The production of Antarctic Bottom Water is influenced by Ice Shelf Water which is formed due to the modification of shelf water masses under huge ice shelves. The coupling of inflow conditions, thermohaline processes at the ice shelf base and the sub-ice shelf circulation is studied with a two-dimensional thermohaline circulation model which has been developed for a section perpendicular to the ice shelf edge. Different boundary conditions appropriate to the Filchner Ice Shelf regime are considered. The model results indicate that, in general, shelf water is transported toward the grounding line, where at the ice shelf base melting occurs with a maximum rate of 1.5 my−1. Accumulation of ice takes place at the end of the melting zone close to the ice shelf edge with a rate on the order of 0.1 my−1. The location of this accumulation zone determines whether or not the density increase by salt rejection causes an upper circulation cell and the separation of the modified water mass from the ice shelf base at mid-range depth. At the ice shelf edge the simulated temperature, salinity, helium and δ18O values for the temperature minimum layer are typical for Ice Shelf Water. However the sub-ice shelf circulation is highly variable as well as sensitive to changes in boundary conditions. Moderate changes in the characteristics of the inflowing water or in sea-floor topography may double the intensity of the circulation. Non-linear processes in the accumulation zone cause variabilities which can be described by an ice shelf edge oscillator influencing the entire circulation regime.

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.

The source function describing the energy transfer between the components of the internal wave spectrum due to nonlinear interactions is derived from the Lagrangian of the fluid motion and evaluated numerically for the spectral models of Garrett & Munk (1972a, 1975). The characteristic time scales of the transfer are found to be typically of the order of some days, so that nonlinear interactions will play an important role in the energy balance of the wave field. Thus implications of the nonlinear transfer within the spectrum for generation and dissipation processes are considered.

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We present a review of fl spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations. A specific technique is then designed and applied to the North Atlantic part of Levitus' (1982) climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat, salt, and potential vorticity. These quantities are determined on a 1 ø grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms. The estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters. The resulting circulation pattern bears strong resemblance to the classical view of the North Atlantic circulation as put foreward by Wrist (1935) and Defant (1941). The upper layers are dominated by the Gulf Stream/North Atlantic current system with a broad subtropical gyre recirculation. In the lower layer a western boundary current is fed from Norwegian Sea overflow penetrating the Gibbs fracture zone and partly circulating around the Labrador Sea. As a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities, in particular in the upper layer. The vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of Defant (1941). Diffusion coefficients were determined fo• an upper layer (depth of mixed layer to 800 m depth) and a lower layer (800 m to 2000 m). The spatial pattern of these coefficients correlates with maps of eddy activity, showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre. Average values in the lower layer of the quiet regions are 10 -5 m2/s and 102 m2/s for the diapycnal and isopycnal diffusivity, respectively, and 10-• m2/s for the vertical diffusivity of vorticity (which yields 102 m2/s for the lateral diffusivity of potential vorticity). Toward the regions of strong currents and in the upper layer these values roughly increase by an order of magnitude.

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