The Reddy maker

A new mechanism for the formation of high-amplitude anticyclonic eddies (lenses) from outflows emptying into the ocean at mid-depth is proposed. The essence of the new mechanism is that, in order for an inviscid outflow to exist as a continuous (uninterrupted) current, the condition g′S/f>α(g′H)1/2 [where g′ is the “reduced gravity”, S the bottom slope, f the Coriolis parameter, α a coefficient of order unity whose value depends on the outflowʹs potential vorticity (it is 2 for a zero potential vorticity outflow and unity for a uniform potential vorticity) and H the maximum thickness] must hold. When the above condition is not met, i.e., when g′S/f<α(g′H)1/2, the outflow can only exist as a chain of propagating lenses. ear analytical considerations leading to the above conclusion are (successfully) compared to numerical simulations which we have conducted (using a reduced gravity layer-and-a-half model). The experiments show that an outflow situated on a bottom whose (uniform) slope gradually varies in the downstream direction is continuous (i.e., is not broken into eddies) where the slope is supercritical [g′S/f>α(g′H)1/2] and discontinuous (i.e., constitutes a chain of eddies) where the slope is subcritical [g′S/f<α(g′H)1/2]. Hence, the eddies are generated by the gradual reduction in the bottom slope rather than by an instability process. Namely, the eddies are not formed by the breakdown of a known steady solution because such a steady solution does not exist. e that after reaching its “balanced depth”, an outflow usually continues to (slowly) descend toward the bottom of the ocean due to frictional effects associated with an energy loss. [Note that the “balanced depth” is the depth at which the outflow has completed its initial adjustment in the sense that it has adjusted to a state where it no longer flows primarily offshore but rather propagates primarily along the isobaths. This depth needs to be distinguished from the (sometimes significantly greater) equilibrium depth corresponding to the point where the outflowʹs density equals the environmental density.] Most of the time, the outflow descent is accompanied by a reduction in the bottom slope S, and an entrainment which causes both a reduction in g′ and an increase in H. All of these alterations bring the outflow closer and closer to the critical condition and it is, therefore, argued that all outflows ultimately reach the critical point (unless diffusion and mixing destroy them prior to that stage). suggested that Reddies (i.e., isolated lenses containing Red Sea water) are formed by the above processes. Namely, we propose that the “Reddy maker” is a combination of three processes, the natural reduction in the bottom slope which the outflow senses as it approaches the bottom of the ocean, the entrainment-induced increase in the outflowʹs thickness, and the entrainment-induced decrease in the outflowʹs density. An animation of the eddy generation process can be viewed at http://doronnof.net/features.html#video (click on “Reddy maker video”).