Effect of the wind on the shelf dynamics: Formation of a secondary upwelling along the continental margin |
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| Authors: | V. Rossi Y. Morel V. Garçon |
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| Affiliation: | 1. Laboratoire d’Études en Géophysique et Océanographie Spatiales, CNRS, 18 Avenue Edouard Belin, 31401 Toulouse Cedex 9, France;2. Service Hydrographique et Océanographique de la Marine, (SHOM), 42 av Gaspard Coriolis, 31057 Toulouse, France;1. School of Civil, Environmental and Mining Engineering, The University of Western Australia, Crawley, Australia;2. The UWA Oceans Institute, The University of Western Australia, Crawley, Australia;3. School of Earth and Environment, The University of Western Australia, Crawley, Australia;4. ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, Crawley, Australia;5. Australian Institute of Marine Science, Townsville, Queensland, Australia;1. IFISC (Institute for Cross-Disciplinary Physics and Complex Systems), CSIC-UIB, Palma de Mallorca, 07122 Spain;2. Climate Change Research Centre, University of New South Wales, Sydney 2052, Australia;3. ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney 2052, Australia;4. Laboratoire d’Etude en Géophysique et Océanographie Spatiales, CNRS/UPS/IRD/CNES, 14 av. E. Belin, Toulouse 31400, France;1. Dipartimento di Matematica, Università di Pisa, Largo Bruno Pontecorvo, 5, 56127 Pisa, Italy;2. Dipartimento di Matematica e Geoscienze, Università degli Studi di Trieste, Via Valerio, 12/1, 34127 Trieste, Italy;3. Université Paris-Est, LAMA (UMR 8050), UPEC, UPEMLV, CNRS, F-94010 Créteil, France;4. IMB, Université de Bordeaux 1, 351, Cours de la Libération, F-33405 Talence, France;1. Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), 18 Avenue Edouard Belin, 31401 Toulouse Cedex 9, France;2. National Oceanography Centre, Southampton, SO14 3ZH, UK;1. LUNAM Université, École Centrale de Nantes, IRCCyN UMR CNRS 6597 (Institut de Recherche en Communications et Cybernétique de Nantes), 1 rue de la Noë - B.P. 92101 - 44321 Nantes Cedex 3, France;2. ETH Zürich, Switzerland;3. BISON group, Automatic Control Laboratory, ETH Zürich Physikstrasse 3, 8092 Zurich, Switzerland |
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| Abstract: | In this paper, the authors study the influence of the wind on the dynamics of the continental shelf and margin, in particular the formation of a secondary upwelling (or downwelling) front along the shelf break.Observations during the MOUTON2007 campaign at sea along the Portuguese coast in summer 2007 reveal the presence of several upwelling fronts, one being located near the shelf break. All upwellings are characterized by deep cold waters close to or reaching the surface and with high chlorophyll concentrations. Simplified numerical models are built in order to study a possible physical mechanism behind this observation. First, a simple shallow water model with three distinct layers is used to study the formation of secondary upwelling fronts. We show that the physical mechanism behind this process is associated with onshore transport of high potential vorticity anomalies of the shelf for upwelling favorable conditions. Sensitivity studies to bottom friction, shelf width, continental slope steepness, shelf “length” are analysed in terms of potential vorticity dynamics. In particular bottom friction is analyzed in detail and we find that, even though bottom friction limits the barotropic velocity field, it enhances the cross-shore circulation, so that no steady state is possible when stratification is taken into account. Bottom friction accelerates the onshore advection of high potential vorticity, but also drastically reduces its amplitude because of diabatic effects. The net effect of bottom friction is to reduce the secondary upwelling development. Based on similar mechanisms, previous results are then extended to downwelling favorable conditions. Finally a more realistic configuration, with bottom topography, wind forcing and stratification set up from observations, is then developed and the results confronted to the observations. Simulations overestimate the velocity amplitude but exhibit good agreement in terms of density ranges brought over the shelf and general isopycnal patterns.The application and extension of the results to more general oceanic regions is discussed and we conclude on the influence of such process on the dynamics of wind driven circulation over a shelf. |
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