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Carlowen A. Smith, Kevin G. Speer, and Ross W. Griffiths

Abstract

A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. A topographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (RoT ~ 10−3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius L ρ is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave–turbulence interaction on a β plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup.

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Kial D. Stewart, Graham O. Hughes, and Ross W. Griffiths

Abstract

The role of externally imposed rates of small-scale mixing in an overturning circulation forced by differential surface buoyancy fluxes is examined in a laboratory experiment. The circulation occupies the full volume and involves a dense turbulent plume against the endwall and a broad upwelling throughout the interior. For strong externally imposed stirring, turbulent diffusion is the primary means of vertical density transport in the flow, and the dependence of the equilibrated circulation on the mixing rate accords with a theoretical model; the overturning rate increases as the ¼ power of the turbulent diffusivity. For weak externally imposed stirring, advection is the dominant mechanism of vertical density transport, and the circulation is independent of the rate of external stirring. The rate of vertical density transport is parameterized as a bulk diffusivity obtained from different methods, including one from a Munk-like advection–diffusion balance and another from the transport of buoyancy across the surface. For strong stirring, the bulk diffusivities returned by the various methods agree with the externally imposed mixing rate. However, the parameterizations implicitly include a nondiffusive component of vertical transport associated with advection of the density field and it is shown that, for weak stirring, the bulk diffusivities exceed the externally imposed mixing rate. For the oceans, results suggest that the primary effect of mixing (with energy sourced from winds, tides, and convection) is to deepen the thermocline, thereby influencing the entrainment and consequent vertical transport of density in the dense sinking regions. It is concluded that this advective transport of density, and not vertical mixing, is crucial for coupling the surface to the abyss.

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Graham O. Hughes, Andrew Mc C. Hogg, and Ross W. Griffiths

Abstract

The overturning circulation of the global oceans is examined from an energetics viewpoint. A general framework for stratified turbulence is used for this purpose; first, it highlights the importance of available potential energy in facilitating the transfer of kinetic energy to the background potential energy (defined as the adiabatically rearranged state with no motion). Next, it is shown that it is the rate of transfer between different energy reservoirs that is important for the maintenance of the ocean overturning, rather than the total amount of potential or kinetic energy. A series of numerical experiments is used to assess which energy transfers are significant in the overturning circulation. In the steady state, the rate of irreversible diapycnal mixing is necessarily balanced by the production of available potential energy sourced from surface buoyancy fluxes. Thus, the external inputs of available potential energy from surface buoyancy forcing and of kinetic energy from other sources (such as surface winds and tides, and leading to turbulent mixing) are both necessary to maintain the overturning circulation.

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Catherine A. Vreugdenhil, Andrew McC. Hogg, Ross W. Griffiths, and Graham O. Hughes

Abstract

The relative roles of advective processes and mixing on the temporal adjustment of the meridional overturning circulation are examined, in particular the effects of mixing in either the abyssal or upper ocean. Laboratory experiments with convectively driven overturning and imposed stirring rates show that the circulation adjusts toward an equilibrium state on time scales governed by mixing in the upper boundary layer region but independent of the mixing rate in the bulk of the interior. The equilibrium state of the stratification is dependent only on the rate of mixing in the boundary layer. An idealized high-resolution ocean model shows adjustment (of a two-cell circulation) dominated primarily by the advective ventilation time scale, consistent with a view of the circulation determined by water mass transformation occurring primarily near the surface. Both the experiments and the model results indicate that adjustments of the circulation are controlled by surface buoyancy uptake (or rejection) and that the nonequilibrium circulation is dominated by advective processes, especially if the average abyssal ocean diffusivity is less than 3 × 10−5 m2 s−1.

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