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The Rest of the Story: Oscillations in Climate Triggered by El Niño

by Stephen Marshak

Long before the modern science of meteorology became established, fishermen from Peru and Ecuador who ventured into the coastal waters west of South America knew that in late December the fish population that provided their livelihood diminished. Because of the timing of this event, it came to be known as El Niño, Spanish for "the Christ child." Why did the fish vanish? Fish are near the top of a food chain that begins with plankton, which live off nutrients in the water. These nutrients increase when cold water upwells from the deep along the coast of South America. During El Niño, warm water currents flow eastward from the central Pacific, and the cold, nutrient-rich water that supports the marine food chain remains at depth. With less nutrients, there are less plankton, and without the plankton, the fish migrate elsewhere to find food.

To understand why El Niño occurs, we need to look at atmospheric flow and related surface ocean currents in the equatorial Pacific. Normally (when El Niño is not in progress), a major equatorial low-pressure cell exists in the western Pacific over Indonesia and Papua New Guinea, while a high-pressure cell forms over the eastern Pacific, along the coast of equatorial South America. This geometry means that a large east-west convection cell exists in the troposphere over the equatorial Pacific—air rises in the western Pacific, flows east, sinks in the eastern Pacific, and then flows west at the surface. The easterly surface winds blow warm surface water westward, so that it pools in the western Pacific. Cold water from the deep ocean rises along South America, to replace the warm water that moved west. It is this rising cold water that brings nutrients to the surface. During El Niño, the low-pressure cells moves eastward over the central Pacific, and a high-pressure cell develops over Indonesia; so two convective cells develop. As a result, surface winds starts to blow east in the western Pacific, driving warm surface water back to South America. This warm surface water prevents deep cold water from rising, sending the fish away. In effect, pressure cells oscillate back and forth across the Pacific, an event now called the southern oscillation.

El Niño gained world notoriety in late 1982 and early 1983, when a particularly large low-pressure cell developed in the eastern part of the Pacific, and surface water currents flowed eastward with vigor, some spreading to the north and south of the equator; as a result, the jet streams stayed farther north than is typical. In effect, El Niño caused a temporary climate change worldwide. Drought conditions persisted in the normally rainy western Pacific, while unusually heavy rains drenched western South America. In North America, rains swamped the southern United States and storms battered California, winters were warmer that usual in Canada, and snowfalls were heavier in the Sierra Nevada Mountains, leading to spring floods.

Climatologists have been working intensely to understand the periodicity of El Niño in the past, and have studied such features as growth rings on trees, shells, and corals to learn when El Niño conditions have occurred. It is clear that strong El Niño conditions take place around once every four years, with even stronger ones possibly happening at other intervals. Meteorologists have come to refer to only the strong events with global climatic effects as El Niño, and intervening "normal" years as La Nina. Whether the oscillation between El Niño and La Nina is changing because of an increase in global temperature remains a subject of debated (see Chapter 23).

For more information see: http://www.elnino.noaa.gov

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