(^) Every 3 to 5 years, the balance changes, signaled months in advance by a decrease
in the Easter–Darwin pressure difference (Quinn et al. 1987). The trade winds
subside, the sloping sea surface begins to level, and warm water in the west flows
east, beginning an El Niño. The Coriolis effect pushes the flow toward the equator,
and it becomes a Kelvin wave traveling east along the equatorial wave guide. It
arrives in the eastern tropical Pacific, raising sea level and pushing down the cold
layers from which upwelling normally supplies nutrients to the surface. Then flow
divides to north and south, continuing along the coasts and held inshore by the
Coriolis effect. Actual water displacement can be a thousand kilometers, the displaced
mass pushing along water downstream from it and thus warming the coastal ocean as
far as Alaska in the north, central Chile in the south. El Niño events vary in intensity.
More or less water can move into the east, coupled with extended or shorter poleward
effects. Some recent events have been among the most dramatic: 1972, 1983, 1997–
1998.
(^) The “double” El Niño of 1982–1983 was notable for the rapid succession of two
events, the later one occurring about 6 months from the usual timing with peaks in
sea-surface temperature (SST) and sea level in Peru during May–June, not December
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