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9.12: Impacts of ENSO Cycles

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    Impacts of ENSO Cycles

    ENSO cycles [El Niño-La Niña Cycles] consist of shifting weather and oceanographic conditions in the tropical Pacific region (refer to Figures 9-24 and 9-25).

    During El Niño:

    High and low atmospheric pressures systems reverse across the equatorial Pacific region. As a result the Walker Cell circulation pattern is very weak.
    • Winds become slack or blow against the west-moving Equatorial Current.
    • The west-moving Equatorial Current mounds warm water on eastern side of Pacific Basin. near Australia and Indonesia.
    • Along the coast of South America, a normally thin temperate thermocline replaced with a thick tropical thermocline.
    • This thick thermocline prevents mixing of deep cold nutrient rich water because of the buoyancy of extra warm surface water.
    • The tropical thermocline shuts down upwelling currents that would otherwise provide nutrients to the base of the food chain in shallow ocean waters, resulting in a collapse of marine fisheries offshore (often resulting in economic and ecological catastrophe along South America's west coast).
    • During El Niño, the warm conditions typically arrive around Christmas, so El Niño refers to the Christ Child in Peruvian weather— El Niño conditions offshore results in both warm and wet conditions on land.

    During La Niña:

    • The Walker Cell circulation intensifies across the equatorial Pacific region.
    • This increase in windy weather condition pulls the thick warm waters away from the coast of South America.
    • As a result, there is increased cooling and more upwelling along the coast, enhancing ocean productivity.
    • Cool conditions offshore results in persisting drought conditions on land in South America.

    Global significance of ENSO cycles:

    These fluctuating cycles of ocean surface water temperatures influence climate factors (warm/wet or cool/dry) conditions around the entire Pacific Basin, if not the entire world. Monitoring for El Niño is conducted by:
    • studies of wind speed and direction on the Equatorial regions.
    • monitoring high and low pressure systems on the Equatorial regions.
    • monitoring water temperature changes on Equatorial regions, mainly warming on east side of Pacific Basin.
    • measuring water heights (mounding) above average sea level along the Equator.

    ENSO Impacts on Coastal California

    During El Niño periods, California's coastal ocean waters are warmer, and a more well-developed thermocline hinders coastal upwelling. This reduces the nutrient supply for sea life, so marine specie either adapt and migrate elsewhere, or in many cases, loose populations due to competition for limited food resources. Southern California typically gets heavier winter rainy periods because the southern tropical jet stream move north from the Central America region. As a result, Southern California gets more tropical moisture which can translate to increased rainfall if conditions are right.

    During La Niña periods, California's coastal ocean waters are cooler, only a weak thermocline can develop. As a result, there is stronger and well developed coastal upwelling. As a result, more food is available, and marine life flourishes in coastal waters. Colder waters offshore translate to drier conditions on land.

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