4.4. Monsoon region
4.4.1. Projections
Global warming is expected to lead to a higher temperature difference between
the land and sea surfaces on the Indian subcontinent. This induces stronger
winds from sea and, in combination with higher evaporation, and transports
greater quantities of moist air. Recent results from global coupled models
generally agree with these expectations.
The global coupled ocean-atmosphere climate model of NCAR produces greater
mean precipitation in the south Asian summer monsoon region (Lal, 1993). With
the same GCM, Meehl and Washington (1993) found that the interannual
variability of monsoon rainfall is enhanced.
The UKMO GCM model (Bhaskaran et al., 1995) predicts a total precipitation
increase of approximately 20% with increased CO2. Specific humidity increases
by 19%, indicating that the increased monsoon rainfall is largely due to
increased water content of the atmosphere. The model also predicts a greater
number of heavy rainfall days during the summer monsoon period, and an
increased interannual variability.
4.4.2. Observations
The annual times series of East Asian precipitation shows a weak tendency for
increasing precipitation over the last 100 years (Hulme and Zhao, 1994). This is
confirmed by Sontakke et al. (1993) analysing the all-India summer monsoon
rainfall series from 1813 to 1991. There was little trend in precipitation up to the
turn of the century and only a marginally significant wetting trend between 1902
and 1991.
Consistent with the model results, observations (20 years) show a trend of
increased interannual variability of Indian monsoon precipitation (Meehl and
Washington, 1993). Rackhecha and Soman (1994) find significant increasing
trends in the extreme rainfall series at stations over the west coast of India and
at some stations in the central part of the subcontinent.
Chowdbury and Debsarma (1992) reveal a significant upward trend in
Bangladesh annual rainfall in the north (by 18%) and in the southwest (since the
early 70s), and a downward trend in the southeast. In addition, a trend towards
more severe tropical cyclones was detected.
Kumar et al. (1993) noticed that annual precipitation has been increasing (by
more than 5%) over India, Pakistan, Nepal, Sri Lanka, and Burma since 1850. On
the other hand, no significant precipitation change has been observed
throughout the Taiwanese region (Cheng et al., 1993)
4.4.3. Flooding
In recent years the frequency of abnormal floods in Bangladesh has increased
substantially (Khalequzzaman, 1994): 1974, 1984, 1987, 1988, 1991 and 1993.
Normally the monsoon floods cover 20% of the territory one year out of two. The
disastrous river floods of 1987 and 1988 inundated 40 and 60% of the country,
respectively. Apart from the reasons given above, other factors that reduce the
water carrying capacity of the drainage system may be important here: change
in the base level of the rivers due to local sea level rise, inadequate sediment
accumulation on flood plains, an increase in the catchment area due to seismic
and neotectonic activities in the region, river bed narrowing due to siltation and
damming, soil erosion, deforestation in the upstream region, and the increase in
the rate of melting of snow pack in the Himalayas (observed over the past
decade). Studies of the World Climate Research Program on the ENSO
phenomenon suggest a strong relationship between variations in Bay of Bengal
tropical cyclones, as well as other weather conditions in Bangladesh, and the
state of development of El Niño (Bruce, 1995).
4.4.4. Analysis of a recent flood
June 1993
The monsoonal floods in Bangladesh were the worst in 58 years, causing serious
damage to lives and property. The heavy monsoon downpour (heaviest rains in
20 years) and synchronisation of flood peaks of the three major rivers are
considered to be the main causes. The monsoon began a few days earlier than
usual. It advanced northwards and also covered the whole of India a few days
early. The total rainfall was equal to the long-term average: it was a "normal"
monsoon year.
