Walkar Circulation and ENSO | Atmospheric Circulation | Geography

In this article we will discuss about the concept of Walker circulation and El Nino southern oscillation (ENSO).

Certain variations are found from the atmos­pheric general circulation patterns e.g., surface trades, westerlies and polar winds circulation and tricellular meridional circulation. Circulation of local and sea­sonal (monsoon) winds may be cited example of such deviations. East-west zonal circulation of tropical winds is an important variant from general atmospheric circulation.

This typical east-west circulation of tropi­cal wind is called Walkar circulation named after fa­mous scientist G.T. Walkar. In fact, Walkar circulation is a convective cell of air circulation, which is formed due to the development of pressure gradient from east to west in the equatorial Pacific Ocean.

After two-three years this general condition of east-west pressure gra­dient is reversed i.e., pressure gradient becomes from west to east (fig. 35.19B). Thus, there are oscillations in pressure gradient and air circulation after the inter­vals of 2-3 years. Walkar called such oscillation as southern osicllation.

Walkar circulation and southern oscillations are driven by the sea surface pressure gradient from the equatorial eastern Pacific ocean (near the western coastal areas of South America, to the equatorial west­ern Pacific ocean (near S-E Asian coasts). In normal conditions high pressure develops on the sea surface of the equatorial east Pacific ocean and the western coastal lands of south America (fig. 35.19A) due to subsidence of air from above and upwelling of cold oceanic water.

On the other hand, low pressure is formed in the equatorial western Pacific ocean due to rise of air from the warm sea surface. This pressure gradient from east to west generates east-west circulation of trade winds on the surface while there is reverse upper air circula­tion i.e., from west to east, (fig. 35.19A) which completes a convective cell. This east-west air circulation drives the ocean water mass from the western coast of South America towards the west.

This phenomenon facili­tates upwelling of cold sea water near the coasts of Peru and Equador resulting in further cooling of air, high air pressure, atmospheric stability and dry weather condition. Contrary to this, east-west air circulation becomes warm north-east trades in the equatorial west Pacific ocean where it, after being heated, rises up­ward, becomes unstable and causes precipitation.

Af­ter rising to certain height it turns eastward and de­scends in the equatorial eastern Pacific ocean to com­plete the convective cell (fig. 35.19A). This is now evident that tropical eastern and western Pacific is characterized by dry and wet weather conditions re­spectively.

By October-November the low air pressure of the tropical western Pacific is shifted to the tropical eastern Pacific causing weakening of trade winds. This reversal in pressure condition facilitates the return of warm sea water which was driven from the coasts of South America westward, towards the tropical east Pacific. Consequently, low air pressure is formed in the south-east Pacific mainly off the coasts of South America (Equador and Peru), upwelling of cold seawater is stopped, warm air rises upward and becomes unsta­ble and ultimately yields rainfall after condensation.

It is evident that the general normal condition (fig. 35.19A) has got reversed (35.19B). This event is called El Nino penomenon. The rising air in the east Pacific cools above and turns westward in the troposphere and ultimately descends in the tropical west Pacific giving birth to high pressure which drives warm air towards the coasts of South America.

Thus, again a complete convective cell is formed. Such condition is called El Nino-Southern Oscillation Event (ENSO Event). In fact, changes in the positions of air pressure in the tropical eastern and western Pacific are called southern oscilla­tions. During El Nino event Walkar circulation is weakened due to the development of equatorial west­erlies on sea surface (fig. 35.19B) but Hadley circulation is activated.

This phenomenon again acti­vates trade winds which again drive sea-water of the tropical eastern Pacific westward resulting in the upwelling of cold water from below, weakening of El Nino event and re-establishment of normal condition (fig. 35.19A).