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

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.  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 “diffusivity” from drifter dispersion and salinity surveys, other estimates of velocity variance contributing to exchange.  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.   

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.  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.  Nevertheless, cross-slope fluxes, and exchange due to motion with periods exceeding two days, are large by global standards and also very variable.  Flux values nearest the shelf break were in the range 0.3 – 3 m²/s, and exchanges were 0.8 – 4 m²/s.  Deeper longer-term moorings and drifters crossing the 500 m depth contour gave much larger fluxes and exchanges up to 20 m²/s.  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.  For the NW European shelf, transports across the shelf edge enable its disproportionately strong CO₂ “pump”.

The small scales of numerous processes enabling cross-slope transports, and the complex context, imply a need for models.  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.  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.   Validated fine-resolution models give the best prospect of coverage and of estimating shelf-sea sensitivities to the adjacent ocean.