We consider shelf sectors from southern Biscay (Spain) to 70ºN (Norway), and other Nordic Sea shelves. This ocean margin has varied orientation and width, canyons, broken and fjordic coastlines.
Typically, shelf or shelf-edge flows are anticyclonic relative to land. Warm, saline North Atlantic Water flows along the continental slope from Biscay to Norway. Beneath this slope current, a down-slope Ekman layer effects exchange O(1 m2/s). The along-slope flow is unstable in places, but estimated shelf-ocean exchange from resulting Biscay eddies is modest: 0.16 m2/s. A possible ″overshoot″ at Goban Spur may exchange O(106 m3/s) locally (comparable with the slope current transport).
Winds are strong; westerlies prevail, but variability and friction enable cross-slope transport O(1 m2/s). Tidal currents exceed 0.1 m/s typically, 0.5 m/s in places. Internal tides with comparable peak currents are generated locally over steep slopes in some places; where large, they can transport O(1 m2/s).
Inputs of freshwater from land and rivers are modest in global terms. Surface buoyancy derives from precipitation - evaporation (within 200mm of net annual balance over much of the region), summer heating and winter cooling. The strong cycle of seasonal stratification is shaped by mixing from tides, winds, surface and inertial-internal waves. Tidal internal waves mix within the thermocline. Dense water in winter-cooled shallow shelf seas may ″cascade″ down the slope; possibly 1 m2/s locally.
Drifters have enabled estimates of cross-slope dispersion O(500 m2/s). Such measured or process-based exchange estimates are needed because LOICZ budgeting methodology is sensitive to the small and uncertain differences in shelf-sea salinities and large variations in along-shelf flow.
Current-induced bed stresses often suffice to move local upper-slope sediment. Hence sediment reaching the shelf edge is liable to export, especially in the down-slope Ekman layer under the slope current.
A spring bloom starts when the shoaling mixed layer meets the deepening (light-determined) critical depth for net algal growth. In general, the bloom progresses northwards and becomes nutrient-limited when the initial near-surface charge is exhausted. Summer growth is limited to that sustained by biologically regenerated nitrogen, nutrients entrained through the thermocline and Ekman suction. On the shelf, tidal mixing also supplies nutrients to the euphotic zone. An autumn bloom may be fuelled by nutrients entrained in the deepening mixed layer. Typical shelf-sea production is O(100-250 gC m-2yr-1); the shelf/ocean distinction is reduced at these latitudes. Upwelling or mixing can increase production.
Organic carbon and/or related budgets have previously been estimated over Goban Spur, sub-areas of the north-west European shelf, the North Sea, Irish Sea, at the Hebrides shelf edge and off northern Norway.
Various shelf characteristics, constituents and flux estimates have been assembled to extend the budgets. |
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