Preprints
https://doi.org/10.5194/os-2021-60
https://doi.org/10.5194/os-2021-60

  13 Jul 2021

13 Jul 2021

Review status: this preprint is currently under review for the journal OS.

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

Matthew Clark1, Robert Marsh1, and James Harle2 Matthew Clark et al.
  • 1Ocean and Earth Science, University of Southampton Waterfront Campus, National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK
  • 2National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK

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.

Matthew Clark et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-60', Hjálmar Hátún, 08 Sep 2021
  • RC2: 'Comment on os-2021-60', Anonymous Referee #2, 11 Oct 2021

Matthew Clark et al.

Matthew Clark et al.

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Short summary
The European Slope Current (SC) is a northward-flowing current running parallel to the UK coastline. It is forced by changes in the density gradient of the wider North Atlantic Ocean. As the North Atlantic has warmed since the late 1990s, these gradients have changed strength and moved: reducing the volume and speed of water feeding into the SC. The SC flows into the North Sea, where changes in the species distribution of some plankton and fish has been seen due to the warming inputs.