Geography Reference
In-Depth Information
are strong and converge over the warm waters of the western trop-
ical Pacific where there is low pressure and lots of rainfall. During
this time the ocean surface in the eastern tropical Pacific is rela-
tively cold and the air above the ocean and the coastal parts of
equatorial South America is cold and dry. However, every few
years for 6 to 18 months the trade winds relax. As air pressure falls
in the east the warmer surface ocean water and heavy rain moves
east while there is a rise in surface air pressure in the west. This
event is known as El Niño and brings drought to Indonesia and
heavy rainfall and floods to coastal areas in equatorial South
America. The movement of the warm water to the east means that
the normal upwelling off Peru is capped with warm water prevent-
ing deep cold water from moving up to the surface. Thus, the
nutrients that support the rich fisheries are cut off.
The effects of El Niño can also be seen across the planet due to
the fact that the whole global climate system is linked and a change
in one location has knock-on effects elsewhere. For example, con-
trary to normal conditions most El Niño winters are warm and dry
over western Canada and wet from Texas to Florida. Damage from
floods and landslides caused by very high rainfall in southern Cali-
fornia has been linked to El Niño along with Indonesian forest
fires, Australian bush fires and drought, and crop failures and
famine in south-central Africa (e.g. Zambia, Zimbabwe, Mozam-
bique and Botswana). Some El Niño events can be more intense
and last longer than others. Forecasters have been trying to estab-
lish the likely nature and severity of each El Niño event in advance
so that governments, farmers and other interested parties can
prepare for the changes (e.g. by sowing different crops than normal
or saving additional water before drought develops).
The El Niño Southern Oscillation is not the only example of
interannual variability. For example, there is the North Atlantic
Oscillation which when in a 'positive' phase increases the rainfall
across northern Europe (with less across southern Europe) and results
in milder northern European winters, while in a negative phase there
will be less rainfall in northern Europe and more in southern Europe
and north Africa. The North Atlantic Oscillation can remain in a
positive or negative phase for several years or even decades.
It is also worth noting that natural events can cause variability in
the Earth's climate. For example, volcanic ash from the eruption of
