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R. Onken, J. Fischer, and J. D. Woods

Abstract

We present and compare results from a two-dimensional numerical frontogenesis model and a field experiment in the eddy field of the North Atlantic Current in order to illustrate and explain the shape and generation mechanisms of frontal finestructure.

The frontogenesis model has been presented in detail in an earlier paper by Bleck et al. We have added to this model simple initial temperature and salinity fields, which are treated as passive tracers on isopycnals. Integrating the model for three days produces an asymmetry of the thermohaline gradient field and the familiar slope of the frontal surface details of which depend on the initial conditions. An analysis of the terms of the tracer advection equation reveals that the asymmetry is due to the divergence of the cross-jet agestrophic mass flux induced by vortex stretching, whereas the slope of the frontal surface is caused by the cross-jet advection of the thermohaline gradient.

Within the eddy field of the North Atlantic Current we observed regions of confluent flow in which mesoscale fronts have been formed, exhibiting dynamical and thermohaline properties similar to those predicted by the model. By applying the model results we construct a dynamically consistent picture of the cross-front circulation, which leads to the observed thermohaline structure. A method is proposed which allows to estimate the magnitude of the cross-frontal flow solely from the finestructure of passive scalar gradients.

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M. J. Roberts and R. A. Wood

Abstract

This paper describes a series of four experiments, each run for 10 years at 1° × 1° resolution on a North Atlantic domain, designed to illuminate the sensitivity of a Bryan–Cox-type ocean model to changes in the representation of the ridges that restrict the flow of dense, deep water out of the Greenland–Iceland–Norway (GIN) basin. In reality, much of the outflow takes place through narrow sills, which are subgrid-scale in the model, and small changes in the model topography to reflect these sills have a large impact on the outflow and on the compensating inflow of warm North Atlantic water. The circulation of the GIN basin is dramatically changed depending on the amount of this inflow; with no inflow, the basin cools and freshens, as would be expected, whereas with too much inflow, it becomes warm, salty, and homogeneous to great depths.

Moreover, the small changes in topography have wider implications for the simulation. The presence or absence of dense overflows has a great impact on the mixed layer development in the subpolar gyre, with mixed layer depths differing by more than 500 m between two of the cases. This has implications for the formation of subpolar mode water, which is nearly shut off in the two cases with significant overflow.

The meridional overturning in the model in year 10 increases by over 50% at its peak between the cases with no dense overflow and those with the greatest overflow, and this partly explains a change in peak heat transport, which increases by around 50% in the cases with significant overflow.

The results in this paper imply that careful “tuning” of the model topography is necessary in ocean/climate models in order to get a reasonable simulation of the conveyor belt and of North Atlantic Deep Water formation.

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