The Physics of the Atlantic Meridional Overturning Circulation (AMOC)

The Atlantic Meridional Overturning Circulation (AMOC) is a large-scale system of surface and deep currents in the Atlantic Ocean that transports heat from southern latitudes to the North Atlantic. The system plays a key role in shaping the climate of the Nordic countries and northern Europe. This system is part of the Gulf Stream system, but the AMOC also encompasses other currents and processes.

Understanding the physics of ocean currents is based largely on the work of oceanographers since the middle of the last century. One individual, Henry Stommel (1920 - 1992), played a major role in explaining both the surface circulation, and the amplification of ocean currents on the western edge of the Atlantic and Pacific Oceans (e.g. the Gulf Stream and the Kuroshio Current), deep-sea currents, and in addition he presented a simple model of vertical ocean circulation, a model often referred to as the "thermal and salinity circulation". One feature of this model is that according to it, the ocean circulation has at least two stable states, one where deep-sea formation occurs in cold places (e.g. near Greenland and Antarctica) and another where deep-sea formation is weaker and occurs in warm areas.

One prediction of the model was that the current state of deep-sea formation near Greenland could therefore be disrupted by reducing the salinity of the sea in that area, e.g. by increasing the flow of freshwater there. In the current state, the surface ocean in the North Atlantic transfers heat northward, and the cooling ocean then releases the heat and warms the Arctic. Results from cores, e.g. ice cores from the Greenland ice sheet and sediment cores from the ocean show that during the last glacial period, deep-sea formation in the North Atlantic Ocean was repeatedly disrupted, and each time the region cooled significantly. These disruptions in the AMOC have been linked to large amounts of freshwater flowing from the North American and Scandinavian ice sheets.

In modern times, it is likely that deep-sea formation in the North Atlantic Ocean was disrupted about 8200 years ago, which reduced the flow of warm water to the north, and as a result, there was a cold snap in the North Atlantic Ocean, including Iceland, for at least 160 years.

Ocean circulation models can be used to simulate these events, and they show that the ocean circulation can be unstable. The simple physics of the Stommel heat and salinity model can be used to explain in a simple way how this cycle becomes unstable. And if the ocean surface is subjected to a slowly increasing forcing (e.g., by freshwater from melting glaciers), the system can respond linearly until a tipping point is reached that quickly shifts it to another state.

Climate change is causing many changes around the world, and among other things, rapid and perhaps irreversible changes to the North Atlantic ocean current system are often discussed. Such changes could reduce warming here on land, or even cause cooling here in a world that is already warming. However, the consequences of this are not well known, as the scenario can be quite complex. The probability is also not well known, but it depends on how much greenhouse gas is emitted and how much disruption there would be to ocean currents. The risk is there, however, and the only known way to reduce it is to reduce greenhouse gas emissions as much as possible. The paper traces the history of this field, discusses the instability of the AMOC cycle, and reviews the current state of knowledge.