2.5 SEASONAL VARIATIONS

• Monsoon circulation is the main agent of seasonal change

• Little variation in average temperature, but large variation in rainfall pattern

• Modest rainfall related to disturbances from the west wind drift during the dry season

• Climate variability and change is studied in order to better assess their impact on the Mekong basin

• Atmospheric pollution (for example burning during the dry season) can be a problem

The most important change in climate over the course of a year is associated with rainfall. The northeast monsoon occurs roughly from November to March. It brings relatively dry, cool air and little precipitation to the mainland.

The southwest monsoon prevails from May to September. Then, the air current reverses, and the dominant flow of warm and moisture laden air is to the northeast. The basin receives up to 75% of the annual rainfall during this period.

The dry and wet monsoons are important in explaining the seasonal distribution of rainfall patterns. But there are other factors, for example relief, land and sea breezes or disturbances from the west wind drift. These factors often are combined with the monsoon circulation and can produce highly variable rainfall patterns, even over relatively short distances. The disturbances of the west wind drift, originating from the middle latitudes, produce only moderate amount of rain. Other disturbances, originating from the equatorial zone, can mature into tropical storms that bring heavy rains and destruction to the areas over which they pass.

Overall there is little temperature variation in the Mekong basin. Temperatures are steady throughout the year, averaging between 25 and 29°C. Monthly averages tend to vary more with increasing latitude. Increasing elevation acts to decrease average temperatures. The greatest seasonal fluctuations are in the north, where frost may occur in December at higher elevations. Snow falls only in extreme northern highlands. Conversely, maritime influences moderate the climate in the south. The cooler, drier air of the northeast monsoon produces frequent morning fogs that generally dissipate by midday in the north and northeast regions. Annual average sea-level temperatures are remarkably close to 27°C. The degree of seasonal variation depends upon proximity to the sea, and also upon elevation.

Temperature distribution over Southeast Asia during the months of January, April, July and October (average of 1961-1990) Source: National Climate Prediction Center, USA

The issue of variation in the climate of the Mekong Basin and the potential effects of climate change on the region are currently under study. For example, with the help of satellite derived information, researchers can map monthly differences or changes in precipitation patterns against the long-term average. Exceptional floods occurred within the Mekong Basin in the year 2000. An example of the rainfall anomalies recorded during that year is given in Jan / April / July / Dec. This series of maps reveals abnormally high precipitation over many regions of the Mekong Basin; high concentrations are noted throughout the year, particularly in its lower portions.

Other satellite instruments are capable to detect pollutants in the atmosphere. In April 2002, Total Ozone Measurement Spectrometer (TOMS) provided some imagery of abnormally high concentrations of pollutants. Smoke from biomass burning in Southeast Asia seemed to be severe toward the end of the dry season as shown on imagery on April 7, 11, 13, 2002.