Interannual variation of the northward movement of the South Asian High towards the Tibetan Plateau and its relation to the Asian Summer Monsoon onset

Abstract

The interannual variation of the northward movement of the upper-level South Asian High (SAH) towards the Tibetan Plateau (TP) in the early summer and its relation to the Asian summer monsoon (ASM) onset are still unclear. This study shows that when the ASM onset occurs earlier, there is an anomalous westerly/easterly in the lower/upper troposphere, accompanying an anomalous ascending motion over the southern part of Asia. Meanwhile, cyclonic circulation anomalies occur over the Arabian Sea, the Bay of Bengal, and the South China Sea and are conducive to the earlier development and spread of the deep convection and atmospheric heating. Consequently, the warm center in the mid-upper troposphere surrounding the TP generates earlier in May, resulting in an earlier reversal of the meridional temperature gradient over the southern slope of the TP. Based on the thermal wind relationship, the vertical shear of the zonal wind reverses earlier and finally the SAH shifts over the TP earlier. The opposite is true when the ASM onset occurs later. Compared with the evolution of the diabatic heating over South Asia from May to June, there is no significant difference in the diabatic heating over the TP between the earlier and later years, indicating that the ASM heating may play a more important role in the northward movement of the SAH than the TP heating. Therefore, the ASM heating is essential to the interannual variation of the northward shift of the SAH towards the TP in the early summer.