The El Niño-Southern Oscillation is a cycle of
wind and ocean temperatures in the tropics. Ordinarily, the trade winds blow
westward across the Pacific, building up large mass of very warm water to the
east of the Philippines. This large-scale wind circulation is called the Walker
circulation (named in honor of the mathematician Gilbert Walker), and results
from cooler, higher-pressure air in the east moving toward warmer,
lower-pressure air in the west. The east-to-west trade winds drive a surface
current which brings water toward the western edge of the basin. But this
pressure differential, and the associated winds, is not constant.
The Pacific Walker circulation has three modes: warm (El Niño),
neutral, and cool (La Niña). In warm mode, the east-west pressure differential shrinks,
the trade winds weaken, and the warm water of the western Pacific flows
eastward and impacts the western coasts of Central and South America. In
neutral mode, the trade winds are at average strength, and the eastward flow of
warm water is limited. In cool mode, the pressure differential is greater, the
trade winds are stronger, and warm water is pushed farther west than average.
El Niño received its name because the effects are typically felt
beginning around Christmas, and last until April. They include heavy rain along
the west coast of Central and South America, and wet weather in California and
along the gulf coast and southeastern US. The warm water shuts down the normal
upwelling of cold water along those coasts, harming fisheries. During La Niña
events, storm tracks are more northerly, so the Pacific Northwest is wetter, as
well as the upper northwest and Great Lakes of the US, while from California to
the southeast it tends to be dry and warm. Upwelling along the west coast of
South America is strong and fisheries there tend to flourish.
The Southern Oscillation is a related atmospheric cycle,
monitored across the Pacific. In El Niño years, pressure to the west
(measured at Darwin, Australia), on the boundary between the Pacific and Indian
Oceans, is greater (negative index), and pressure in the central Pacific (at
Tahiti) is lower. La Niña episodes feature higher pressure in Tahiti and lower
pressure in Australia (positive index).
Tomorrow: the structure of the ocean.
Be well!
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