Climate change impacts on ocean circulation relevant to the UK and Ireland

Ocean circulation, including ocean currents and systems of ocean currents,
such as ocean gyres and the meridional overturning circulation, play a key
role in the climate system through the redistribution of heat, freshwater,
carbon, and ecosystem-relevant quantities. Some of these systems of ocean
currents are on a large spatial scale and of global climate relevance. For
example, the basin-scale Atlantic Meridional Overturning Circulation
(AMOC) plays an important role in the climate of north-west Europe
(Bellomo et al., 2021; McCarthy et al., 2015). Other ocean circulation
features are on a smaller spatial scale and still have an important climate
relevance. For example, the regional-scale exchanges across the north-west
European shelf are large and enable a disproportionately large carbon
transport that plays an important role in the ocean’s sequestration of
anthropogenic carbon (Legge et al., 2020). How these systems will change
as the climate changes is a key focus of research.
The large-scale North Atlantic circulation consists mainly of the winddriven gyres and the AMOC which is partly wind-driven and partly driven
by differences in density of water masses (mainly dependent on salinity and
temperature, Figure 1). Changes in the wind-driven circulation can impact
the thermohaline circulation and vice versa. This overlap of drivers makes
distinct definitions of AMOC and subpolar gyre strength impossible.
Traditionally, the AMOC has been defined by the maximum of the
overturning stream function—the maximum balance of upper water flowing
to the north balanced by deeper water flowing south, which occurs in the
subtropics. In the subpolar North Atlantic, warmer water flows northwards
on the eastern side of the basin and colder water flows southwards at the
western side of the basin. On the eastern boundary of the gyre, at the
continental shelf, the European Slope Current flows, with its origins
traceable to the Iberian Peninsula and extending all the way around the
European shelf towards Scandinavia. On the continental shelf, several
coastal currents flow (see Figure 3 for details) that are driven by wind, tides,
and thermohaline factors.