The first transition of the Asian summer monsoon, intraseasonal oscillation, and Taiwan Meiyu

This study reveals the close relationship between the first transition of the Asian summer monsoon, intraseasonal oscillation and the Meiyu 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 circulation and convection in the sharp onset years is characterized by an eastward-propagating tropical intraseasonal oscillation (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 occurs, and a channel supplying moist air in the lower troposphere is well established across the Indian Ocean. 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 South China Sea (SCS) and the western North Pacific. This efficient and persistent transport of moisture to the SCS and surrounding areas provides a favorable condition for the maintenance of the Meiyu front and the development of mesoscale convective complexes (MCCs). This also marks the onset of the Taiwan Meiyu season. Most wet (active) Taiwan Meiyu years are sharp onset cases, while only a few dry (inactive) Taiwan Meiyu years are characterized as sharp onset. Whether the moisture supply channel is well established or not seems to be an important factor affecting the interannual variability of Taiwan Meiyu. Since a strong TISO is often observed prior to the onset of the first transition and Taiwan Meiyu, a close monitoring of the TISO can be very informative for weather forecasters in Taiwan to predict the initiation of the Taiwan Meiyu.