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Keywords:

  • Indirect circulations;
  • Oceanic eddies;
  • Transient weather systems

Abstract

The meridional circulation in the ocean and the atmosphere, when averaged over longitude and time at constant height, shows a number of cells. Most of these appear as direct circulations, with ascent in response to forcing which reduces the density. There are several indirect circulations, particularly the Deacon cell in the Southern Ocean and the Ferrel cells in the mid-latitude atmosphere, which appear to act against the mean density-gradient in regions of no apparent mean density-forcing. When the zonal-mean circulation is calculated in density coordinates, both the Deacon cell and the Ferrel cells disappear. A transformation of the zonal-mean circulation as a function of height is used to give the residual mean circulation, which is remarkably similar to the zonal-mean circulation in density coordinates in both the Southern Ocean and the atmosphere. This shows that the existence of the Deacon and Ferrel cells is the result of correlations of zonal variations of density and meridional flow, and not of zonal-mean density-forcing. Zonal variations associated with the time-mean eddies in the Southern Ocean are the main contributors to the Deacon cell, while correlations in transient weather systems are the major factor leading to Ferrel cells.