Atmospheric response to wintertime Tibetan Plateau cold bias in climate models

Abstract. Central Asia orography sets important features of the winter climate over East Asia and the Pacific. By deflecting the mid-latitude jet polewards it contributes to the formation of the Siberian High and, on the lee side, to the advection of dry cold continental air over the East Asian coast and the Pacific Ocean, where atmospheric instability and cyclogenesis thrive. While the mechanical forcing by the orography is assessed by a number of modelling studies, it is still not clear how near-surface temperature over the two most prominent orographic barriers of the Central Asian continent, namely the Tibetan and Mongolian plateaux, influences the winter climate downstream. Moreover, a well known issue of state-of-the art climate models is a cold land temperature bias over the Tibetan Plateau related with the difficulty in modelling land processes and land–atmosphere interaction over complex orography. Here we take advantage of the large spread in representing near surface temperature over the Central Asia plateaux among climate models taking part in the Coupled Model Inter-comparison Project, Phase 6 (CMIP6) to study how temperatures over these regions impact the atmospheric circulation. Based on composites of the CMIP6 models' climatologies showing a cold bias over the Tibetan Plateau, we find that negative temperature anomalies over Asian orography intensify the East Asia winter monsoon and, by enhancing the low-level baroclinicity in the region of the East China Sea, reinforce the southern flank of the Pacific jet. The results of the CMIP6 composite analysis are supported by the response of an intermediate-complexity atmospheric model to a similar pattern of cold surface temperatures over the Central Asia plateaux; we also distinguish the relative influence of the Tibetan and the Mongolian Plateau surface conditions. Thereby, based on the intensification of the East Asia winter monsoon in models characterised by a cold land temperature (bias) over Central Asia plateaux, we prospect that advances in the modelling of the land energy budget over this region may improve the simulation of the mean climate over the Asia/Pacific sector, together with the reliability of climate projections and the performance of shorter term forecasts.