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

Abstract

The physical context for ocean margin exchange at Goban Spur is described. Observations adjacent to, prior to and during the Ocean Margin EXchange (OMEX) project of 1993–1996 are used. They include currents measured on moorings, drogued-buoy tracks; temperature and other data from CTD profiles, especially as indicators of vertical mixing; evidence from models, particularly for turbulence causing vertical mixing. These data are combined in estimates of (seasonally dependent) mean flow, tidal currents, other current variability, exchange and mixing over the main cross-slope section studied in OMEX and in nearby and contrasted locations (aided by the use of earlier and adjacent measurements). Causative physical processes are discussed: potentially northward flow along the continental slope, effects of Goban Spur topography, eddies, wind-driven transport, cascading, tides, fronts, internal tides, internal waves, surface waves. Among these, there is evidence that • the along-slope flow, typically O(0.05 m s −1 ), is reduced or even reversed in spring, is generally weaker than at some other margin sectors owing to the non-meridional alignment and indentations in the Celtic Sea slope, and may sometimes overshoot rather than follow the depth contours around Goban Spur; • tidal currents are O(0.2 m s −1 ) on the adjacent shelf but O(0.1 m s −1 ) or less over most of Goban Spur; they increase to the southeast; • other (wind- and eddy-forced) contributions to the currents are typically O(0.1 m s −1 ) or less, except on the shelf, and decrease with depth; • wind-, tide- and wave-forced currents are probably the most consistent agents of cross-slope exchange O(1 m 2 s −1 ), with topographic effects being important locally (canyons, spurs); • stratification starts intermittently until early June, becomes shallower through June and deepens by September. In 1995, one storm on 5–8 September roughly doubled the upper mixed-layer depth to >40 m and reinstated maximal primary production in the upper mixed layer; • vertical mixing is intermittent, dominated by surface inputs (wind and waves); 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 mixed layer, diffusion through the summer thermocline appears to be small unless internal waves strongly increase mixing.