Abstract
The physical elements of the circulation of the Antarctic Circumpolar Current (ACC) are reviewed. A picture of the circulation is sketched by means of recent observations from the WOCE decade. We present and discuss the role of forcing functions (wind stress, surface buoyancy flux) in the dynamical balance of the flow and in the meridional circulation and study their relation to the ACC transport. The physics of form stress at tilted isopycnals and at the ocean bottom are elucidated as central mechanisms in the momentum balance. We explain the failure of the Sverdrup balance in the ACC circulation and highlight the role of geostrophic contours in the balance of vorticity. Emphasis is on the interrelation of the zonal momentum balance and the meridional circulation, the importance of diapycnal mixing and eddy processes. Finally, new model concepts are described: a model of the ACC transport dependence on wind stress and buoyancy flux, based on linear wave theory; and a model of the meridional overturning and the mean density structure of the Southern Ocean, based on zonally averaged dynamics and thermodynamics with eddy parametrization.
Highlights
75% of the world ocean volume has temperatures below 4°C, connected with only 2% of the ocean surface
A global perspective of the zonal balance is presented in the experiments which Bryan (1997) has performed with a non-eddy-resolving OGCM. In his findings the balance between zonal wind stress and bottom form stress prevails everywhere in the world ocean and likewise, we find in all experiments the approximate cancellation of the barotropic and baroclinic form stresses which individually are an order of magnitude larger than the wind stress
They are woven into the physics governing this extraordinary current, attempting to contribute to an answer of the most interesting and most urgent questions about the Antarctic Circumpolar Current (ACC): what is the balance of the zonal momentum; what mechanism and forcing functions determine the transport; how do watermasses and the substances they carry penetrate the strong and deep-reaching zonal flow? We base our discussion on the research on these topics which has accumulated in the last decade and partly answer these questions
Summary
75% of the world ocean volume has temperatures below 4°C, connected (at polar latitudes) with only 2% of the ocean surface. The ‘sliced cake’ view of the Southern Ocean watermasses and their propagation shown in Fig. 4 (Gordon 1999) presents this overturning circulation by the distribution of salinity and temperature of Antarctic Intermediate Water (AAIW), Circumpolar Deep Water. We will address this question of mass and property balances, which is closely connected to the question of the dynamical balance of the ACC (treated in Section 3): what are the forces driving the zonal current, which act as brake and what physical mechanisms are responsible for the deep reaching current profile?. We should point out that this review does not cover all the current research on the ACC, e.g. we do not address regional properties of the ACC system and its temporal variability; we not report on teleconnections and links of the Southern Ocean with the global ocean circulation and the possible dependence of the stratification and transport of the ACC on remote conditions and mechanisms
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