Deep ocean exchange with west-European shelf seas

Abstract

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 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: 2.5×10 6 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, CO 2 take-up by primary production, settling below the thermocline before respiration, and then off-shelf transport, make an effective shelf-sea "pump" (for CO 2 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.

Oceanic controls on the primary production of the northwest European continental shelf: model experiments under recent past conditions and a potential future scenario

“…The ocean-shelf shelf exchange considered here (of 2.6Sv, adding inflow and outflow separately; see Table 3 and Huthnance et al, 2009) corresponds to ∼1.3 m 2 s −1 of exchange per length of the slope (∼2000 km). This falls within the range estimated for slope currents (Ekman drain) of 0.5-2 m 2 s −1 by Huthnance et al (2009), but is a significant underestimate (by a factor of 2) when other processes that are more variable, (e.g. internal tides) are added together.…”

“…internal tides) are added together. The salinity estimate of bias suggests these processes are not simply additive, as is also noted by Huthnance et al (2009).…”

“…River discharge plays an important role in near coastal regions, leading to regions of freshwater influence; although compared with other shelf sea regions globally, river flows are low. The large scale ocean-shelf exchange is controlled by seasonal upwelling in the south of the region (see GomezGesteira et al, 2011 and references therein), and the poleward slope current and Ekman transport in the North Huthnance et al, 2009). This exchange is supplemented by sub-mesoscale processes (e.g.…”

“The Ekman Drain”: a conduit to the deep ocean for shelf material

“…This persistent down-slope transport which circumvents the bathymetric constraint and provides a conduit for export to the deep ocean, we refer to as the "Ekman Drain" following the use of the term during the SES project (Souza et al2001) and subsequently by others (e.g. Huthnance et al 2009). If this flow is present along the whole west European shelf where the slope current is observed (~1,000 km), the total volume transport would be~1.6 Sv.…”

“…Shelf-ocean exchange is also considered to play a vital role in supplying inorganic nutrients from the deep ocean to support primary production on the shelf. Consequently, there is an increasing focus on the processes involved in cross-slope exchange with a number of substantial observational programmes (Shelf Edge Exchange Processes experiments (SEEP I and II), Ocean Margin EXchange (OMEX), Shelf Edge Study (SES)) to investigate the dynamics of the slope region, within which, the very different oceanographic regimes of shelf and deep ocean adjust to each other (Huthnance 1995;Huthnance et al 2009). …”