Effect of marginal topography around the Tibetan Plateau on the evolution of central Asian arid climate: Yunnan-Guizhou and Mongolian Plateaux as examples

Mountains are believed to have played an important role in the evolution of modern arid climate over central Asia. The main topography of the Tibetan Plateau (TP) suppresses the regional atmospheric rainfall by both the modulation of atmospheric circulation and blocking of water vapor transport from the ocean. In this study, the effect of Yunnan-Guizhou and Mongolian Plateaux (YGP and MP, respectively), two marginal topographies around the main TP, on the central Asian aridity are evaluated using general circulation model experiments. The results show that the precipitation over central Asia is significantly decreased by these two topographies. Compared to the whole TP-induced annual precipitation decrease of 0.45 mm/d, the contributions of the YGP and MP reach 0.14 mm/d and 0.08 mm/d, respectively. These two marginal mountains occupy approximately one half of the total change by the TP although they are much smaller in heights and sizes. The orographic forcing of the TP suppresses the precipitation significantly throughout the year while those of YGP and MP are mainly effective in boreal winter. A moisture budget analysis shows that all the mountains examined drive increases in subsidence and resulting decreases in humidity over central Asia, with smaller or opposing roles for changes in horizontal winds. These subsidence changes dominate the drying of Central Asia due to the TP and MP, and are largely driven by the influences of the topography on stationary waves. In contrast, the YGP dries Central Asia primarily through altering transient eddies. The forcing of YGP and MP originate from the mechanical blocking of the tropical easterly and mid-latitude westerly, respectively, which exert significant changes in atmospheric circulation. This implies that the effect of small-scale mountains on arid climate over central Asia might be underestimated and a considerable proportion of mechanical effect of the TP on the Asian aridity actually comes from its margins.