Walker circulation

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Main article: Atmospheric circulation

The Walker circulation is an atmospheric circulation of air at the equatorial Pacific Ocean, responsible for creating ocean upwelling off the coasts of Peru and Ecuador. This brings nutrient-rich cold water to the surface, increasing fishing stocks. It was discovered by Jacob Bjerknes in 1969 and was named after the English physicist Sir Gilbert Walker who discovered the Southern Oscillation.

[edit] El Niño

Main article: ENSO

The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern pacific ocean, and a low pressure system over Indonesia. When the Walker circulation weakens or reverses, an El Niño results, causing the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all. An especially strong Walker circulation causes a La Niña, resulting in cooler ocean temperatures due to increased upwelling.

A schematic diagram of the quasi-equilibrium and La Niña phase of the southern oscillation. The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the western Pacific as the result of this motion. The water and air are returned to the east. Both are now much cooler, and the air is much drier. An El Niño episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific.
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A schematic diagram of the quasi-equilibrium and La Niña phase of the southern oscillation. The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the western Pacific as the result of this motion. The water and air are returned to the east. Both are now much cooler, and the air is much drier. An El Niño episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific.

A scientific study published in May 2006 in the journal Nature indicates that the Walker circulation has been slowing in recent decades. The authors speculate that global warming is a likely causative factor in the weakening of the wind pattern.

[edit] References

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