined.Finally,the influence mechanism of the IPWPon the summer water vapor transport of the TP is revealed.The main conclusions include:1.Temporal and spatial characteristics of water vapor transport and cycle over the Tibet Plateau（TP）in summerWater vapor transport is an important external driving force of the water vapor cycle system in the TP.Accurate estimation of the TP’s water vapor budget is crucial for analyzing the temporal and spatial evolution of the water vapor cycle process.However,there is currently a lack of reliable research on water vapor transport at the TP and basins’scales and their spatiotemporal relationship with the water vapor cycle.Based on multi-source reanalysis data and refined plateau and basins’boundaries,the temporal and spatial evolution characteristics of water vapor transport and water vapor cycle over the plateau in summer were quantitatively analyzed.The results show that the ensemble mean of the climatic state of the net summer water vapor budget in the TP from 1979 to 2020 was 75.9×10⁶kg s^-1,while the previous results obtained by using a rectangular or simple boundary obviously overestimated by 50-100%.The ability of different reanalysis data to describe the water vapor transport is significantly different.The performance of CRA-40,ERA5 and JRA-55 in climatic state and interannual variability is relatively reliable,while other reanalysis data show the characteristics of overestimation or underestimation.From 1950 to 2020,the climatic state of the water vapor budget in summer was 86.2×10⁶kg s^-1,and the budget showed a weak increasing trend.On the interdecadal scale,the summer water vapor budget experienced two interdecadal transitions,the water vapor budget was larger during 1950-1968 and 1995-2020,smaller during 1969-1994.After combining the 20CR and CRA-40 reanalysis data,the time series of the TP’s summer water vapor budget from 1836 to 2020 was obtained,and the water vapor budget has quasi 4-6 years,and multi-decadal variability in 26,62 and 93 years.When the water vapor budget of the TP was strong in summer,the water vapor transport channel in the northeast direction of the Hengduan Mountains（Yajiang-Nujiang-Lancang-Jinsha Rivers）was strengthened.The water vapor transport and budget increased in the basins eastern of the monsoon line of the TP,while the results were the opposite in the basins western of the monsoon line.The elements of the water vapor cycle in the TP have strong consistency and correlation.The water vapor budget is significantly positively correlated with the total water vapor input and precipitation.The total water vapor input is the main factor driving the changes in the water vapor budget and precipitation.The climatic state of the summer precipitation recycling rate in the TP was 18.96%during 1950-2020,and the precipitation recycling rate is significantly positively correlated with the basin area,which has the characteristics of basin scale.2.Temporal and spatial variation characteristics of hydrothermal parameters of the Indo-Pacific Warm Pool（IPWP）Regarding the air-sea change of the IPWP,many previous studies have studied the temporal and spatial distribution and evolution characteristics of the sea sureface temperature,as well as the correlation and influence mechanism between the Pacific and Indian Ocean variability patterns.Focusing on key hydrothermal parameters,this paper analyzed the temporal-spatial variability and associated patterns of hydrothermal activity in the IPWP on interannual and interdecadal scales,as well as the impact mechanism on the warm pool water vapor cycle.The results showed that the warming rate of the SST in the IPWP during 1951-2020 was twice that of 1870-1950,and the SST has experienced four significant interdecadal transitions.Since 1950,the surface latent heat flux average index of the IPWP showed an interdecadal weakening trend during 1971-1993.Although the ERA5 reanalysis data overestimated the outgoing longwave radiation of the IPWP,the temporal and spatial evolution characteristics of ERA5 and remote sensing data were generally consistent,namely,there exhibited a positive east-west negative dipole mode between the equatorial central Pacific,oceanic continents,and the eastern Indian Ocean warm pool.The freshwater flux average and intensity index have consistent interannual and interdecadal variation characteristics.On the interannual scale,the average index of precipitable water in summer showed a weak negative correlation with the average index of surface latent heat flux,outgoing longwave radiation and freshwater flux,and a significant positive correlation with the water vapor budget.There is a significant positive correlation between SST,latent heat flux,outgoing longwave radiation,precipitable water intensity index and water vapor budget,while freshwater flux intensity index has no significant correlation with other indices.On the multi-decadal scale,the average index of the SST has the characteristics of multi-decadal variability of～30 years and～90 years,and the average indices of surface latent heat flux,precipitable water,freshwater flux,and outgoing longwave radiation showed oscillation periods of 35 to 42 years.For the eight-reanalysis data,NCEP2,JRA-55,NCEP1 and 20CR significantly overestimated the water vapor budget.In terms of climate state,the water vapor budget of the 29°C isotherm in the IPWP is the largest,and the budget of the 28°C isotherm is the least.The multi-year ensemble mean of summer water vapor budget within the line is 1266.9×10⁶kg s^-1,and the interannual variation rate is-12.8×10⁶kg s^-1（10a）^-1.3.Influence mechanism of the IPWP air-sea process on the summer water vapor transport over the TPAs the water vapor source for precipitation of the TP,changes in the air-sea conditions of the IPWP directly affect the water vapor transport channels and budget of the TP.However,previous studies have mostly focused on the effect of monsoon-westerly synergy on water vapor transport in the TP.Clarifying the evolution characteristics of the whole process of water vapor transport from IPWP to TP will help us to systematically reveal the external influence mechanism of water vapor transport in the TP.From the perspective of water vapor source-transport channel-water vapor sink,this study examined the direct driving mechanism of the changes in air-sea conditions of the IPWP affecting the summer water vapor transport over the TP on interannual and interdecadal scales.The results showed that the summer water vapor budget of TP was significantly positively correlated with the average and intensity indices of the surface latent heat flux and water vapor flux divergence in the IPWP during the same period.The four intensity indices（SST,surface latent heat flux,outgoing longwave radiation,and water vapor content）of the IPWP were synchronized with the time-lag relationship of the TP’s water vapor budget.The correlation between the TP’s summer water vapor budget and the IPWP’s indices showed a significant interdecadal strengthening around the 1990s.On the interannual and interdecadal scales,the possible mechanism by which the IPWP affects the water vapor transport over the TP can be summarized as follows:When the Indian Ocean warm pool is colder（the Pacific warm pool is warmer）,the atmosphere is vertically rising（sinking）,the ocean transport fresh water flux to the atmosphere（atmosphere to ocean）,and the precipitble water increases（decreases）,strengthening the water vapor transport in the water vapor source and the middle and upper reaches（inhibiting the downstream water vapor transport and reducing the output to the Indo-China Peninsula）,jointly drive the overall strengthening of the water vapor transport channel and the northward entry into the Bay of Bengal,and increase the water vapor entering the plateau from the southern border.At the same time,the anomalous combination of cyclones（anticyclones）and positive（negative）geopotential heights in the northwestern TP（Mongolia Plateau）make more water vapor enter the plateau from the south,west and north boundaries,which together lead to the enhanced water vapor transport,budget and precipitation of the TP.