The ocean is responsible for much of the transport and storage of energy on Earth and as such has a huge influence on our climate. The ocean also acts as a major carbon sink, taking in CO2 at the surface and transporting it to the deep ocean on time scales of 1000’s of years. The distribution of nutrients in the ocean is also largely set by the global ocean circulation.

The UK and the rest of Western Europe enjoy a relatively mild climate, particularly compared to other regions at the same latitude, such as Ontario in Canada. A significant contributor to this mild climate is the Meridional Overturning Circulation (MOC) (part of the global thermohaline circulation) which brings warm, salty equatorial water northwards into the North Atlantic, helping to keep our coastline ice free during winter.

Figure 1. The Meridional Overturning Circulation (thermohaline current) from: (Kuhlbrodt et al. 2007, Rev. Geophys., 45, RG2001, 2007, doi:10.1029/2004RG000166)

As warm surface ocean waters arrive in the polar region they rapidly lose heat to the cold, overlying atmosphere. When the surface of the ocean cools sufficiently it begins to freeze, forming sea ice. This locks freshwater away within the ice and causes the salinity of the surface waters to increase. The cold temperatures and high salinity both act to increase the density of surface waters, causing them to sink to a depth of many kilometres below the surface, forming what is referred to as North Atlantic Deep Water (NADW). Warm waters from the equator continue to fill the space left by this sinking water, resulting in an ocean current moving from the equator to the poles at the surface and from the poles to the equator at depth, redistributing heat in the process. However, the MOC is sensitive to changes in surface temperature and salinity caused by factors external to the ocean. A warming atmosphere or an influx of freshwater from melting sea or land ice may reduce the formation of sinking deepwater, leading to a weakening or even complete shutdown of the MOC. Such a weakening will lead to large impacts on the climate of Western Europe, potentially even leading to rapid regional cooling.

Meanwhile, it is in fact the Southern Ocean that has the most influential role in carbon uptake, accounting for 40% of the total ocean carbon uptake: the largest of all water bodies on Earth. The ocean encircles Antarctica and possesses a unique upwelling of NADW, increasing productivity in the region by bringing up cold nutrient-rich waters. Just north of here, the convergence of cold Antarctic Surface Water (AASW) and the warmer Subantarctic Water to the north forces the AASW to sink to form Antarctic Intermediate Water (AATW), transporting dissolved carbon into the deep ocean. Furthermore, deep water formation in the Ross and Weddell seas within the Southern Ocean also acts as a site of carbon uptake. Like in the Arctic, decreasing temperatures caused by icy surface wind from the Antarctic continent, and increased salinity from sea ice formation, promotes sinking to form Antarctic Bottom Water (AABW), transporting dissolved carbon with it. Just as in the North Atlantic, there remains the fear of a slowing of carbon uptake in the Southern Ocean as a result of rising temperatures and freshwater input. Due to the mammoth role of the Southern Ocean in the uptake of atmospheric CO2 and therefore its influence in balancing human emissions, these could have huge consequences on the global climate.

Read more about the role of Ocean physics in UKESM1