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Long-Term Coastal Upwelling over a Continental Shelf–Slope

Nobuo SuginoharaGeophysical Institute, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, 113 Japan

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Yoshiteru KitamuraGeophysical Institute, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo, 113 Japan

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Abstract

Long-term coastal upwelling over a continental shelf-slope with emphasis on the planetary dispersion of Rossby waves is studied with numerical models. The ocean is forced by a wind stress with a limited longshore extent. The thermocline intersects the shelf–slope and the internal radius of deformation is smaller than the width of the shelf-slope. Two case studies with and without the β effect are studied. lie early-stage response within a week or so is similar between the two. For an f-plane case, the whole-stage response is accounted for by coastal-trapped wave dynamics, and only equatorward flow exists over the shelf–slope at an advanced stage. However, inclusion of the β effect leads to a significant change in the alongshore flow pattern at the advanced stage. Due to the westward Rossby dispersion of the equatorward flow associated with the first and second coastal-trapped mode responses, the poleward undercurrent develops in the thermocline layer over the shelf. The time for the undercurrent to appear depends on the speed of the westward dispersion of the second-mode response. The undercurrent obtained in the present model may represent the one observed off Oregon and California.

Abstract

Long-term coastal upwelling over a continental shelf-slope with emphasis on the planetary dispersion of Rossby waves is studied with numerical models. The ocean is forced by a wind stress with a limited longshore extent. The thermocline intersects the shelf–slope and the internal radius of deformation is smaller than the width of the shelf-slope. Two case studies with and without the β effect are studied. lie early-stage response within a week or so is similar between the two. For an f-plane case, the whole-stage response is accounted for by coastal-trapped wave dynamics, and only equatorward flow exists over the shelf–slope at an advanced stage. However, inclusion of the β effect leads to a significant change in the alongshore flow pattern at the advanced stage. Due to the westward Rossby dispersion of the equatorward flow associated with the first and second coastal-trapped mode responses, the poleward undercurrent develops in the thermocline layer over the shelf. The time for the undercurrent to appear depends on the speed of the westward dispersion of the second-mode response. The undercurrent obtained in the present model may represent the one observed off Oregon and California.

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