On the atmospheric dynamical responses to land-use change in East Asian monsoon region

This study aims at (1) exploring dominant atmospheric dynamical processes which are responsible for climate model-simulated land-use impacts on Asian monsoon; and (2) assessing uncertainty in such model simulations due to their skills in simulating detailed monsoon circulations in the region. Firstly, results from a series of the Australian Bureau of Meteorology Research Centre (BMRC) global model simulations of land-use vegetation changes (LUC) in China are analysed. The model showed consistent signals of changes in atmospheric low-level vertical profile and regional circulations responding to LUC. In northern winter, the model-simulated rainfall reduction and surface cooling are associated with an enhanced southward penetration of dry and cold air mass, which impedes warm and humid air reaching the region for generating cold-front rainfall. In its summer, an enhanced cyclonic circulation responding to LUC further blocks the northeast penetration of southwestly summer monsoon flow into the region and results in rainfall decreases and a surface warming. Secondly, we have explored uncertainties in the proposed mechanism operating in the global model. By comparing its results with a set of high-resolution regional model simulations using the same vegetation datasets, it reveals similar changes in winter rainfall but opposite features in summer rainfall responses. In the global model, there is a cyclonic low-level circulation pattern over the South China Sea and adjacent region, an unsatisfactory feature commonly seen in other global climate models. With the reduction in surface roughness following LUC, such a deficiency becomes more prominent which further results in a weakened south/southwestly summer monsoon flow and rainfall reduction. In contrast, in the regional model, its southwestly summer monsoon flow is further enhanced due to the same process as reduced surface roughness. The enhanced monsoon flow further pushes the East Asian monsoon rainfall belt more northward and increases summer rainfall in the Yangtze River region. This study highlights the need for better monsoon simulations in climate models to produce reliable climate change projections in the region.