Pre-monsoon rains and onset of monsoon over the Indochina Peninsula
Jun Matsumoto (1), Masashi Kiguchi (1)
(1) Dept. of Earth & Planetary Sci., Univ. Tokyo
It has been pointed out that the start of rainy season is earlier in the inland region of the Indochina Peninsula in a climatological field. It is because periodical rainfall events occur there in the pre-monsoon season with the influence of the disturbances in the mid-latitude westerly (Kiguchi and Matsumoto, 2002). This paper further elaborates on the characteristics of pre-monsoon rains over the Indochina Peninsula and discusses the role of these rains on the onset of Southeast Asian summer monsoon.
The data utilized in the present study are: daily mean OLR provided by NOAA (National Oceanic and Atmospheric Administration), reanalysis of daily mean of various meteorological elements by NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research), daily rainfall of stations in Thailand, Laos and Cambodia obtained from the GAME-T dataset for the period 1979-2002, and daily mean precipitable water in the atmosphere derived from GPS observations conducted by the GAME-T project in the GAME-IOP year, 1998.
Analyses on the height fields, moisture transport and convergence in the inland area of the Indochina Peninsula, and precipitable water at Bangkok and Chaing Mai observed by GPS and area averaged precipitation show that in the pre-monsoon season before May 15, 1998, the rainfall events well correspond to the passage of upper level westerly trough coming from the southern periphery of the Tibetan Plateau in conjunction with the low level moisture convergence. In these cases, moisture inflow is mainly from the east, not from the west. This situation drastically changes after the monsoon onset, when moisture inflow from the west is the main feature of the rainfall events. In accordance with these premonsoon rains, the land surface gets wetter and the latent heat release begins to increase after March, far before the monsoon onset. This is very different from the situation of the onset of the Indian summer monsoon.
Long-term analysis on the monsoon onset and pre-monsoon rainfall reveals that there is little relationship between them, which implies that local wetness over the Indochina Peninsula in the premonsoon season affects little on the monsoon onset.