Surface warming in summer over the Tibetan Plateau: Local and atmospheric circulation processes1

Abstract

Both ground surface temperature (Ts) and near-surface air temperature (T2m) in summer over the Tibetan Plateau (TP) are not only a critical control of the cryosphere, but also an important indicator of atmospheric heat sources, which have a significant impact on regional water resources and broader climate systems. However, their characteristics and mechanisms have not been systematically studied. Based on gridded observations and ERA5 reanalysis we use diagnostic methods such as surface energy budget, longwave radiation decomposition, and water vapor budget, to analyze the characteristics and mechanisms of summer Ts and T2m changes over the monsoon and westerly regions of the TP during 1961–2022, as well as the possible impact of atmospheric circulation on the changes. Summer Ts (T2m) over the TP has increased at a rate of 0.20 (0.29) °C/decade, with rates of 0.21 (0.31) and 0.19 (0.28) °C/decade in the monsoon and westerly regions, respectively. For summer Ts warming, the increase in downward longwave radiation caused by an increase in water vapor is the primary contributor, and snow-albedo feedback is the secondary contributor. The former accounts for 58.3 %/62.3 % of the warming in the monsoon/westerly regions, and the corresponding proportions for the latter are 31.3 %/15.1 %, respectively. These surface-based local processes also contribute to T2m warming through the regulation of surface turbulent fluxes. Besides, atmospheric circulation changes (mainly manifested as the weakening of westerly winds) have not only increased water vapor, but also enhanced anomalous warm advection near the surface, resulting in an extra T2m warming over the TP. This study shows that local feedback and atmospheric circulation are controlling the summer warming over the TP, in both monsoon and westerly regions.