Analysis of Basic Features of the Onset of the Asian Summer Monsoon

In this paper, a relatively systematic climatological research on the onset of the Asian tropical summer monsoon (ATSM) was carried out. Based on a unified index of the ATSM onset, the advance of the whole ATSM was newly made and then the view that the ATSM firstly breaks out over the tropical eastern Indian Ocean and the middle and southern Indo-China Peninsula was further documented, which was in the 26th pentad (about May 10), then over the South China Sea (SCS) in the 28th pentad. It seems that the ATSM onset over the two regions belongs to the different stages of the same monsoon system. Then, the onset mechanism of ATSM was further investigated by the comprehensive analysis on the land-sea thermodynamic contrast, intraseasonal oscillation, and so on, and the several key factors which influence the ATSM onset were put forward. Based on these results, a possible climatological schematic map that the ATSM firstly breaks out over the tropical eastern Indian Ocean, the Indo-China Peninsula,and the SCS was also presented,namely seasonal evolution of the atmospheric circulation was the background of the monsoon onset; the enhancement and northward advance of the convections,the sensible heating and latent heating over the Indo-China Peninsula and its neighboring areas, the dramatic deepening of the India-Burma trough, and the westerly warm advection over the eastern Tibetan Plateau were the major driving forces of the summer monsoon onset, which made the meridional gradient of the temperature firstly reverse over this region and ascending motion develop. Then the tropical monsoon and precipitation rapidly developed and enhanced.The phase-lock of the 30-60-day and 10-20-day low frequency oscillations originated from different sources was another triggering factor for the summer monsoon onset. It was just the common effect of these factors that induced the ATSM earliest onset over this region.