Eddy Lateral Vorticity Transport and the Equilibrium of the North Atlantic Subtropical Gyre

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  • 1 Department of Meteorology and Physical Oceanography, MIT, Cambridge. MA 02139
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Abstract

Oceanic gyres defined by the mean zero wind-stress curl lines have been the focus of wind-driven ocean circulation theory since its beginnings. In the face of single-signed vorticity input from the curl of the wind stress over a gyre, a mechanism to balance the wind vorticity input is required if an equilibrium is to be established. Traditional models have tended to restrict their allowable physics so that a dissipation mechanism is required for equilibrium. However, dissipation is not necessary, in principle, for equilibrium for a general fluid system. In fact, it has recently been shown that lateral eddy vorticity transport between gyres can provide an important part of the vorticity tendency required for equilibration in model oceanic systems. This note examines the possibility that in the North Atlantic subtropical gyre this process also might be important. After a brief review of the equilibration mechanisms possible in a primitive equation fluid, attention is focussed on estimates of the eddy lateral transport of relative vorticity in the North Atlantic. It appears that there could be sufficient eddy transport across the Gulf Stream to balance the wind vorticity input over the gyre. Equilibrium might thus be possible, without any vorticity dissipation mechanism or without invoking higher order dynamical processes. If this mechanism is important in the ocean there should be interesting effects on the subpolar gyres, details of which depend on the circumstances surrounding the vorticity exchange process.

Abstract

Oceanic gyres defined by the mean zero wind-stress curl lines have been the focus of wind-driven ocean circulation theory since its beginnings. In the face of single-signed vorticity input from the curl of the wind stress over a gyre, a mechanism to balance the wind vorticity input is required if an equilibrium is to be established. Traditional models have tended to restrict their allowable physics so that a dissipation mechanism is required for equilibrium. However, dissipation is not necessary, in principle, for equilibrium for a general fluid system. In fact, it has recently been shown that lateral eddy vorticity transport between gyres can provide an important part of the vorticity tendency required for equilibration in model oceanic systems. This note examines the possibility that in the North Atlantic subtropical gyre this process also might be important. After a brief review of the equilibration mechanisms possible in a primitive equation fluid, attention is focussed on estimates of the eddy lateral transport of relative vorticity in the North Atlantic. It appears that there could be sufficient eddy transport across the Gulf Stream to balance the wind vorticity input over the gyre. Equilibrium might thus be possible, without any vorticity dissipation mechanism or without invoking higher order dynamical processes. If this mechanism is important in the ocean there should be interesting effects on the subpolar gyres, details of which depend on the circumstances surrounding the vorticity exchange process.

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