| In this study,the long-term trend of water vapor over the Tibetan Plateau(TP)in boreal summer is investigated by using observation and reanalysis data from 1979 to 2020.The historical experiment simulations of 19 models whose scenarios and variables are complete that participated in the Coupled Model Intercomparison Project phase6(CMIP6)are evaluated,and the future variation tendency under four emission scenarios is projected.The interdecadal variability of the net water vapor budget over the TP in boreal summer and the differences in circulation and water vapor transport over the TP and its surrounding areas before and after the interdecadal transition are analyzed.The SST(surface sea temperature)data from the Hadley Centre are used to analyze the mechanism that interdecadal variability of SST in the Indian Ocean affect the interdecadal water vapor anomalies over the TP.The results indicate that:(1)In recent 42 years,the water vapor content and the net water vapor budget over the TP show notable increasing trends,which are mainly manifested by a significant increase in the net water vapor import and a significant decrease in the water vapor export on the eastern boundary of the TP.This is mainly due to an anomalous anticyclone from Lake Baikal to the Mongolian Plateau.There is a high correlation between net water vapor income and precipitation over the TP in boreal summer.The CMIP6 multi-model ensemble can well simulate the variation characteristics of the TP net water vapor budget.The projection results indicate that by the end of the twenty-first century,the water vapor content,the net water vapor import and precipitation over the TP will increase.Under a high-emissions scenario and compared with the current period(1991–2014),these three variables will increase by 47.99%,59.77% and 18.59% in the long term(2081–2100),respectively.The significant enhancement of zonal water vapor transport over the western TP may be the main reason for the increase in humidity over the TP.(2)There is a significant interdecadal cycle of 11 years and a significant interdecadal transition around 1996 for both the net water vapor budget and the regional average water vapour content over the TP in boreal summer,and the timing of the interdecadal transition for precipitation over the TP and the regional mean SST in the critical zone of Indian Ocean in boreal summer is generally consistent with the interdecadal transition for the net water vapor budget over the TP.After the interdecadal transition,the overall humidification,external water vapor import over the TP and the precipitation over the TP except the north-eastern region increase,which indicate a close correlation among the three.In addition,the SST in the critical zone of Indian Ocean also increases and its interdecadal component is highly correlated with the interdecadal component of the net water vapor import over the TP.(3)Increased shortwave radiation absorption due to the significantly reduced clouds is likely to be the main cause of the SST increase in the critical zone of Indian Ocean.Enhanced evaporation over the critical zone of Indian Ocean combined with anomalously enhanced lowlevel dispersion results in enhanced southeasterly winds over the northwestern of the critical area,enhanced southwesterly winds over the northeastern of the critical area,and increased humidity and precipitation over the equatorial Indian Ocean,Indonesia and western New Guinea.Enhanced convection and trans-equatorial flow over these areas further transport water vapor to the TP via the Indian peninsula and western Pacific.In the upper levels,the Indian peninsula and the western Pacific are centers of dispersion,so that the upper and lower levels form a closed anomalous circulation loop.The interdecadal SST anomaly in the critical zone of Indian Ocean affects the interdecadal variability of net water vapor budget over the TP mainly by causing anomalous meridional circulation,resulting in increased water vapor income on the southern and eastern boundary of the TP. |
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