In this article, we focus on the analysis of the climate variability of the Southeastern Asian Summer Monsoon (SEAM) region encompassing Myanmar, Thailand, Cambodia, Vietnam, Laos and parts of southern China. This region is climatologically found to have one of the longest wet seasons in the Asian monsoon region (of nearly 160 d) and also exhibits one of the strongest interannual variations in the length of the monsoon (wet) season. The interannual variations of the length of the SEAM are characterized by corresponding variations in the onset and demise pentad dates of the wet season, with the former dominating slightly over the latter except over Myanmar. Our study reveals that the pentad of late onset of SEAM is characterized by anomalous increase in remote moisture source from Bay of Bengal and Arabian Sea while a substantial decrease of moisture source from the near Andaman Sea and Gulfs of Martaban and Thailand. Furthermore, anomalously strong June–August Somali Jet is found to be associated with earlier than normal onset of the SEAM. Similarly, the pentad of late demise of the SEAM features excess moisture source from the South China Sea associated with a slow eastward withdrawal of the north Pacific subtropical high.
We suggest on the basis of the findings of this study that careful monitoring of the onset the SEAM season will provide important information on the evolution of an ongoing SEAM. Likewise observing low level winds over the northern equatorial Indian Ocean, Bay of Bengal, Gulfs of Martaban and Thailand and South China Sea could be very useful in understanding the seasonal variability of the SEAM. Finally, monitoring of the demise would be equally helpful in characterizing the variation of the concluded SEAM as the length of the wet season seems to be a very robust climate feature of the region. © 2013 Royal Meteorological Society