The Climate of Southeast Asia

Southeast Asia lies between the continental influence of the rest of Asia to the north and the more oceanic influence of the Indian and Pacific Oceans to the south and the east respectively. While its overall net energy balance is very much determined by its latitudinal position, which is approximately between 20°N and 10°S, the locational factors referred to above largely give the regional climate its distinctive character. Within the broad latitudinal extent defined above, the Southeast Asian region has often been conveniently separated into two sub-areas: continental and insular Southeast Asia. In some ways these sub-regions represent a valid delineation into the more seasonal climatic region influenced by the monsoon system of winds and the uniformly humid equatorial climate. The former comprises Myanmar, Thailand, Lao PDR, Cambodia, and Viet Nam, while the latter includes Malaysia, Singapore, Indonesia, and the Philippines. The continental Southeast Asia experiences greater seasonality, more extremes in both temperature and rainfall, and more pronounced dry spells; whereas the insular parts, termed the ‘maritime continent’ (Ramage 1968), with a much greater expanse of sea than land (the sea area of Indonesia, for example, is four times its land area), have more equable climate. The northern and southern continental interactions in winter and summer and the differential heating due to the asymmetric character of the two sub-regions give rise to the monsoon development (Hastenrath 1991), which, to a large extent, influences the rainfall characteristics of the region as a whole. In a sense, more than temperature variations, this monsoonal influence gives the Southeast Asian climate its distinctive character. Figure 5.2 shows the two monsoon wind systems that affect Southeast Asia. In addition to these annual reversals of the monsoon winds, the seasonal migration of the Intertropical Convergence Zone (ITCZ)—closest to the Equator during the northern hemispheric winter and farthest north during summer—is a significant factor in influencing the monthly weather regime of the region. Being a belt of low-pressure trough coinciding with the band of highest surface temperature, the ITCZ attracts the moist easterlies from both hemispheres towards its trough resulting in uplift of air, intense convection, and precipitation. This whole process provides a mechanism for the transfer of latent heat from the low to the higher latitudes (Houze et al. 1981; Hastenrath 1991).