Download El Niño and La Niña revision notes

El Niño and La Niña revision notes
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El Niño and La Niña are the opposite extremes of the ENSO cycle. Changes in air
pressure and ocean temperatures in the Equatorial Pacific result in large-scale
changes in global weather and climate patterns.
Under normal conditions the air pressure at Darwin (in the west) is lower than at
Tahiti (in the east). The ocean is warmer in the west and cooler in the east.
The warm ocean phase, El Niño, occurs when there is higher air pressure in the
western Pacific, i.e. a reversal in the air pressure and in ocean temperature
The cold ocean phase, La Niña, occurs when there is lower air surface pressure than
normal in the western Pacific.
El Niño and La Niña episodes last about 9 - 12 months and occur about every 3 to 5
years. El Niño occurs more frequently than La Niña.
Normal Conditions
Low pressure in the east
Wet in the east
Higher pressure in the west
Colder temperatures and ocean in the
west
Trade winds blow off shore in the east
so cold, nutrient rich water upwells off
the South American coast, resulting in
productive fisheries and a more
shallow thermocline.
Trade winds push water over to west, which piles up in west. This results in a deeper surface
layer and a deeper thermocline.
El Niño Conditions
Higher pressure in the east
Dry in the east
Lower pressure in the west
Warmer temperatures and ocean in
the west
The trade winds weaken and may
reverse direction.
The normal flow of water away from
South America decreases and ocean water piles up (relatively) off South America.
The thermocline becomes deeper and upwelling decreases. The sea surface temperature
increases to higher than normal in the Eastern Pacific.
The rainfall is now more common in the Central Pacific while the Western Pacific becomes
dry. Heavy rainfall and flooding occur over Peru and drought over Indonesia and Australia.
The reduced up–welling, adversely affects fisheries off the coast of South America. The
tropical cyclones in the South Pacific shift eastward.
La Niña Conditions
Even lower pressure in the east
Wetter in the east
Even higher pressure in the west
Colder temperatures and ocean in
the west
Trade winds blow more strongly
Even shallower thermocline and
increased upwelling.
Lower than normal sea surface
temperatures.
During El Niño, New Zealand has:
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stronger or more frequent winds from the
west in summer
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drought in east coast areas and more rain in
the west.
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In winter, the winds are more from the
south, bringing colder conditions
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In spring and autumn south–westerly winds
are more common.
During La Niña, NZ has:
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More north–easterly winds occur
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Moist, rainy conditions to the north–east of
the North Island
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Reduced rainfall to the south and south–west
of the South Island.
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Some areas, such as central Otago and South
Canterbury, may have a drought
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Warmer than normal temperatures