Thermohaline Finestructure and Its Relation to Frontogenesis Dynamics

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  • 1 Institut für Meereskunde an der Universität Kiel, Kiel, FRG
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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.

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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