Winter dynamics on the northern Portuguese shelf. Part 2: bottom boundary layers and sediment dispersal |
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| Authors: | J Vitorino A Oliveira J M Jouanneau T Drago |
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| Institution: | 1. Instituto Hidrográfico, Rua das Trinas, 49, 1200 Lisbon, Portugal;2. DGO-UMR 5805 CNRS, Av. Des Facultes, 33405 Talence Cedex, France;3. IPIMAR-DAA, Av. Brasília, 1449-006 Lisbon, Portugal;1. EarthByte Group, School of Geosciences, University of Sydney, Sydney NSW 2006, Australia;2. Climate Change Research Centre, University of New South Wales, Sydney NSW 2052, Australia;3. ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney NSW 2052, Australia;4. Sydney Informatics Hub, University of Sydney, Sydney NSW 2006, Australia;1. Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China;2. National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China;1. State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, 310012, China;2. China-Pakistan Joint Ocean Research Center, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, 310012, China;3. Institute of Port, Coastal and Offshore Engineering, Ocean College, Zhejiang University, Hangzhou, 310058, China;4. National Institute of Oceanography, Karachi, 75600, Pakistan;1. School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, China;2. School of Resource and Environmental Management, Guizhou University of Finance and Economics, Guiyang 550002, China |
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| Abstract: | The northern Portuguese shelf, between 41°N and 42°N, is characterised by the presence of a mid-shelf mud deposit, the Douro Mud Patch (DMP). Observations conducted between July 1996 and June 1999 under the framework of project OMEX II–II, are used to examine the impact of the dynamic processes in the lower water column, particularly on the DMP. The typical wave conditions observed during the winter maintain a highly energetic environment, capable of promoting the erosion of fine sediments at mid-shelf depths. However, the bottom nepheloid layers generated beneath these waves only extend a few meters above the bottom and are contained within rocky outcrops with similar heights that fringe the outer shelf. Each year there are about ten storms, mostly associated with southerly winds that create downwelling conditions over the shelf. The waves associated with these storms produce shear velocities over 3 cm/s at mid-shelf and bottom nepheloid layers which extend a few tens of meters above the bottom and spread offshore, over the outer shelf and upper slope. A rough estimate suggests that these events account for an offshore export of about 20×106 kg of fine sediments each year (equivalent to 1–2% of sediments trapped at DMP). |
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