The first transition of the Asian summer monsoon, intraseasonal oscillation and Taiwan Mei-yu

This study reveals the close relationship between the first transition of the Asian summer monsoon (ASM), the tropical intraseasonal oscillation (TISO), and the mei-yu in Taiwan, which occurs climatologically between mid-May and mid-June. For about half of the years in 1958-2002, the first transition of the Asian summer monsoon can be classified as a sharp onset, which is characterized by an abrupt reversal of the monsoon flow from northeasterly to southwesterly. The evolution of the large-scale monsoon circulation and convection in the sharp-onset years is characterized by an eastward-propagating TISO from eastern Africa and the western Indian Ocean to the Maritime Continent. Upon the arrival of the TISO in the Maritime Continent, a sharp onset of the ASM occurs, and a channel supplying moist air in the lower troposphere is well established across the Indian Ocean to the South China Sea (SCS). This channel consists of the Somali jet, transporting the moisture from the Southern Hemisphere to the Northern Hemisphere, and the southwesterly monsoon, delivering the moisture across the Indian Ocean to the SCS and the western North Pacific. This efficient and persistent transport of moisture to the SCS and surrounding areas presumably provides a favorable condition for the maintenance of the mei-yu front and the development of convective systems. This also marks the onset of the Taiwan mei-yu season. Because a strong TISO signal, which tends to occur concurrently with the sharp onset of the ASM, is often observed prior to the onset of the first transition and Taiwan mei-yu, a close monitoring of the TISO can be informative for the weather forecasters in Taiwan to project the initiation of the Taiwan mei-yu.