Sir Gilbert Walker provided an important clue concerning El Niño when he discovered that air pressures at sea level in the South Pacific seesaw back and forth between two distinct patterns. In the "high index" phase of what Walker referred to as the "Southern Oscillation" (upper map for November 1988) pressure is higher (darker red) near and to the east of Tahiti than farther to the west of Darwin. The east-west pressure difference along the equator causes the surface air to flow westward, as indicated by the long arrow. When the atmosphere switches into the "low index" phase (lower map for November 1982) barometers rise in the west and fall in the east, signalling a reduction, or even a reversal of the pressure difference between Darwin and Tahiti. The flattening of the seasaw causes the easterly surface winds to weaken and retreat eastward as shown. We now know that the "low index" phase is usually accompanied by El Niño conditions.

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