Walker Circulation Southern Oscillation- El Nino and La Nina

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By moderator July 8, 2019 16:52

El Nino and La Nina

For understanding the phenomena of El Nino and La Nina, first, we need to understand Walker Circulation. Trade winds, Westerlies, and Polar Easterlies form the general circulation patterns of the atmosphere. But there are some exceptions. One of the El Nino and La Nina is Walker Circulation. The typical East-West circulation of the tropical and equatorial winds comprises the Walker Circulation, named after G.T. Walker.

It is basically a convective cell of air circulation, most predominantly formed over the Pacific Ocean, which consists of easterly winds at the lower troposphere, westerly winds at the upper troposphere, rising motion over the western Pacific, and subsidence over the eastern Pacific. It is the result of pressure gradient development from east to west in the equatorial Pacific Ocean. This general condition of the east to west pressure gradient is reversed after every two to three years. This kind of oscillations in the pressure gradient, causing reversals of winds also, is termed as Southern Oscillation.

Ocean-Atmosphere Coupling

Bjerknesinterpreted the Walker Circulation as an atmospheric circulation driven by the gradient of sea surface temperature along the Equator and suggested that the characteristics of the Walker circulation were largely determined by the coupling between the tropical atmosphere and oceans.

In normal conditions, there is high pressure on the sea surface of the equatorial eastern Pacific Ocean and western coastal lands of South America. These regions witness upwelling of cold oceanic water and subsidence of air from above. This is the time when low pressure is formed over the western equatorial Pacific Ocean due to the rise of air from the warm sea surface. The pressure gradient is from east to west causing easterly circulation of trade winds and a reverse westerly upper air circulation (as shown in the figure below). Due to east-west air circulation, the warm water pool over gets driven away from the western coast of South America towards the west. The upwelling over the Peruvian and Ecuador coasts causes further cooling of the air, high air pressure, atmospheric stability, and dry weather condition. On the contrary, the equatorial western Pacific Ocean witnesses atmospheric instability and precipitation.

Read Also: Cyclone Vayu

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El-Nino

The term El Niño translates from Spanish as ‘the boy-child’ or ‘the Christ Child’. Peruvian fishermen originally used the term to describe the appearance, around Christmas, of a warm ocean current off the South American coast. During an El Nino, a relaxation of the lower level easterlies, signaling a weakening of the Western Circulation is accompanied by weaker upwelling in the eastern Pacific.

The low air pressure of the tropical western Pacific is shifted to the tropical eastern Pacific causing weakening of trade winds. Around the coasts of Peru and Ecuador, the upwelling of cold seawater stops along with the formation of low air pressure. The warm air rises above causing rainfall after condensation. The upper air pattern also gets reversed and follows a westerly flow from Eastern Pacific finally descending in the Western Pacific regions.

During an El  Nino, the weakening Walker circulation causes widespread drought in Indonesia/maritime continent, drought in northeastern   Brazil, severe floods in Peru and Ecuador, and in south-easternBrazil and northern Argentina. It also affects Indian Monsoon negatively and average monsoonal rainfall in India decreases. These changes in the Pacific Ocean and its overlying atmosphere occur in a cycle known as the El Niño–Southern Oscillation (ENSO).

el-nino

La-Nina

La Niña translates as ‘girl-child’ and is the opposite ENSO phase to El Niño. During a La Nin ̃a, theWalker Circulation intensifies and leads to rainfall anomalies with reverse sign compared to El Nino. Places like Indonesia and Australia can get much more rain than usual. However, the cold water in the eastern Pacific causes fewer rain clouds to form there. So, places like the southwestern United States can be much drier than usual. Generally, La Nina years are marked with a good Indian Monsoon also.

la-nina

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moderator
By moderator July 8, 2019 16:52