The climate of Tropical Asia is dominated by the two monsoons: The summer southWest monsoon influences the climate of the region from May to September, and the winter northeast monsoon controls the climate from November to February. The monsoons bring most of the region's precipitation and are the most critical climatic factor in the provision of drinking water and water for rain-fed and irrigated agriculture.
As a result of the seasonal shifts in weather, a large part of Tropical Asia is exposed to annual floods and droughts. The average annual flood covers vast areas throughout the region; in India and Bangladesh alone, floods cover 7.7 million ha and 3.1 million ha, respectively (GOI, 1992; Mirza and Ericksen, 1996). At least four types of floods are common: riverine flood, flash flood, glacial lake outburst flood, and breached landslide-dam flood (bishayri); the latter two are limited to mountainous regions of Nepal, Bhutan, Papua New Guinea, and Indonesia. Flash floods are common in the foothills, mountain borderlands, and steep coastal catchments; riverine floods occur along the courses of the major rivers, broad river valleys, and alluvial plains throughout the region.
Tropical cyclones also are an important feature of the weather and climate in parts of Tropical Asia. Two core areas of cyclogenesis exist in the region: one in the northWestern Pacific Ocean, which particularly affects the Philippines and Viet Nam, and the other in the northern Indian Ocean, which particularly affects Bangladesh. Other extreme events include high-temperature winds, such as those that blow from the northWest into the Ganges valley during January.
In the megacities and large urban areas, high temperatures and heat waves also occur. These phenomena are exacerbated by the urban heat-island effect and air pollution.
Geographically much more extensive is the El Ni�o-Southern Oscillation (ENSO) phenomenon, which has an especially important influence on the weather and interannual variability of climate and sea level, especially in the western Pacific Ocean, South China Sea, Celebes Sea, and northern Indian Ocean. Indeed, the original historical record of El Ni�o events compiled by Quinn et al. (1978) considered the relationships among Indonesian droughts, the Southern Oscillation, and El Ni�o. For more recent analyses of historical ENSO teleconnections in the Eastern Hemipshere-based on teak tree-ring data from Java and a drought and famine chronology from India-see Whetton and Rutherford (1994, 1996).
The strength of such connections has been demonstrated in several other studies. Suppiah (1997) has found a strong correlation between the Southern Oscillation Index (SOI) and seasonal rainfall in the dry zone of Sri Lanka; Clarke and Liu (1994) relate recent variations in south Asian sea-level records to zonal ENSO wind stress in the equatorial Pacific. The influence of Indian Ocean sea-surface temperature on the large-scale Asian summer monsoon and hydrological cycle and the relationship between Eurasian snow cover and the Asian summer monsoon also have been substantiated (Sankar-Rao et al., 1996; Zhu and Houghton, 1996). Kripalani et al. (1996) studied rainfall variability over Bangladesh and Nepal and identified its connections with features over India.
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