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11.2. Regional Climate
11.2.1. Present Climate Characteristics
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 Niņo-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 Niņo events compiled by Quinn et al. (1978) considered
the relationships among Indonesian droughts, the Southern Oscillation, and El
Niņo. 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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