VARIABILITY OF THE SUMMER MONSOON RAINFALL OVER THAILAND - COMPARISON WITH FEATURES OVER INDIA

In this study, 20 years (1961-1980) of rainfall data for 34 stations over Thailand are used to investigate and understand the intraseasonal and interannual variability of the summer monsoon. Dominant structures of 5-day and seasonal rainfall are determined through empirical orthogonal functions (EOFs). Monthly and seasonal spatial patterns and a map showing the coefficient of variation are also presented. On an intraseasonal time-scale regions with high 30-60-day variances (Madden Julian oscillations-MJOs) and 10-20-day variances (Quasi-biweekly oscillations-QBWOs) are identified using a band-pass Butterworth filter for 5-day rainfall. It is seen that the MJOs are dominant over the Indian region and the QBWOs are dominant over the Thailand region. Extended EOFs (EEOFs) have shown that the most important evolutionary feature over India is the northward propagation associated with the MJOs, whereas over the Thailand region it is westward, associated with the QBWOs. Northward propagation of rainfall anomalies is not observed over the Thailand region. On the interannual scale the area over north-west Thailand is related well with rainfall variation over west-central India. The Darwin pressure tendency (DPT) and the subtropical ridge over India shows significant relation with rainfall over northwest Thailand. However, the Northern Hemisphere surface temperature (NHST), Quasi-biennial Oscillation (QBO), and West Pacific ridge (WPR) show no significant relation.