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Ocean Science An interactive open-access journal of the European Geosciences Union

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© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
22 Aug 2016
Review status
A revision of this discussion paper was accepted for the journal Ocean Science (OS) and is expected to appear here in due course.
Large-scale forcing of the European Slope Current and associated inflows to the North Sea
Robert Marsh1, Ivan D. Haigh1, Stuart A. Cunningham2, Mark E. Inall2, Marie Porter2, and Ben I. Moat3 1Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK
2Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll, PA37 1QA, UK
3National Oceanography Centre, European Way, Southampton SO14 3ZH, UK
Abstract. Drifters drogued at 50 m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely ''recruited'' from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50 % reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40 % of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a clear decline in this Atlantic inflow over 1988–2007. The influence of variable Slope Current transport on the northern North Sea is also expressed in salinity, which declines through the hindcast period, and there is evidence for a similar freshening trend in observational records. A proxy for Atlantic inflow may be found in sea level records. Variability of Slope Current transport is implicit in mean sea level differences between Lerwick (Shetland) and Torshavn (Faeroes), in both tide gauge records and a longer model hindcast spanning 1958–2013. Potential impacts of this variability on North Sea biogeochemistry and ecosystems, via associated changes in seasonal stratification and nutrient fluxes, are discussed.

Citation: Marsh, R., Haigh, I. D., Cunningham, S. A., Inall, M. E., Porter, M., and Moat, B. I.: Large-scale forcing of the European Slope Current and associated inflows to the North Sea, Ocean Sci. Discuss., doi:10.5194/os-2016-61, in review, 2016.
Robert Marsh et al.
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version      Supplement - Supplement
RC1: 'Minor Revisions', Anonymous Referee #1, 26 Sep 2016 Printer-friendly Version 
RC2: 'Review', Anonymous Referee #2, 14 Nov 2016 Printer-friendly Version 
AC1: 'Response to Referee 1', Robert Marsh, 11 Dec 2016 Printer-friendly Version 
AC2: 'Response to Referee 2', Robert Marsh, 11 Dec 2016 Printer-friendly Version 
Robert Marsh et al.
Robert Marsh et al.


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Short summary
To the west of Britain and Ireland, a strong ocean current follows the steep slope that separates the deep Atlantic and the continental shelf. This ''Slope Current'' exerts an Atlantic influence on the North Sea and its ecosystems. Using a combination of computer modelling and archived data, we find that the Slope Current weakened over 1988–2007, reducing Atlantic influence on the North Sea, due to a combination of warming of the subpolar North Atlantic and weakening winds to the west of Scotland.
To the west of Britain and Ireland, a strong ocean current follows the steep slope that...