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Abstract
We investigate the effects of mesoscale eddies on the scalings for the stratification depth in a primitive equation ocean model in two configurations: (1) a double gyre, basin-scale model with a flat bottom and solid boundaries and (2) a basin-scale model with a periodic channel at the poleward end. In the first case, the low resolution runs are in the familiar two-thermocline regime with an upper, advective thermocline (ventilated thermocline) and a diffusive boundary layer (internal thermocline) beneath. In the eddying model's ventilated thermocline, the dominant balance in the buoyancy equation is the same as in the low resolution model, and the depth of the thermocline continues to depend on the downwelling to the half power, although the dependence on the windstress is somewhat steeper. In the internal thermocline, the boundary region becomes a three-way balance between vertical advection, diffusion, and eddy flux convergence. Near the surface, convergent eddy fluxes create a source of buoyant water mass which is subducted into the internal thermocline by eddy fluxes. As the eddy fluxes converge, they redeposit mass along the isopycnals, thereby thickening the front. This suggests that as the diffusion goes to zero, the internal thermocline thickness may remain finite, although we were unable to explicitly test this.
In the domain with a circumpolar channel, the effects of the eddies on the stratification depth are more pronounced. Within the channel, the eddy circulation largely balances the mean Ekman circulation so that the residual circulation is small. Based on this observation, scaling theories suggest the depth of the channel stratification should depend on the windstress to the 2/5 power. However, the background diffusion tends to pull down isopycnals, so that the diffusion and zero-residual-circulation conditions compete to set the stratification depth, and the actual dependence on the windstress is shallower than the 2/5 dependence. For low windstresses, the diffusive condition prevails and the depth varies little with the windstress. For higher windstresses, the depth tends to depend on the windstress to the 1/4 power.
