Q. J. R. Meteorol. SOC. (1997), 123, pp. 519-526 Similarities of the Deacon cell in the Southern Ocean and Ferrel cells in the atmosphere By D. J. KAROLY1*, P. C. McINTOSH', P. BERRISFORD3, T. J. McDOUGALL' and A. C. HIRSTZ 'Hadley Centre for Climate Prediction and Research, Meteorological Ofice, UK 2Commonwealth Scientific and Industrial Organization, Australia University of Reading, UK (Received 20 March 1996; revised 24 June 1996) SUMMARY 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 Southem 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. KEYWORDS: Indirect circulations Oceanic eddies Transient weather systems 1. ZONAL-MEAN MERIDIONAL CIRCULATION IN HEIGHT COORDINATES A simplified picture of the mean meridional (north-south) circulation in the ocean and in the at- mosphere can be obtained by averaging the meridional flow over time and around a latitude circle at constant height to give the time-mean zonal-mean circulation as a function of latitude and height. This representation of the flow at fixed locations is often called the Eulerian mean flow. It is common to rep- resent the mean meridional flow using a streamfunction $, where the flow is parallel to contours of the streamfunction and proportional to the gradient of the streamfunction. This approach has been used to represent the zonal-mean meridional circulation in the atmosphere for many years and is now being used with data from global ocean models (Manabe et nl. 1990; DOOs and Webb 1994). Figure 1 shows the Eulerian mean meridional mass-transport streamfunction in the atmosphere in the two extreme seasons (from eight years of operational weather analyses from the Meteorological Office (MO)) and in the Southern Ocean (from one year of simulation with the FRAMt high resolution ocean model (FRAM Group 1991)). We use the FRAM model simulation as this is an eddy-resolving ocean model and there are no global ocean observational data-sets which resolve ocean eddies. The annual cycle in the meridional circulation is marked in the atmosphere but is weaker in the ocean (Doos 1996). The major features of the meridional circulation in the atmosphere are the Hadley cells, with maximum ascent in the summer tropics and descent in the winter subtropics, and the reversed Ferrel cells, with ascent in high latitudes and descent in the subtropics (Lorenz 1967; Holton 1992). In the Southern Ocean, a major meridional circulation is the Deacon cell, with descent around 35"s and ascent around 55"S, reaching depths in excess of 2000 m (Doos and Webb 1994). The Hadley cells are direct meridional circulations, with ascent in the tropics in regions of mean atmospheric-heating and sinking motion in regions of net cooling. However, the Ferrel cells and the Deacon cell are indirect or reversed circulations, with apparent ascent or descent across strong density-gradients in regions of no apparent mean density-forcing. To understand the mean meridional circulation in the atmosphere and the ocean, it is useful to consider the equations for fluid motion on the earth after averaging around a latitude circle (zonal mean). We consider local Cartesian coordinates (x, y, z) on the surface of the earth, with x eastward, y northward and z upward, and fluid velocity (u, v, w). Let s' = s - S be the departure of an arbitrary variable s from its zonal-mean value S. The equation for zonally averaged density may be written * Corresponding author, present address: Cooperative Research Centre for Southem Hemisphere Meteorology, Monash University, Clayton, VIC 3168, Australia. Fine ResolutionAntarctic Model. 519