Atmospheric conditions associated with cold East-Asian orography in climate models

East Asia orography, i.e. the elevated Tibetan and Mongolian Plateaux, is of fundamental importance for the mid-latitude climate over the Eastern part of the continent and the Pacific. During winter it supports and strengthens the Siberian (surface-pressure) High and the East-Asia Winter Monsoon, contributing to the advection of dry and cold air over the Asian coast and the Pacific Ocean. While the role of the mechanic forcing by Asian orography has been thoroughly addressed by scientific literature, here we identify the near-surface temperature over East-Asia high plateaux as an additional and important ingredient for setting the winter climate on the lee side of the orography : a cold surface-based heat source on the elevated Asian Plateaux amplifies the atmospheric response to orography (as observed in former modelling studies), causing an intensification of the East Asia winter monsoon and a strengthening of the southern flank of the Pacific jet. This finding is based on the analysis of a composite of climate-model climatologies characterised by cold East-Asian orography (from the Coupled Model Inter-comparison Project, Phase 6 - CMIP6), and is confirmed by running simulations forced by cold land-surface temperatures over the same orographic region, using an intermediate-complexity atmospheric model in perpetual-winter conditions. Our results, ranging over different degrees of model complexity, are in line with the outcomes of a highly idealised analysis of the interaction of the atmosphere with orography and superposed heat sources, in Ringler and Cook (1999). Moreover, the problem is particularly relevant in view of the well known cold bias in state-of-the art climate models - such as the CMIP6 ensemble - in proximity of the Tibetan Plateau.

T. D. Ringler and K. H. Cook. Understanding the seasonality of orographically forced stationary waves: Interaction between mechanical and thermal forcing. Journal of the atmospheric sciences, 56(9):1154–1174, 1999