Over the past 100 years, mean surface temperatures have increased by 0.3-0.8°C
across the region; the general increase is clearly evident in Figure
11-2a. Although there is no definite trend discernible in the long-term
mean for precipitation for the region (Figure 11-2b)
or in individual countries during this time period, many countries have shown
a decreasing trend in rainfall in the past three decades. The southWest monsoon
in India has shown definite changes in the period 1961-1990, in comparison with
either 1901-1930 or 1931-1960, though no consistent longer-term trend is detectable.
In Viet Nam, mean annual temperature has increased over the period 1895-1980,
with net warming estimated at 0.27°C over the past two decades. Variations in
rainfall also have been observed, with geographical variation. Since the 1960s,
annual rainfall has been increasing in the north of Viet Nam and decreasing
in the south (Granich et al., 1993).
|Figure 11-2: Time series of climate observations in Tropical Asia-a) observed annual temperature anomalies and b) observed precipitation anomalies, both relative to the 1961-90 means [longer term variations of the annual anomalies are emphasized by the smooth curve using a nine-point binomial filter (see Annex A)]; c) annual number of tropical cyclones over the Bay of Bengal, Arabian Sea, and land (Gadgill, 1996); and d) annual number of tropical cyclones that cross the coast of Viet Nam from the South China Sea (data supplied by Marine Hydrometeorological Center, Hydrometeorological Service, Hanoi, S.R. Viet Nam).|
The frequencies of tropical cyclones in the core regions of cyclogenesis in Tropical Asia are quite different. Data for a 27-year period (1958-84) indicate that the average number of cyclones per year in the northern Indian Ocean is 5.5, whereas the northWestern Pacific experiences an average of 26.1 cyclones per year-representing 7% and 33%, respectively, of the total number of global cyclones (Climate Impact Group, 1992).
No identifiable changes in the number, frequency, or intensity of tropical cyclones or depressions have been observed in the northern Indian Ocean cyclone region (Bay of Bengal and Arabian Sea) over the past 100 years, although Gadgill (1995) has shown decadal-scale variations-with a rising trend during 1950-75 and a declining trend since that time (see Figure 11-2c). Similarly, there is evidence of substantial multidecadal variability in the northWestern Pacific, as shown by data from Viet Nam (see Figure 11-2d), though no clear evidence of long-term trends (Henderson-Sellars and Zhang, 1997). In Bangladesh, during the period 1948-1988, a total of 418 depressions and storms were formed, of which 79 were severe cyclonic storms (wind speed 69-117 km/hr). A time-series prepared by GOB (1989) indicates that, over the 40-year period, the highest number of depressions and storms formed in the 1960s, whereas the 1980s showed a decreasing trend. These findings are in broad agreement with regional trends.
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