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Eric P. Chassignet, Rainer Bleck, and Claes G. H. Rooth

Abstract

It is shown that the main characteristic of the Persons western boundary current separation mechanism, namely a separation south of the zero wind stress curt line (ZWCL), is no longer present when diabatic effects are included through the addition of a mixed layer to the model structure. When diabatic processes are taken into account, the separation is associated with the outcropping of interior layers into the mixed layer where the horizontal density gradient is a maximum. The maintenance of this maximum results from a balance between potential vorticity conservation and diabatic processes. In spite of wide variations in either forcing functions or parameters, the flow patterns of the thermally forced experiments described herein remain very similar from one experiment to the other. With a separation latitude located at the ZWCL. It is only when salinity is allowed to vary significantly in space that one observes a significant move in the jet separation latitude.

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Tamay M. Özgökmen, Eric P. Chassignet, and Claes G. H. Rooth

Abstract

As the salty and dense Mediteranean overflow exits the Strait of Gibraltar and descends rapidly in the Gulf of Cadiz, it entrains the fresher overlying subtropical Atlantic Water. A minimal model is put forth in this study to show that the entrainment process associated with the Mediterranean outflow in the Gulf of Cadiz can impact the upper-ocean circulation in the subtropical North Atlantic Ocean and can be a fundamental factor in the establishment of the Azores Current. Two key simplifications are applied in the interest of producing an economical model that captures the dominant effects. The first is to recognize that in a vertically asymmetric two-layer system, a relatively shallow upper layer can be dynamically approximated as a single-layer reduced-gravity controlled barotropic system, and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity.

Two sets of computations are presented within the 1½-layer framework. A primitive-equation-based computation, which includes the divergent flow effects, is first compared with the equivalent quasigeostrophic formulation. The upper-ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the β effect until the flow encounters the western boundary. In the steady state, the circulation pattern consists of bidirectional zonal flows with a limited meridional extent: eastward to the south of the sink and westward to the north of the sink. The localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink’s strength. It is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component.

A set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation, driven both by wind forcing and “thermohaline” flow through the open southern and northern boundaries. When a sink near the eastern boundary is superimposed on the idealized midlatitude circulation, it is shown to alter significantly the upper-ocean flow and induce an eastward zonal current, which resembles the Azores Current in location and transport. This mechanism also generates a westward current to the north of the sink location, which could be associated with the Azores Countercurrent. An extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes, sink strength, sink distribution, model resolution, and wind forcing. The magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline.

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