The Spatiotemporal Patterns of the Upper-Tropospheric Water Vapor over the Tibetan Plateau in Summer Based on EOF Analysis

Using empirical orthogonal function (EOF) analysis, we investigate the spatial pattern and temporal variation of the upper-tropospheric water vapor (UTWV) over the Tibetan Plateau (TP) in summer based on the fifth-generation ECMWF atmospheric reanalysis (ERA5). The main factors affecting UTWV in different regions over the TP are also discussed. The results show that the spatial distribution of the UTWV over the TP mainly displays the so-called uniform, east-west dipole, and north-south dipole modes. The three modes show significant periods of 5, 8, and 4 years, respectively. The first mode exhibits a small but significant positive trend over the TP, which can be mainly attributed to the intensification of the South Asian summer monsoon and accounts for 31% of the UTWV variance, followed by the South Asian high and the Indian low with 17% and 16%, respectively. The second mode shows opposite variation of water vapor over the eastern and western TP, mainly resulting from the weakened westerly wind over the eastern TP and the enhanced westerly wind over the western TP, which accounts for 57% of the UTWV variance. The third mode exhibits a positive trend of water vapor over the northern TP, mainly attributed to the ascending motion associated with local anticyclonic circulation over the northeastern TP and the decrease of the static stability over the northern TP, which account for 45% and 11% of the UTWV variance, respectively. The tropopause folds also contribute to the positive trend of water vapor shown in the third mode.