Onset of the Rainy Seasons in the Eastern Indochina Peninsula

The onset dates of rainy season over the eastern Indochina Peninsula (8.5 degrees-23.5 degrees N, 100 degrees-110 degrees E) are objectively determined for individual years from 1958 to 2007 using the empirical orthogonal function (EOF) analysis. On average, the onset of the summer rainy season (SRS) determined by EOF1 is 6 May, with a standard deviation of 13 days. The autumn rainy season (ARS) indicated by EOF2 has a mean onset and standard deviation of 16 September and 12 days, respectively. The SRS onset is characterized by the evolution of summer monsoon westerlies and the northward propagation of strong convection from the equatorial region. Conversely, the withdrawal of the summer monsoon over northeastern Indochina in late summer-early autumn favors the ARS onset. Both onsets are strongly associated with intraseasonal oscillation on 30-60- and 10-20-day time scales. Examination of the precursory signals associated with the early/late onsets of both SRS and ARS implies that ENSO has a significant impact on their year-to-year variations. In La Nina years, the subsequent SRS tends to have early onsets. Simultaneously, the western Pacific subtropical high (WPSH) weakens and retreats eastward earlier. In contrast, advanced ARS onset generally occurs during an El Nino-developing autumn with weakened equatorial easterlies and suppressed convection over the central Indian Ocean from the preceding summer, as evident in weakened Walker circulation. However, robust precursory signals in SST are observed only from midsummer (July-August). An earlier ARS onset is also associated with the development of an anomalous Philippine Sea anticyclone and a westward-extended WPSH from midsummer. However, no coherent correlation is found between the late onset and La Nina.