Why is it important?
Currents can transport nutrients, contaminants (organic and inorganic), passively floating organisms (such as phytoplankton and zooplankton), and fibres and other micro-particles over great distances. Ocean currents also transport large quantities of heat and salt. In the Atlantic Ocean, the Atlantic Meridional Overturning Circulation (AMOC) describes the circulation pattern where warm, saline water moves northwards in the surface layers, and cold, fresh water flows towards the equator close to the sea bed (Figure 1). These surface ocean currents transfer heat to the overlying atmosphere, and the prevailing south-westerly winds in the North Atlantic bring this heat towards the UK, making its climate milder than is typical for its latitude (Figure 1).
Figure 1: Schematic map of the Atlantic Meridional Overturning Circulation demonstrating its impact on European climate. Arrows show schematic circulation: red ones show near-surface currents and blue ones show flows at depth (adapted from UK Met Office, 2019). The contours show the difference between the observed temperature at a given location and the mean temperature at that latitude averaged across the Earth. For example, in Lerwick (approximately 60° N in latitude), the value is calculated by taking the observed annual mean air temperature and removing the average value of all locations at a latitude of 60°N). Reds show temperatures warmer than expected for their latitude; blues show colder than expected for their latitude (adapted from Rahmstorf & Ganopolski, 1999).
What is already happening?
Currently, there is no evidence that the ocean currents in Scottish waters are significantly changing due to climate change (Berx et al., 2013; Østerhus et al., 2019). Although recently there have been some studies that present evidence of a slowing of the AMOC (Smeed et al., 2014), there is little evidence that this can be attributed to anthropogenic climate change, with several studies suggesting this is due to internal variability (McCarthy et al., 2020).
The subpolar North Atlantic is also shifting towards a cooler phase (with temperatures 1 °C colder than the 1981 - 2010 mean), although this is driven by changes in the atmosphere causing increased heat loss (Josey et al., 2018). Simultaneously, the eastern subpolar North Atlantic Ocean has also seen a widespread freshening event (a reduction in salinity) caused by changes in the prevailing wind patterns influencing the circulation of the subpolar North Atlantic (Holliday et al., 2020). These changes caused more water of sub-polar origin (which has lower salinities) to be mixed in with the sub-tropical waters (higher salinity) that flow in the surface layers of the North Atlantic.
Both events are likely due to natural variability in the ocean and atmosphere, rather than the external greenhouse gas forcing. Attribution studies and other research are needed to investigate the impacts of anthropogenic climate change on these cycles and the occurrence of these extreme events.
The cooling and freshening in the subpolar North Atlantic Ocean highlights the additional year-to-year variability which overlies the decadal and longer term trends in the AMOC (McCarthy et al., 2020).
What is likely to happen in future?
The AMOC will very likely slow down by the end of the century due to the external greenhouse gas forcing. This is due to warmer atmospheric temperatures making it harder for the ocean to lose heat at high latitudes (IPCC, 2014; McCarthy et al., 2020; Pörtner et al., 2019). A collapse of the process where deep water is formed by convection may also occur due to increased Greenland Ice Sheet melt, although this will likely have an impact on a limited region in the subpolar North Atlantic (McCarthy et al., 2020).
Although most climate models agree on the trend, there are differences in the magnitude of this change. Considerable uncertainty in these model predictions is due to natural variations in volcanic and solar activity, and natural internal variability in the ocean (McCarthy et al., 2020). The latest climate models (Climate Model Intercomparison Project 6) indicate that the AMOC may weaken between 34-45% by the end of the 21st century, which is a larger decline than previous (CMIP5) models predicted (Weijer et al., 2020). The sudden collapse of the AMOC by 2100 has been assessed by the IPCC (Collins et al., 2013) as being very unlikely (defined as an event with a probability between 0% and 10%).
A recent study by Holt et al. (2018) suggests that the changes in temperature and salinity due to anthropogenic greenhouse gas emissions may also cause changes to the exchange of Atlantic water in the Northern North Sea. The predicted changes in salinity are due to changes in ocean circulation pathways. Numerical models predict a decline in this exchange before the end of the century causing widespread change to circulation, temperature, salinity, dissolved oxygen concentrations and stratification in the North Sea. These changes are likely to have an impact on marine ecosystems in the region.