Page 94 - The World About Us

Infrared imaging
1.1.1
1.1.1
2.2.32.3.6

What is the El Ni Southern Oscillation?ño

Perhaps the most important global Peruvian fisherman who no ced a warm Using historical evidence and other
climate phenomenon a er the annual current appearing along their shores just climate data (see pages 74-75)
change in the seasons has its origins in before Christmas. Its original name, El climatologists and oceanographers now
the Pacific Ocean. The El Niño Southern Niño de Navidad, was a reference to the recognise that a full El Niño Oscilla on
Oscilla on (ENSO) has the ability to birth of the Christ child. event may last any me between two and
change global atmospheric circula on, seven years.
In the 1920s, meteorologist Gilbert
which in turn can alter the weather across Walker iden fied a repeated, if Each phase may be considered to be
the en re planet.
unpredictable, shi in the atmospheric strong or weak, measured using a range
ENSO is a single phenomenon which circula on across the Pacific region. This of atmospheric and oceanic indicators,
links both the ocean and atmosphere became known as the Walker Circula on. called the Mul variate ENSO Index.
across the South Pacific. It has three It was not un l much later, in the The El Niño events of 1982-83, 1997-
phases through which it changes: El Niño 1960s, that meteorologist Jacob Bjerknes 98 and 2015-16 are considered to be the
and La Niña, as well as a Neutral phase, suggested a physical mechanism three strongest on record. The impacts of
somewhere in the middle of the ENSO connec ng the atmospheric changes of ENSO can be felt across the globe
con nuum.
the Walker Circula on with changes in sea thousands of miles from the Pacific
El Niño was first given its name by water temperatures found with El Niño. Ocean.
The Neutral phase
Walker Circula on
In the western Pacific Ocean,
warm tropical waters at average
temperatures lead to rising air,
low pressure and rain over
eastern Australia, New Guinea
and Indonesia. Trade winds of
average strength blow across the
Pacific Ocean from the east and
warm water accumulates in the
western Pacific. On the South
American coast an upwelling of
cold, nutrient rich water
dominates the coastal area and
suppresses evapora on. Fig.251 The south and central Pacific during the neutral phase of ENSO.

The El Niño event

The El Niño phase of ENSO starts with a warming
of the ocean surface in the central and eastern
tropical Pacific Ocean. The Trade Winds, which
normally blow from east to west, are reduced or
even reversed; now blowing from west to
east. Rainfall decreases over Indonesia
and Australia. Warm surface water
flows across the Pacific Ocean,
accumula ng off the coast of
South America.
This warm water prevents
the upwelling of cold water
along this coast associated with Fig.252 The south and central Pacific during the El Niño phase of ENSO.
the Humboldt Current. Without the upwelling of cold,
nutrient rich waters, the food chains in these The warm water results in warmed air rising off its surface,
normally rich waters begin to break down, affec ng the crea ng an area of low pressure. This may bring heavy rain
shoals of anchovies, predatory fish, sea birds and sea lions. and flooding to Peru and Ecuador.

94

Drought can be devastating for people and the environment.