Seasonality of the European slope current (Goban Spur) and ocean margin exchange

Abstract

Eight years of current data, 5.5 years of Eulerian current meter data and 2 years of Lagrangian data, are analysed in the context of seasonality along the continental slope margin of NW Europe, across shelfbreak flow and continental shelf flow. Long-term records from the continental slope at the ∼1000 m depth contour on Goban Spur and Little Sole Bank Spur show maximum recorded currents in the range 39–43 cm s -1, but typical maximum monthly mean values are ∼10 cm s -1 in the upper half of the water column. On the slope, the flow is predominantly poleward in the general alongslope direction with maximum flow in winter (December–January). The spur situation can result in four preferred directions of alongslope flow which due to topographic control is less evident near the sea floor (∼50 m up), where the flow is predominantly poleward. Equatorward upper layer flow tends to occur in the spring (March–April) period but can extend to the sea floor and into the summer period nearer the sea surface. Some seasonal signatures correlate with sea level anomalies seen in altimeter data in the continental slope and shelf region. May infrared remote sensing images show cool water separating from the upper slope (∼1 km water depth contour) region of Goban Spur both in a WNW and SSW direction (i.e. the off-slope preferred flow directions) reflecting both poleward and equatorward flows. The seasonal results are discussed in terms of the SOMA (September–October/March–April) seasonal response. The long-term or near annual Eulerian residual mean flows were typically 2 cm s -1 throughout the water column and poleward in the alongslope direction. The upper layer (30 m depth) shelf currents, ∼30 km onshelf from the Goban Spur shelfbreak, were markedly onshelf with a Eulerian mean of 6 cm s -1 over a 129-day period. A carefully prepared drogued Argos buoy (drogued at 45 m depth), and deliberately placed for subsequent along continental shelf margin movement, released a further 100 km onshelf, moved northward and parallel to the western Celtic shelfbreak towards the south of Ireland. Here the drogued buoy found the Valentia Coastal Current, passed west of Ireland on the mid-shelf, then along the continental shelfbreak (200 m contour) and through the LOIS SES site, reaching a maximum speed of 66 cm s -1 (12 h average). The drogued buoy entered the Northern North Sea, north of the Shetland Isles, travelling a distance of ∼1600 km over a period of 5 months. Drogued buoys were deployed in the ocean and on the shelf. Overall, it is shown that the continental slope is an insulator for ocean shelf exchange but the ocean margin is a conductor for advective influences from the south.