Environmental Engineering Reference
In-Depth Information
drought has been established (Folland
et al.
1986;
Owen and Ward 1989). When southern
hemisphere ocean temperatures exceed those in
the northern hemisphere, rainfall in the Sahel is
below average. The warmer southern oceans may
reduce the strenghth of the ITCZ leading to less
uplift and therefore less precipitation. When the
world began warming in the 1980s, the southern
oceans warmed first and fastest, and the years
with record global temperatures—1983, 1987
and 1990—were also years in which the rains
failed. Only 1988 did not fit the pattern, and
Pearce (1991b) has drawn attention to the
possibility that if global warming continues as
expected, drought in the Sahel might only get
worse.
The links established between SSTs and
drought in sub-Saharan Africa have allowed the
UK Meteorological Office, through the Hadley
Centre for Prediction and Research, to issue
rainfall forecasts for the area (Owen and Ward
1989). These involve multi-stage predictions. To
be useful, the forecasts have to be supplied by
April, but it is the SSTs in June and July that
correlate most closely with the rainfall. Thus the
precipitation forecast is based on SSTs predicted
two months ahead, and any changes between
April and June will reduce the quality of the
forecast. Apart from 1988, when a strong La
Niña caused a major cooling of the tropical
Pacific and ruined the forecast, the predictions
have been remarkably accurate (Pearce 1991b).
Although the lead time is short, the approach is
one of the most promising yet developed for
drought prediction.
ENSO events in the south Pacific have also
received considerable attention as potential
precursors of drought. It has long been known
that following an El Niño changes occur in wind
fields, sea surface temperatures and ocean
circulation patterns. The large shifts of air and
water, associated with these developments, cause
major alterations to energy distribution patterns.
Zonal energy flow replaces the meridional flow
which is normal in tropical Hadley cells, and,
because of the integrated nature of the
atmospheric circulation, the effects are eventually
felt beyond the tropics. Reduced rainfall has been
noted in a number of semi-arid areas following
such episodes and teleconnections have been
established between ENSO events and
precipitation in areas as far apart as Brazil, India,
Indonesia and Australia. Drought in north-
eastern Brazil commonly occurs in conjunction
with an El Niño event, and India receives less
monsoon rainfall during El Niño years. The
relationship is well-marked in India, where
monsoon rainfall over most of the country was
below normal in all of the 22 El Niño years
between 1871 and 1978 (Mooley and
Parthasarathy 1983). Similarly, in Australia 74
per cent of the El Niño events between 1885 and
1984 were associated with drought in some part
of the interior of the continent (Heathcote 1987).
Such figures suggest the possibility of El Niño
episodes being used for drought prediction in
some parts of the world.
There is no clear relationship between drought
in the Sahel and the occurrence of ENSO events
(Lockwood 1986). Semazzi
et al.
(1988) have
suggested that sub-Saharan rainfall is linked to
ENSO events through the influence of the latter
on SSTs in the Atlantic. However, SST anomalies
do occur in years when El Niños are poorly
developed or absent. Thus, although an ENSO
episode may be a contributory factor in the
development of drought in the Sahel, it does not
seem to be a prerequisite for that development.
Associated with ENSO is La Niña, which has
physical characteristics completely opposite to
those of El Niño. It is a cold current rather than
a warm one and flows west instead of east. The
climatological impact of La Niña also seems to
be opposite to that of El Niño. At the time of the
last major La Niña—in 1988—heavy rain caused
flooding in Bangladesh, Sudan and Nigeria. The
drought forecast for the Sahel that year did not
come to pass. Instead, the region experienced one
of its wettest years in recent decades (Pearce
1991b). There is as yet too little data to establish
a link between La Niña and rainfall variability,
but it is a relationship that merits additional
investigation.
Teleconnection links remain largely
