Physical structures, advection and mixing in the region of Goban spur

The physical context for ocean margin exchange at Goban Spur is described. Observations adjacent to, prior to and during the Ocean Margin EXchange (OM EX) project of 1993-1996 are used. They include currents measured on moorin gs, drogued-buoy tracks; temperature and other data from CTD profiles, espe cially as indicators of vertical mixing; evidence from models, particularly for turbulence causing vertical mixing. These data are combined in estimat es of (seasonally dependent) mean flow, tidal currents, other current varia bility, exchange and mixing over the main cross-slope section studied in OM EX and in nearby and contrasted locations (aided by the use of earlier and adjacent measurements). Causative physical processes are discussed: potenti ally northward flow along the continental slope, effects of Goban Spur topo graphy, eddies, wind-driven transport, cascading, tides, fronts, internal t ides, internal waves, surface waves. Among these, there is evidence that the along-slope flow, typically O(0.05ms(-1)), is reduced or even reversed in spring, is generally weaker than at some other margin sectors owing to t he non-meridional alignment and indentations in the Celtic Sea slope, and m ay sometimes overshoot rather than follow the depth contours around Goban S pur; tidal currents are O(0.2ms(-1)) on the adjacent shelf but O(0.1ms(-1)) or l ess over most of Goban Spur; they increase to the southeast; other (wind- and eddy-forced) contributions to the currents are typically O (0.1ms(-1)) or less, except on the shelf, and decrease with depth; wind-, tide- and wave-forced currents are probably the most consistent agen ts of cross-slope exchange O(1m(2)s(-1)), with topographic effects being im portant locally (canyons, spurs); stratification starts intermittently until early June, becomes shallower th rough June and deepens by September. In 1995, one storm on 5-8 September ro ughly doubled the upper mixed-layer depth to > 40m and reinstated maximal p rimary production in the upper mixed layer; vertical mixing is intermittent, dominated by surface inputs (wind and wave s); towards the southeast, internal waves of tidal origin are increasingly important for mixing across the thermocline; in the context of nutrient provision for primary production in the upper mi xed layer, diffusion through the summer thermocline appears to be small unl ess internal waves strongly increase mixing. (C) 2001 Elsevier Science Ltd. All rights reserved.