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Large-scale forcing of the European Slope Current and associated inflows to the North Sea

Large-scale forcing of the European Slope Current and associated inflows to the North Sea
Large-scale forcing of the European Slope Current and associated inflows to the North Sea
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.
1812-0792
315-335
Marsh, Robert
702c2e7e-ac19-4019-abd9-a8614ab27717
Haigh, Ivan D.
945ff20a-589c-47b7-b06f-61804367eb2d
Cunningham, Stuart A.
b7d3a27c-733f-4bd7-80ef-23825ee56bdf
Inall, Mark E.
c0126931-82eb-4c4c-8f7e-22fd6279a7bf
Porter, Marie
6c2668f8-6ca3-469c-ac4b-e9edfcd86702
Moat, Ben I.
497dbb18-a98f-466b-b459-aa2c872ad2dc
Marsh, Robert
702c2e7e-ac19-4019-abd9-a8614ab27717
Haigh, Ivan D.
945ff20a-589c-47b7-b06f-61804367eb2d
Cunningham, Stuart A.
b7d3a27c-733f-4bd7-80ef-23825ee56bdf
Inall, Mark E.
c0126931-82eb-4c4c-8f7e-22fd6279a7bf
Porter, Marie
6c2668f8-6ca3-469c-ac4b-e9edfcd86702
Moat, Ben I.
497dbb18-a98f-466b-b459-aa2c872ad2dc

Marsh, Robert, Haigh, Ivan D., Cunningham, Stuart A., Inall, Mark E., Porter, Marie and Moat, Ben I. (2017) Large-scale forcing of the European Slope Current and associated inflows to the North Sea. Ocean Science, 13 (2), 315-335. (doi:10.5194/os-13-315-2017).

Record type: Article

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.

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Accepted/In Press date: 22 August 2016
e-pub ahead of print date: 21 April 2017
Organisations: Ocean and Earth Science, Physical Oceanography, National Oceanography Centre, Marine Physics and Ocean Climate

Identifiers

Local EPrints ID: 399823
URI: http://eprints.soton.ac.uk/id/eprint/399823
ISSN: 1812-0792
PURE UUID: 200e5265-19ac-49e0-a3d3-17fbdf715dba
ORCID for Ivan D. Haigh: ORCID iD orcid.org/0000-0002-9722-3061

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Date deposited: 30 Aug 2016 13:00
Last modified: 28 Apr 2022 01:57

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Contributors

Author: Robert Marsh
Author: Ivan D. Haigh ORCID iD
Author: Stuart A. Cunningham
Author: Mark E. Inall
Author: Marie Porter
Author: Ben I. Moat

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