Association between regional summer monsoon onset in South Asia and Tibetan Plateau thermal forcing

By using multiple data sources and numerical simulation results from the atmospheric general circulation model (AGCM) of CAM4.0 (Community Atmosphere Model, version 4), we investigated the effect of thermal forcing over the Tibetan Plateau (TP) on the onset of the summer monsoon in the Arabian Sea (AS) and India. Results indicate that the spatial distribution of diabatic heating over the TP in May is a southeastern-northwestern inverse pattern. This diabatic heating shows a robust negative relationship with the onset date of the summer monsoon over the AS and India. A positive diabatic heating seesaw pattern can enhance the ascending (descending) motion over the southeastern (northwestern) TP, corresponding to above (below) normal in- situ precipitation. Temperature budget diagnosis reveals that the adiabatic heating due to anomalous vertical motion and relevant horizontal advection of temperature in the mid-upper troposphere are responsible for the warming over the TP. Consequently, the reversal of the meridional temperature gradient from winter to summer over South Asian regions occurs earlier than the climate-mean state. Furthermore, the diabatic heating over the TP induces an enhanced and westward-extended South Asian High (SAH), which together with the easterly along the southern flank of the SAH superimpose on the low-level westerly flow over the AS and India, results in intensive upper-level divergence-pumping and upward motion. This anomalous circulation configuration in lower and upper levels further facilitates an earlier onset of the summer monsoon in AS and India. These findings are corroborated in the sensitivity runs based on CAM4.0.