In vertical cross section, the Pacific may be likened to an asymmetrically heated basin with rising air and precipitation in the west, and the reverse in the east.

What happens when this system is perturbed? Anoma­lies in ocean forcing by the winds could initiate Kelvin and Rossby waves. The former, travelling eastward along the equatorial wave guide, deepen the thermocline in the eastern Pacific, interrupting normal upwelling, and pro­ducing the anomalously warm sea-surface temperatures characteristic of El Nino; the latter seem to propagate to higher latitudes, with implications for temperate zone climate. The impacts of El Nino on biological systems such as fisheries have attracted some attention.

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El Nino was originally described purely as a local biological anomaly. There is no strong evidence that El Nino alone had been responsible for major long-term changes in the highly productive Humboldt Current ecosystem in recent decades. Rather, a complex set of interactions among physical, biological, and social/economic factors may be operating. Between the 1960’s and the 1970’s, the fisheries saw a major collapse in anchoveta landings, and a shift from an anchoveta dominated system to a mixed system of sardines and other pelagic fishes. El Nino has clearly demonstrable economic effects on the fisheries.

The impact of El Nino depends on the existing state of the ecosystem, and may have highly signifi­cant second, third, or higher order effects.