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Creating an
Earth System I

Creating an Earth System:

El Niņo

El Niņo is the result of an on-going "dialog" between the ocean and atmosphere in the tropical Pacific Ocean. It's part of a natural, combined oceanic-atmospheric cycle referred to as El Nino-Southern Oscillation (ENSO)

  • El Niņo is one extreme (warm phase)

  • La Niņa is the other extreme (cold phase)

"Normal" Patterns in the Pacific

1. In normal years when there is no El Niņo or La Niņa, atmospheric pressure is greater in the eastern Pacific than in the western Pacific. Because wind flows from higher to lower pressure, the trade winds blow from east to west.

2. Warm water "piles up" in the western Pacific and sea level is higher (by ~ 30 cm) than it is in the east.

3. Upwelling of cold, nutrient-rich water occurs along the western coast of South America.
4. In some years, this pattern intensifies, so that sea-surface temperatures are colder than usual in the central and western Pacific. This condition is referred to as La Niņa and is similar to normal patterns, except that circulation is increased and convection is enhanced over Indonesia.

Southern Oscillation

The Southern Oscillation (S0) is an irregular "see-saw" in which atmospheric pressure and wind patterns shift across the Pacific. When normally high pressure in the eastern Pacific decreases and normally low pressure over Australia and northern Indonesia rises, conditions are right for an El Niņo event to develop.

As warm water shifts eastward, so do the convection and heavy rains caused by the increased buoyancy of air warmed by the underlying water. As warm water piles up in the east, upwelling of cold, nutrient-rich water is inhibited.

Latent heat of condensation further warms the air, which further decreases atmospheric pressure in the east, etc. The thunderstorms that have shifted from the western to the central and eastern Pacific disrupt high-level jet stream circulation by pumping warm air and moisture high into the atmosphere. This has a far-reaching effect on weather patterns.

Hazard/Impact

Although El Niņo (and La Niņa) are generated in the tropical Pacific, their effects are felt all over the world. The process by which Earth system events in one location are related to events in a different part of the world is called teleconnections.

  • Because each El Niņo event is different, their effects vary.
  • At temperate latitudes, the effects show up most clearly during the winter.

General impacts:

  • Heavy rains on islands of Pacific and the west coast of South America
  • Drought in Australia, Indonesia, and the Philippines
  • Warmer-than-normal winters in northern US and Canada
  • Drought in Africa and India
  • Weakening of Atlantic hurricanes
  • Greater precipitation in SW United States

Impact on California

El Niņo's storm track affects the location of jet streams, which are a major factor in producing winter weather patterns at mid-latitudes. Instead of coming ashore in the Pacific Northwest as usual, the southern jet stream hits California, carrying moisture and storms. In general, the effect of El Niņo on California is increased rainfall with accompanying floods, landslides, and coastal erosion. The effects are variable across the state and are more predictable in Southern California.

Effects of El Niņo on the Biosphere

  • Diminished upwelling of cold, nutrient-rich water upon which phytoplankton depend. This affects fish, birds, etc.
  • Coral bleaching
  • Human health: famine, water pollution, diseases such as malaria, dengue fever, and cholera

All images courtesy of USGS and ENSO (http://www.pmel.noaa.gov)

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