Deep ocean exchange with west-European shelf seas

Abstract

We review mechanisms and studies of exchange between the north-east Atlantic and the adjacent shelf seas. Well-developed summer upwelling and associated filaments off Portugal and north-west Spain give exchange O(3 m²/s per unit length of shelf). Prevailing westerly winds further north drive exchange O(1 m²/s). Poleward flow along most of the upper slope has associated secondary circulation O(1 m²/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 m²/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 m²/s). These various physical exchanges amount to an estimated 2–3 m²/s per unit length of shelf, between ocean and shelf. A numerical model estimate is comparable: 2.5×10⁶ m³/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.