INFLUENCES OF THERMAL FORCING OVER THE SLOPE/PLATFORM OF THE TIBETAN PLATEAU ON ASIAN SUMMER MONSOON: NUMERICAL STUDIES WITH THE WRF MODEL

Abstract

With the global warming, the evolution of Asian summer monsoon (ASM) becomes more complicated, while the thermal effect of the Tibetan Plateau (TP) is an important forcing to the variability of weather and climate in ASM region. Regional climate model (RCM) is a useful tool in the regional climate change research and has higher resolutions that can represent topography and land surface processes more accurately comparing with atmospheric general circulation models (AGCMs). In this study, the impact of thermal forcing over different terrains of TP on ASM (including South and East ASM (SASM and EASM)) is investigated using the Weather Research and Forecasting (WRF) model. Results indicate that the local circulation and precipitation around the Himalayas (HIM) are significantly influenced by the surface heating over the HIM's southern slope, which is a dominant factor for the formation of the north branch of SASM. Meanwhile, the climbing moist airflow and precipitation over the southern slope of TP are mainly induced by HIM's thermal forcing. Due to HIM's sloping heating, the upper-level troposphere warm center is steadily located over the HIM area and the EASM is also intensified obviously (Characterized both by the enhanced low-level southwesterly over East China and the enhanced southward anomalous dry-cold northerly). As to the surface heating over TP's platform region (PL), although its influence on the summer monsoon circulation and precipitation is weaker than the HIM's, it induces a wider response of SASM and a stronger influence on the meridional Hadley circulation. The PL's heating is able to regulate low-level southwesterly over the remote tropical ocean. Furthermore, a comparison of multiple monsoon indices reveals that both HIM sloping heating and PL heating can intensify SASM or EASM, but the impacts of thermal forcing over different terrains are distinct on the two subsystems of ASM.