Deep ocean exchange with west-European shelf seas

• Published in: Ocean Science
• Main Authors: Huthnance, J.M., Holt, J.T., Wakelin, S.L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2009
• Subjects: Marine Sciences; Norway; North East Atlantic
• Online Access: http://nora.nerc.ac.uk/id/eprint/8719/; https://nora.nerc.ac.uk/id/eprint/8719/1/DOESjmhOsdNORA.pdf; https://nora.nerc.ac.uk/id/eprint/8719/2/DOESjmhf01.pdf; https://nora.nerc.ac.uk/id/eprint/8719/3/DOESjmhf02.pdf; https://nora.nerc.ac.uk/id/eprint/8719/4/DOESjmhf03.pdf; https://nora.nerc.ac.uk/id/eprint/8719/5/DOESjmhf04.pdf; https://nora.nerc.ac.uk/id/eprint/8719/6/DOES2009-28.pdf; http://www.ocean-sci.net/5/621/2009/os-5-621-2009.pdf

We review mechanisms and studies of exchange between the north-east Atlantic and the adjacent shelf sea. Well-developed summer upwelling and associated filaments off Portugal and north-west Spain give exchange O(3 m**2/s per unit length of shelf). Prevailing westerly winds further north drive exchange O(1 m**2/s). Poleward flow along most of the upper slope has associated secondary circulation O(1 m**2/s), meanders and eddies. Eddies are shed from slope waters into the Bay of Biscay, and local exchanges occur at shelf spurs and depressions or canyons (e.g. dense-water cascading of order 1 m**2/s). Tidal transports are larger, but their reversal every six hours makes exchange largely ineffective except where internal tides are large and non-linear, as in the Celtic Sea where solitons carry water with exchange O(1 m**2/s). These various physical exchanges amount to an estimated 2-3 m**2/s per unit length of shelf, between ocean and shelf. A numerical model estimate is comparable: 2.5 x 106 m**3/s onto and off the shelf from Brittany to Norway. Mixing controls the seasonal thermocline, affecting primary production and hence fluxes and fate of organic matter. Specifically, CO2 take-up by primary production, settling below the thermocline before respiration, and then off-shelf transport, make an effective shelf-sea “pump” (for CO2 from the atmosphere to the deep ocean). However, knowledge of biogeochemical fluxes is generally sparse, giving scope for more measurements, model validation and estimates from models.