Baroclinically Unstable Geostrophic Turbulence in the Limits of Strong and Weak Bottom Ekman Friction: Application to Midocean Eddies

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Abstract
This paper examines the plausibility of mesoscale eddy generation through local baroclinic instability of weak midocean gyre flows. The main tool is a statistically steady, two-layer quasigeostrophic turbulence model driven by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction. A wide range of friction strengths is investigated. In the weakly damped limit, flow is nearly barotropic, and the horizontal length scale of barotropic energy increases with decreasing friction, consistent with previous studies. The strongly damped limit, explored here for the first time, is equivalent barotropic (lower-layer velocities are nearly zero) and features an increase in the horizontal scale of potential energy with increasing friction. Current-meter data suggest that midocean eddies lie between the barotropic and equivalent barotropic limits. In accord with this suggestion, the moderately damped regime of the model compares well to observations of eddy amp... Abstract This paper examines the plausibility of mesoscale eddy generation through local baroclinic instability of weak midocean gyre flows. The main tool is a statistically steady, two-layer quasigeostrophic turbulence model driven by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction. A wide range of friction strengths is investigated. In the weakly damped limit, flow is nearly barotropic, and the horizontal length scale of barotropic energy increases with decreasing friction, consistent with previous studies. The strongly damped limit, explored here for the first time, is equivalent barotropic (lower-layer velocities are nearly zero) and features an increase in the horizontal scale of potential energy with increasing friction. Current-meter data suggest that midocean eddies lie between the barotropic and equivalent barotropic limits. In accord with this suggestion, the moderately damped regime of the model compares well to observations of eddy amp...