The Influence of Layer Outcropping on the Separation of Boundary Currents. Part I: The Wind-driven Experiments

Eric P. Chassignet RSMAS/MPO, University of Miami, Miami, Florida

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Rainer Bleck RSMAS/MPO, University of Miami, Miami, Florida

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Abstract

The influence of outcropping isopycnal layers on the separation of western boundary currents is investigated in a series of wind-driven eddy-resolving multilayer primitive equation numerical experiments. The outcropping mechanism of Parsons allows the midlatitude jet to separate south of the zero wind-stress curl line (ZWCL), an important property when one considers that most realistic numerical experiments to date exhibit an over-shooting subtropical western boundary current.

If the inertial terms are removed from the momentum equations, the Sverdrup relation for the interior flow emerges as the dominant constraint on the placement of the upper-layer jet separation latitude. As long as the upper/lower layer ratio is small enough, a good agreement is obtained with the analytical theory, namely a separation south of the ZWCL. If the ratio is large, the resulting flow pattern changes drastically by favoring a configuration that satisfies the Sverdrup relation and maintains a jet separation at the ZWCL.

As soon as the inertial terms are included, the Sverdrup constraint becomes less dominant, allowing the upper-layer midlatitude jet separation latitude to shift southward whenever the upper layer is chosen sufficiently shallow to cause large-scale outcropping. The degree to which this southward shift depends on the amount of mass in the top layer and on the parameterization of the wind-induced stress profile in the water column is explored in detail.

Abstract

The influence of outcropping isopycnal layers on the separation of western boundary currents is investigated in a series of wind-driven eddy-resolving multilayer primitive equation numerical experiments. The outcropping mechanism of Parsons allows the midlatitude jet to separate south of the zero wind-stress curl line (ZWCL), an important property when one considers that most realistic numerical experiments to date exhibit an over-shooting subtropical western boundary current.

If the inertial terms are removed from the momentum equations, the Sverdrup relation for the interior flow emerges as the dominant constraint on the placement of the upper-layer jet separation latitude. As long as the upper/lower layer ratio is small enough, a good agreement is obtained with the analytical theory, namely a separation south of the ZWCL. If the ratio is large, the resulting flow pattern changes drastically by favoring a configuration that satisfies the Sverdrup relation and maintains a jet separation at the ZWCL.

As soon as the inertial terms are included, the Sverdrup constraint becomes less dominant, allowing the upper-layer midlatitude jet separation latitude to shift southward whenever the upper layer is chosen sufficiently shallow to cause large-scale outcropping. The degree to which this southward shift depends on the amount of mass in the top layer and on the parameterization of the wind-induced stress profile in the water column is explored in detail.

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