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El Niño and La Niña
Understanding extreme weather
David Redfern
Introduction
This presentation supports the article ‘El Niño and La Niña: understanding extreme weather’ in
GEOGRAPHY REVIEW Vol. 32, No. 1, pages 35–37. Note that the phenomenon is often referred
to as El Niño Southern Oscillation, or ENSO.
•
An ENSO event takes place every 3 to 7 years, and may last for up to 18 months. The
impact can be worldwide.
•
Although the precise causes of these events are not fully understood, the impacts are welldocumented.
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A normal year
Normal conditions are:
•
high pressure exists in the eastern
Pacific, low pressure in the western
Pacific
•
surface trade winds blow east to west
across the Pacific Ocean, with
compensatory upper atmosphere
westerly winds (the Walker
Circulation)
•
these push warm water across the
sea surface from east to west
•
upwelling of cold water takes place off
the west coast of Peru, together with
more cold water supplied by the cold
Peruvian ocean current
•
dry weather exists in coastal Peru,
with rain in northern Australia.
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An El Niño year
In an El Niño year:
•
the trade winds become weaker,
and may even reverse
•
low pressure develops over the
eastern Pacific, with high pressure
to the west
•
warm water replaces cold water
off the coast of Peru
•
waters off the coast of northern
Australia become cooler
•
rainfall totals in northern Australia
and Indonesia reduce, and the dry
conditions often spread to India
and southeast Asia
•
rain falls in northwestern South
America
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El Nino impacts
In South America, there are a variety of impacts:
•
an increased possibility of flooding on the western coast of northern South America
•
drier conditions east of the Andes, in Amazonia
•
wetter conditions in southern Brazil and northern Argentina
In Australia and the western Pacific basin:
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weaker monsoons across much of Asia
•
reduction in number and intensity of tropical storms
•
an increase in wildfires with the drier conditions
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A La Niña year
La Niña events act independently – they are
not associated with El Nino events:
•
they involve the build-up of cooler-thanusual subsurface water in the eastern
Pacific Ocean
•
very warm waters build up in the western
Pacific
•
they create severe drought conditions in
the eastern Pacific coastlands/western and
southern South America
•
very wet weather can be experienced in
northern Australia and Indonesia
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Global changes to rainfall and
drought in a La Niña year
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La Niña impacts
A variety of global phenomena have been linked to La Niña events:
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flooding in Queensland, Australia in 2010/11 – more than 80 people were killed
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heavy snowstorms in northern USA/southern Canada in 2010
•
strong tornadoes in the southern USA in 2011
•
increases in transmissible diseases in wetter areas, e.g. malaria in southeast Asia and
Australian encephalitis (or Murray Valley encephalitis), in southeast Australia
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Managing El Niño and La Niña
impacts
Managing the impacts of these events is difficult because:
•
they cannot be predicted with accuracy
•
they affect large parts of the globe, not just the Pacific
•
some of the countries affected do not have the resources to cope
•
there are indirect impacts on other parts of the world though trade and aid
However, increasing awareness and research should make it possible to anticipate the
impacts and enable decision-making to reduce any negative impacts or take advantage of
positive impacts.
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This resource is part of GEOGRAPHY REVIEW, a magazine written for A-level
students by subject experts. To subscribe to the full magazine go
to: http://www.hoddereducation.co.uk/geographyreview
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