Temporal variability of evapotranspiration and its response to westerly and monsoon circulation over the Tibetan Plateau

Westerlies and monsoons converge over the Tibetan Plateau, yet their influence on evapotranspiration in the area is largely unknown. We analyzed the variability of evapotranspiration (ET) over the Tibetan Plateau and its response to the large-scale westerly and monsoon circulation. The results show that the annual mean ET is 414.2 ± 18.32 mm over the Tibetan Plateau. The annual ET has large interannual variability and exhibits trends that differ significantly among sub-regions (southwest, southeast, north regions). The Webster-Yang index (WYI) (Westerly index, WI) showed a fluctuating decreasing (increasing) trend with an interannual variability of 22.1% (6.4%) during the study period. The ensemble empirical mode decomposition analysis (EEMD) demonstrates that the WI dominates the 5-year scale variation of ET in the southeast, the WYI dominates the 5-year scale variation of ET in the southwest and the interannual scale variation of ET in the north, and the interaction between WYI and WI dominates the inter-decadal scale variation of ET in the southwest and southeast. WYI and WI regulate ET by influencing climatic environmental conditions. In the southeast region, the enhanced WYI significantly increases the temperature, which induces the depletion of soil moisture and thus reduces ET. These processes play a dominant role in regulating ET, while the temperature increase, improving vegetation greenness and consequently accelerating ET, plays a secondary role. In the southwest, the increase in WYI increases temperature and decreases the vapor pressure difference (VPD), as well as the rising temperature increases soil moisture and consequently ET. These processes dominate in WYI regulating ET, while the VPD decrease, reducing ET, plays a secondary role. In the northern region, the enhanced WYI decreases VPD, and thus reduces ET.