The shelfward penetration of western boundary currents

Abstract

Penetration of the western boundary current onto a continental shelf, and a marginal sea is investigated numerically to understand the observed shelfward penetration of the Kuroshio in the East China and Japan Seas. The work described in this thesis is mainly divided into two parts.

The first is to study the response of the circulation on a continental shelf to a steady western boundary current forcing using a numerical model in a simple domain. The model domain consists of a rectangular shelf and a deep ocean with a step-like cross section. The model predicts an anticyclonic penetration of the WBC onto the shelf, which is driven by the meridional pressure gradient inherent in the WBC. A southward intensification of the on-shelf flow takes place due to a vorticity constraint. The fundamental features are insensitive to different numerical model assumptions including boundary conditions and the presence of a passive lower layer. The amount of penetration, however, depends on frictional parameterisation, location of the shelf on a β-plane, depth of the shelf, and latitudinal distance of the shelf.

Non-linear effects are found to be important in setting the shelf circulation. Inertial effects broaden the length scale of the penetration boundary layer, resulting in an increased on-shelf transport. This occurs due to vorticity advection, which reduces the frictional effects in the boundary layer by balancing part of the topographic vorticity tendency. Zonal and meridional penetration scales also significantly increase, compared with those in the linear models. Changes in the circulation pattern are also investigated as the inertial effects increase. It turns out that the inertial effects are of little importance for a shelf located to the western boundary of a subpolar gyre, while they are important for a shelf located to the western boundary of a subtropical gyre like the East China Sea.

The second part of the thesis is an investigation of the effects, which changes in the large scale circulation of the deep ocean have, on the influx to a marginal sea.

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