Walker circulation

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.

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.[1] These changes in surface temperature reflect changes in the depth of the thermocline.[2]

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.[3]

The Walker circulation was discovered by Gilbert Walker. The term "Walker circulation" was coined in 1969 by the Norwegian-American meteorologist Jacob Bjerknes.[4]

  1. ^ Bureau of Meteorology. "The Walker Circulation". Commonwealth of Australia. Retrieved 1 July 2014.
  2. ^ Zelle, Hein; Gerrian Appledoorn; Gerritt Burgers; Geert Jan Van Oldenborgh (2004). "Relationship Between Sea Surface Temperature and Thermocline Depth in the Eastern Equatorial Pacific". Journal of Physical Oceanography. 34 (3): 643–655. Bibcode:2004JPO....34..643Z. CiteSeerX 10.1.1.12.3536. doi:10.1175/2523.1. S2CID 16785385.
  3. ^ Xie, Shang-Ping (1 February 1998). "Ocean-Atmosphere Interaction in the Making of the Walker Circulation and Equatorial Cold Tongue". Journal of Climate. 11 (2): 189–201. Bibcode:1998JCli...11..189X. doi:10.1175/1520-0442(1998)011<0189:OAIITM>2.0.CO;2. JSTOR 26242917. INIST 2154325.
  4. ^ Bjerknes, J. (March 1969) "Atmospheric teleconnections from the equatorial Pacific," Monthly Weather Review, 97 (3) : 163–172. From pp. 167–168: "It seems reasonalble to assume that it is the gradient of sea temperature along the Equator which is the cause of the thermal circulation entered in figure 8. Hereafter, in the present article that circulation will be referred to as the "Walker Circulation" since it can be shown to be an important part of the mechanism of Walker's "Southern Oscillation." " Available at: N.O.A.A.