Walker circulation

The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical directions. This circulation, which is roughly consistent with observations, is caused by differences in heat distribution between ocean and land. In addition to motions in the zonal and vertical direction the tropical atmosphere also has considerable motion in the meridional direction as part of, for example, the Hadley Circulation.

The Walker circulation is associated with the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, and a low pressure system over Indonesia. The Walker circulations of the tropical Indian, Pacific, and Atlantic basins result in westerly surface winds in northern summer in the first basin and easterly winds in the second and third basins. As a result, the temperature structure of the three oceans display dramatic asymmetries. The equatorial Pacific and Atlantic both have cool surface temperatures in northern summer in the east, while cooler surface temperatures prevail only in the western Indian Ocean. These changes in surface temperature reflect changes in the depth of the thermocline.

Changes in the Walker circulation with time occur in conjunction with changes in surface temperature. Some of these changes are forced externally, such as the seasonal shift of the sun into the Northern Hemisphere in summer. Other changes appear to be the result of coupled ocean-atmosphere feedback in which, for example, easterly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by the southerlies. From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. Were the Earth climate symmetric about the equator, cross-equatorial wind would vanish, and the cold tongue would be much weaker and have a very different zonal structure than is observed today.

The term "Walker Circulation" refers to Gilbert Walker who discovered the Southern Oscillation during the early twentieth century. The term was first used in 1969 by the Norwegian-American meteorologist Jacob Bjerknes to refer to his proposed east-west overturning circulation close to the Equator in the Pacific. The region of sinking air, which he called the arid zone, was flanked to the north and south by the cloud bands of the Inter-Tropical Convergence Zones. These he proposed were "the equatorial edges of the Hadley Circulation of the Northern and Southern Hemispheres".

As originally proposed, the Walker Circulation is weaker during El Niños, when the temperature gradient across the Pacific and the strength of the easterlies are both reduced, and stronger during La Niñas, when the opposite is true. In modern use, the term "Walker Circulation" may refer to a much greater range of latitudes and longitudes, and may include more than one region of overturning circulation.