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El Nino A warm ocean current setting south along the coast of Ecuador, so called because it generally develops just after Christmas. In exceptional years, concurrently with a southerly shift in the tropical rain belt, the current may extend along the coast of Peru to 12° S. See references and notes in ART-24. This is an important concept for understanding how the weather patterns of the world are related to its ocean circulation, but from a practical point of view, we will always have our maps to tell us what is happening for the moment. The issue is more one of climatology. In modern meteorology, the El Nino is intimately tied to the so called Southern Oscillation Index (SOI). As used by Sir Gilbert Walker, a single number, empirically derived, which represents the distribution of pressure and temperature over a wide ocean area. Basically, the process is one of weighting pressure and temperature values for selected island and coastal stations, and algebraically combing them. These numbers were originally employed in correlations with single station values. Three such "oscillations" were derived: the North Atlantic oscillation; the North Pacific oscillation; and the southern oscillation.

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