Relative Roles of Large-Scale Orography and Land Surface Processes in the Global Hydroclimate. Part I: Impacts on Monsoon Systems and the Tropics

Abstract

Abstract A series of numerical simulations by an atmospheric general circulation model (AGCM) was conducted to evaluate the relative roles of Tibetan Plateau (TP) and continental-scale land surface processes on the Asian monsoon and hydroclimates in other regions in the Tropics. Four boundary conditions were used to define experiments: a flat and bare-rock (nonvegetated) surface (NMR), a realistic TP with bare-rock surface (MR), MR conditions but with a water-holding soil layer (with 20 cm of field capacity) (MS), and TP with a vegetated surface with specified albedo and roughness of the current vegetation and the soil layer (MVS). Systematic increases of precipitation (P) over the Asian monsoon region and other land areas in the Tropics were noticed from NMR, MR, MS, to MVS runs. Precipitation over the warm tropical western Pacific, in contrast, decreased from NMR to MVS runs although the same SST climatology was prescribed in all the experiments. The effect of TP orography produces wet (dry) climate regions to the west (east) of TP as shown in other GCM studies. In the Asian monsoon region both land surface effect (LSE) and TP effect (TE) contribute nearly equally to increase P, but in the West African monsoon region, TE contributes to decrease P while LSE plays a major role to increase P. LSE also enhances the subtropical anticyclone over the North Pacific through enhanced land–ocean thermal contrast. In most of the monsoon regions and Tropics, the albedo (and roughness) effect of vegetation substantially increases available radiation, which in turn increases both evaporation (E) and atmospheric moisture convergence (C), and, as a result, significantly increases P. This positive feedback between E and C is likely to be a characteristic nature when and where vegetation albedo effect is significant under sufficient radiative energy, suggesting a prerequisite for Charney’s original hypothesis.