Geoscience Reference
In-Depth Information
Darwin, Australia. In the east Pacific, a layer of warm
water extending 50-100 m below the ocean surface
swamps cold water that is normally at the surface and
maintaining the high-pressure cell off the South
American coast (Figure 2.7C). As a result the thermo-
cline deepens. As low pressure replaces high pressure
over this warm water in the east Pacific, the easterlies
fail altogether because there is no pressure difference
across the Pacific to maintain them. Westerly winds
may even begin to flow at the equator. So strong is
the change that the Earth's rotation speeds up by
0.5-0.7 milliseconds as the braking effect of the easterly
trades diminishes.
Cyclone development follows the location of the
pool of warm water as it moves eastward towards Tahiti
and Tonga. The 1982-1983 event caused widespread
destruction in these areas, with Tonga receiving four
tropical cyclones, each as severe as any single event in
the twentieth century. The Tuamotu Archipelago, east
of Tahiti, was devastated by five cyclones between
January and April 1983. Not since 1906 had a cyclone
occurred this far east in the tropics. In unusual cir-
cumstances, paired tropical cyclones may develop on
each side of the equator in November. Because winds
in tropical cyclones in each hemisphere rotate in
opposite directions, coupled cyclones can combine
(like an egg beater) to generate, along the equator,
strong westerly wind that may accelerate water
movement eastward.
At the peak of an ENSO event, a
Kelvin wave
is
trapped along the east Pacific coast. Warm water and
elevated sea levels spread north and south, reaching as
far north as Canada. In the 1982-1983 event, tempera-
tures were 10°C above normal and sea levels rose by
60 centimetres along the South America coastline, and
by 25-30 cm along the United States west coast. The
intense release of heat by moist air over the central
Pacific Ocean causes the westerly jet stream in the
upper atmosphere, and the position of the wintertime
Aleutian Low, to shift towards the equator (Figure 2.7D).
Westerly surface winds and the jet stream, rather than
being deflected north around the Rocky Mountains,
cross the continent at mid-latitudes. This can lead to
dramatic changes in climate. For example, in the
1982-1983 ENSO event, coastal storms wreaked havoc
on the luxurious homes built along the Malibu coastline
of California while associated rainfall caused widespread
flooding and landslides. The storms were made all the
more destructive because sea levels were elevated by
25-30 cm along the entire United States west coast,
from California to Washington State. Heavy snowfalls
fell in the southern Rocky Mountains, record-breaking
mild temperatures occurred along the American east
coast and heavy rainfall fell in the southern United
States.
Finally, with the spread of warm water in the
northern Pacific, easterly air circulation begins to re-
establish itself in the tropics. The depression of the
thermocline in the east Pacific during an ENSO event
causes a large wave, also called a Rossby wave, to prop-
agate westward along the thermocline boundary. This
wave is reflected off the western boundary of the
Pacific and, as it returns across the Pacific, it slowly
raises the thermocline to its pre-ENSO position.
However, the return to Walker circulation can be
anything but normal. The pool of warm water now
moving back to the west Pacific drags with it excep-
tional atmospheric instability, and a sudden return to
rainfall that breaks droughts in the western Pacific. In
1983, drought broke first in New Zealand in February,
one month later in eastern Australia with rainfalls of
200-400 mm in one week, and then - over the next two
to three months - in India and southern Africa.
The Southern Oscillation causes extreme, short-
term climate change over 60 per cent of the globe,
mainly across the southern hemisphere and certainly
in North America. However, some events influence
Europe. For example, the El Niño of 1997-1998
caused widespread flooding in central Europe con-
comitantly with flooding in central Africa, South-East
Asia, and Peru. The climatic effects of an ENSO event
are widespread, and its influence on both extreme
climate hazards and short-term climate change is
significant. Of South American countries, southern
Peru, western Bolivia, Venezuela, and north-eastern
Brazil are affected most by ENSO events. The
1997-1998 ENSO event destroyed $US1 billion in
roads and bridges in Peru alone. The widest impact
occurs over southern Africa, India, Indonesia, and
Australia. For instance, in Indonesia, over 93 per cent
of monsoon droughts are associated with ENSO
events, and 78 per cent of ENSO events coincide with
failure of the monsoon. In Australia, 68 per cent of
strong or moderate ENSO events produce major
droughts in the east of the continent. ENSO events
have recently been shown to contribute to drought in
the African Sahel region, especially in Ethiopia and
Sudan.
