Variations of Vegetation and Rainfall in Thailand

While variations in vegetation are dependent on climatic factors such as rainfall and temperature, recent studies have shown a possible feedback of changing surface conditions on climate. For example, Sarkar et al. (2005) show variations of Asian summer monsoon rainfall is preceeded by vegetation changes in the spring over cultivated croplands. In this study, we further examine this feedback over Thailand. Thailand daily rain data collected over 100 gauges are examined and compared with gauge analyses provided by the Global Precipitation Climatology Center (GPCC). Climatologically, Thailand monsoon rainfall is influenced by the prevailing seasonal wind direction. In the northern region, most rainfall occurs during JJA. In the southern region rain occurs mostly in SON. Normalized Difference Vegetation Index (NDVI) derived from AVHRR for the period 1982-2000 is used to examine the vegetation changes. The seasonal cycle of NDVI lags behind that of precipitation by about 2-3 months. The non-seasonal changes in NDVI show minima in 1982 and 1992, which may be related to the volcanic eruptions of El Chichon in Mexico in 1981 and Mt. Pinatubo in the Philippines in 1991, respectively. Aerosol indices such as Aerosol Optical Depth (AOD) and Aerosol Index (AI) derived from the Total Ozone Mapping Spectrometer (TOMS) will be used to delineate the effect of the volcanoes. Other non-seasonal effects, such as that due to the El Nino/Southern Oscillation (ENSO) will be incorporated in our analysis.