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Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes

Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes
Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes
Oceanic influences on shelf seas are mediated by flow along and across continental slopes, with consequences for regional hydrography and ecosystems. Here we present evidence for the variable North Atlantic influence on European shelf seas over the last four decades, using ocean analysis and reanalysis products, and an eddy-resolving ocean model hindcast. To first order, flows oriented along isobaths at the continental slope are related to the poleward increase of density in the adjacent deep ocean that supports a geostrophic inflow towards the slope. In the North Atlantic, this density gradient and associated inflow has undergone substantial, sometimes abrupt, changes in recent decades. Inflow in the range 10–15 Sv is identified with eastward transport in temperature classes at 30° W, in the latitude range 45–60° N. Associated with major subpolar warming around 1997, a cool and fresh branch of the Atlantic inflow was substantially reduced, while a warm and more saline inflow branch strengthened, with respective changes of the order 5 Sv. Total inflow fell from ~ 15 Sv pre-1997 to ~ 10 Sv post-1997. In the model hindcast, particle tracking is used to trace the origins of poleward flows along the continental slope to the west of Ireland and Scotland, before and after 1997. Backtracking particles up to 4 years, a range of subtropical and subpolar pathways is identified from a statistical perspective. In broad terms, cold, fresh waters of subpolar provenance were replaced by warm, saline waters, of subtropical provenance. These changes have major implications for the downstream shelf regions that are strongly influenced by Atlantic inflow, the northern North Sea in particular, where “subtropicalization” of ecosystems has already been observed since the late 1990s.
1812-0792
Clark, Matthew
05adc776-9263-48b6-8b73-6ad8b8385c1d
Marsh, Robert
702c2e7e-ac19-4019-abd9-a8614ab27717
Harle, James
b59d8925-1e59-42d4-b08f-823fec3b701e
Clark, Matthew
05adc776-9263-48b6-8b73-6ad8b8385c1d
Marsh, Robert
702c2e7e-ac19-4019-abd9-a8614ab27717
Harle, James
b59d8925-1e59-42d4-b08f-823fec3b701e

Clark, Matthew, Marsh, Robert and Harle, James (2021) Weakening and warming of the European Slope Current since the late 1990s attributed to basin-scale density changes. Ocean Science. (doi:10.5194/os-2021-60).

Record type: Article

Abstract

Oceanic influences on shelf seas are mediated by flow along and across continental slopes, with consequences for regional hydrography and ecosystems. Here we present evidence for the variable North Atlantic influence on European shelf seas over the last four decades, using ocean analysis and reanalysis products, and an eddy-resolving ocean model hindcast. To first order, flows oriented along isobaths at the continental slope are related to the poleward increase of density in the adjacent deep ocean that supports a geostrophic inflow towards the slope. In the North Atlantic, this density gradient and associated inflow has undergone substantial, sometimes abrupt, changes in recent decades. Inflow in the range 10–15 Sv is identified with eastward transport in temperature classes at 30° W, in the latitude range 45–60° N. Associated with major subpolar warming around 1997, a cool and fresh branch of the Atlantic inflow was substantially reduced, while a warm and more saline inflow branch strengthened, with respective changes of the order 5 Sv. Total inflow fell from ~ 15 Sv pre-1997 to ~ 10 Sv post-1997. In the model hindcast, particle tracking is used to trace the origins of poleward flows along the continental slope to the west of Ireland and Scotland, before and after 1997. Backtracking particles up to 4 years, a range of subtropical and subpolar pathways is identified from a statistical perspective. In broad terms, cold, fresh waters of subpolar provenance were replaced by warm, saline waters, of subtropical provenance. These changes have major implications for the downstream shelf regions that are strongly influenced by Atlantic inflow, the northern North Sea in particular, where “subtropicalization” of ecosystems has already been observed since the late 1990s.

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os-2021-60
Available under License Creative Commons Attribution.
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e-pub ahead of print date: 13 July 2021
Additional Information: Pre-print, under review

Identifiers

Local EPrints ID: 450753
URI: http://eprints.soton.ac.uk/id/eprint/450753
ISSN: 1812-0792
PURE UUID: 1339c057-6aac-4574-99e4-e5ee6f19ee74
ORCID for Matthew Clark: ORCID iD orcid.org/0000-0002-5199-6632

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Date deposited: 10 Aug 2021 16:30
Last modified: 11 Aug 2021 01:53

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Contributors

Author: Matthew Clark ORCID iD
Author: Robert Marsh
Author: James Harle

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