Ocean Shelf Exchange, NW European Shelf Seas: measurements, estimates and comparisons.

<p>We describe estimates of overall transport across three contrasted sectors of the north-west European shelf edge: the Celtic Sea south-west of Britain, the Malin-Hebrides shelf west of Scotland and the West Shetland shelf north of Scotland.&#160; The estimates derive from a variety of measurements in the project FASTNEt (Fluxes across sloping topography of the North East Atlantic): drifters and moored current meters, effective &#8220;diffusivity&#8221; from drifter dispersion and salinity surveys, other estimates of velocity variance contributing to exchange.&#160; Process contributions include transport by along-slope flow, internal waves and their Stokes drift, tidal pumping, eddies and Ekman transports, in a wind-driven surface layer and in a bottom boundary layer.&#160; &#160;</p><p>Estimated overall exchange across the shelf edge is several m²/s (Sverdrups per 1000 km) and thereby large compared with many other locations, large compared with oceanic transports if extrapolated globally and potentially important to the shelf-sea and adjacent oceanic budgets.&#160; However, the large majority of this is in tides and other motion with periods of order one day or less; such exchange is only effective for water properties that evolve on time-scales of a day or less.&#160; Nevertheless, cross-slope fluxes, and exchange due to motion with periods exceeding two days, are large by global standards and also very variable.&#160; Flux values nearest the shelf break were in the range 0.3 &#8211; 3 m²/s, and exchanges were 0.8 &#8211; 4 m²/s.&#160; Deeper longer-term moorings and drifters crossing the 500 m depth contour gave much larger fluxes and exchanges up to 20 m²/s. &#160;Significance of these transports depends on distinctive properties of the water, or its contents, and on internal shelf-sea circulation affecting the further progress of these transports.&#160; For the NW European shelf, transports across the shelf edge enable its disproportionately strong CO₂ &#8220;pump&#8221;.</p><p>The small scales of numerous processes enabling cross-slope transports, and the complex context, imply a need for models.&#160; Measurements remain limited in extent and duration, but a wide variety of contexts, particular conditions, events and behaviours is now available for model validation, especially around the north-west European continental shelf edge.&#160; Variability continues to render observations insufficient for stable estimates of transports and exchanges, especially if partitioned by sector and season; indeed, there may be significant inter-annual differences.&#160; &#160;Validated fine-resolution models give the best prospect of coverage and of estimating shelf-sea sensitivities to the adjacent ocean.</p>