Abstract: The surface wind can be a better indicator of the onset of the Asian Summer Monsoon (ASM) system than the 850-hPa wind. The use of the new ASM onset index and analysis of pentad-isochrones, based on both the seasonal reversal of surface wind and the evident enhancement of rainfall, demonstrated that the onset of the Tropical Asian Summer Monsoon (TASM) first occurs over the southeastern Bay of Bengal (BOB) in May. It then propagates eastward to the Indochina Peninsula and reaches the South China Sea (SCS) in mid-May and the tropical North West Pacific (NWP) in early June. The surface depression of the Indian summer monsoon originates near the equatorial Arabian Sea, and then propagates northward to South Kerala in southwestern India in early June, indicating the onset of the Indian Summer Monsoon (ISM). In addition, the Subtropical Asian Summer Monsoon (STASM) is first formed over the NWP southeast of Honshu, Japan, and then it expands westward and merges into the precipitation zone of the SCS monsoon in early June, forming a northeast-southwest rainy belt. Almost at the same time, the summer monsoon reaches southeastern China and the Baiu in Japan also starts. In mid-June, the rainfall belt shifts northward to the Yangtze River and the Korean peninsula corresponding to the start of the Meiyu and Changma.
This paper also reviews some recent progress in dynamics studies on the ASM onset. In spring, an evanescent but strong warm pool is formed in the central-eastern BOB due to the combined forcing of the Tibetan Plateau and the large-scale land-sea distribution in South Asia. Due to the coupling between this warm pool at the surface and the pumping effect of the South Asia High (SAH) in the upper troposphere, a Monsoon Onset Vortex (MOV) usually develops over the eastern BOB, breaking the ridgeline of the subtropical high that is continuous in winter, resulting in the first onset of TASM over the BOB. Furthermore, the vertical easterly/westerly shear over the eastern/western BOB prompts/inhibits the convection and increases/decreases the surface sensible heat transfer from ocean to atmosphere, resulting in a monsoon onset barrier over the west coast of the BOB that prevents the westward propagation of the monsoon onset. Hence, the monsoon onset can expand only eastward followed by the successive onset of the SCS and the tropical West Pacific summer monsoon. The strong latent heat released by the monsoon rainfall induces the westward development of the SAH and the conspicuous strengthening of zonal asymmetric potential vorticity (PV) forcing. Accompanied by the mid-troposphere anticyclone over the Arabian Peninsula stimulated by strong local surface sensible heating, the depression near the equatorial Arabian Sea moves northward and grows to a monsoon onset vortex, causing the ISM onset. Overall, the three phases (i.e., the BOB, the SCS, and the Indian summer monsoon onset) of the TASM onset that persist for about one month can be considered as a consequential process driven by certain dynamic-thermodynamic rules covering a specific geographic environment, including the Tibetan Plateau and the land-sea distribution in South Asia.