Ocean Physics

The ocean is responsible for most of the transport and storage of energy on Earth

The ocean is responsible for most of the transport and storage of energy on Earth and as such has a huge influence on our climate. Around 90% of the energy that reaches the Earth’s surface ultimately enters the ocean. This energy is then circulated around the ocean, sometimes taking hundreds or thousands of years to remerge at the ocean surface. The ocean acts as a major carbon sink, taking in CO2 at the surface and transporting it to the deep ocean on time scales of thousands of years. The distribution of nutrients in the ocean is largely determined by the global ocean circulation. 

The ocean: A shallow surface layer overlying a deep ocean interior

The ocean can be nominally split into an upper layer that warms and cools on seasonal timescales and is actively mixed by turbulence from the wind. This layer is often referred to as the mixed layer. This mixed layer (varying in depth from tens of metres in the summer, to hundreds of metres in the winter) overlays the vast ocean interior where exchange with the surface is far less frequent.

The ocean has two primary circulation features; (i) near surface currents driven by the wind and (ii) deeper currents driven by density contrasts across the ocean. In addition to these two current systems, the ocean also mixes through tidal currents driven by the gravitational effects of the moon and the sun.

The Atlantic Meridional Overturning Circulation: Keeping Europe warm

One of the best known surface currents is the Gulf Stream, which flows north from the Gulf of Mexico along the North American coastline. The Gulf Stream leaves the U.S. coast around Cape Hatteras and then meanders across the Atlantic, bringing warm, salty waters towards the coast of the UK and Western Europe. This is the surface manifestation of the Atlantic Meridional Overturning Circulation (AMOC) with warm, salty waters travelling north at the surface and cold water travelling southwards at depth. As the warm surface water arrives in the polar regions it rapidly loses 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 in a few localized regions 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 and salinity in the process.

The AMOC 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 deep-water by decreasing the surface water density, leading to a weakening of the AMOC. Such a weakening can lead to large impacts on the climate of Western Europe, potentially even causing a rapid regional cooling. 

a schematic showing the Atlantic Meridional Overturning Circulation
A schematic of the AMOC with relatively warm, salty surface waters from the equator (red) flowing north into the Arctic where they lose heat and mix with cold water before sinking to depth and flowing southward (blue). Illustration: S. Rahmstorf (Nature 1997), Creative Commons BY-SA 4.0.

The Southern Ocean: where deep waters find their way back to the surface

The Southern Ocean plays a major role in marine heat and carbon uptake. It accounts for 40% of the total uptake of carbon by the global ocean, while between 60-80% of the excess heat associated with recent human-induced climate change has been taken up in the Southern Ocean.

The Southern Ocean encircles Antarctica, with a strong surface current (the Antarctic Circumpolar Current, ACC) driven by intense westerly winds at the ocean surface. As a result of this circumpolar current and its response to a rotating Earth (the Coriolis Force), water in the upper layers of the ocean move poleward away from the ACC on its equatorward side.

This transport of surface water sees deep water gradually brought up to the surface to replace the divergent surface water. This “upwelling” brings to the surface water that descended to depths in the northern Atlantic as part of the surface AMOC, hundreds or even thousands of years earlier. The upwelling brings cold, nutrient rich water back to the ocean surface driving intense biological activity along the ACC. It is the combination of the cold water (increasing the solubility of atmospheric CO2) and the biological activity that makes the Southern Ocean have such a large uptake of both CO2 and heat.

Map showing Southern Ocean around Antarctica