Long‐term variations of clouds and precipitation on the Tibetan Plateau and its subregions, and the associated mechanisms

The changes in clouds and precipitation are essential to understand the variation of water cycle under a changing climate and have not been well investigated on the Tibetan Plateau (TP). In this study, the long‐term variations and the associated mechanisms of total cloud amount (TC), low cloud amount (LC), the number of precipitation days (PN) and precipitation amount (PA), as well as the number of snow cover days (SN) and mean snow depth (SD) on the TP and its subregions of northeast (NE), central south (SC) and southeast (SE) are investigated with observational data in 1979–2016. The results indicate that TC has an overall decreasing trend, while LC has a significant increasing trend on the whole TP. PA in the NE has a significant increasing trend, and that in the SE has an insignificant decreasing trend. Light precipitation events tend to decrease and heavy precipitation events tend to increase. SN and SD have significant interannual decreasing trends. It is found that the atmospheric thermodynamic condition on the TP tends to become stable under a warming climate. The northward shifting of the westerly winds in association with the Hadley cell expanding in cold seasons and the strengthening South Asian high and the South Asian monsoon in warm seasons should be directly responsible for the generally decreasing trends in TC and related precipitation events on the south TP, and the increasing trends in LC and associated warm precipitation on the whole TP. In cold seasons, the significant expanding of the Hadley cell causes the westerlies and the subtropical high to shift toward the south TP, and significantly reduce the middle and high clouds, and snow events. In warm seasons, the strengthening and northward shifting anticyclonic system significantly reduces the convective clouds and precipitation on the central TP. On the other hand, the strengthening South Asian monsoon and enhanced easterly winds in the south flank of the strengthening anticyclone may bring more water vapour from the Indian Ocean and the Pacific Ocean into the low levels of the TP and enhance low clouds and precipitation. Schematic of the links of the trends in clouds and precipitation to the anomalous atmospheric thermodynamics, dynamics, synoptic circulations and water vapour on the TP.