The influence of the Asian summer monsoon onset on the northward movement of the South Asian high towards the Tibetan Plateau and its thermodynamic mechanism

Abstract

ABSTRACTThe influence of the Asian summer monsoon (ASM) onset on the northward movement of the South Asian high (SAH) in the upper troposphere towards the Tibetan Plateau (TP) in early summer and its thermodynamic mechanism are studied using ERA‐Interim reanalysis data for the period 1979–2014. Results show that the SAH shifts northward from the southern slope of the TP and moves onto the TP on the 33rd pentad with strengthened intensity. As the SAH moves northward towards the TP, divergence at upper troposphere over the TP region, ascending motions and diabatic heating over South Asia and the southern slope of the TP increase significantly. The onset dates of the ASM, including the summer monsoon over the eastern Bay of Bengal, South China Sea, and Indian Peninsula, are earlier than the mean date of the SAH shifting onto the TP. Accompanied by the ASM onset occurring over South Asia from May to June, much condensation heating is released to the atmosphere. The diabatic heating related to ASM rainfall is the major reason for warming over the southern slope of the TP, leading to the northward movement of the temperature ridgeline towards the TP. Consequently, the meridional temperature gradient in the mid‐upper troposphere over the southern slope of the TP reverses. According to the thermal wind relationship, the sign in the vertical shear of the zonal wind changes, implying that the SAH moves onto the TP. After the SAH shifts onto the TP, the divergence pumping effect caused by the enhancement of the SAH over the TP is favourable for the further prevalence of the summer monsoon over South Asia and East Asia.