The Relative Importance of Wind Strength and Along-Shelf Bathymetric Variations on the Separation of a Coastal Upwelling Jet

Renato M. Castelao College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon

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John A. Barth College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon

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Abstract

A high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet.

Corresponding author address: Renato M. Castelao, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvallis, OR 97331-5503. Email: castelao@coas.oregonstate.edu

Abstract

A high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet.

Corresponding author address: Renato M. Castelao, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvallis, OR 97331-5503. Email: castelao@coas.oregonstate.edu

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