Why does the AMOC vary
and why does it matter?

We often think of the Atlantic Meridional Ocean Circulation (AMOC) as a continuous oceanic conveyor belt, constantly moving vast quantities of warm water in and cold water out of the subpolar North Atlantic. But the truth, as always, is more complicated.

After direct continuous observations of the subtropical AMOC began in 2004, scientists were surprised to see how much the strength of the AMOC fluctuated, even from day to day! This short-term variability is actually quite normal in the climate system. We experience it every day, as the air temperatures around us can easily vary by 10 degrees or more over the course of 24 hours. In contrast, the differences in the global mean temperature from one year to the next are much smaller, at only about a tenth of a degree. Generally speaking, the larger the system and the longer the timescale, the smaller the fluctuations. But some large-scale systems, like the AMOC, are subject to large fluctuations on very long timescales such as decades or even millennia.

These fluctuations can have both internal and external drivers. External forcings include, for example, greenhouse gases, aerosols and volcanic components, all of which enter the atmosphere and influence how much the sun’s energy warms or is reflected by the earth at a given location. This can result in changes in ocean density and changes in the wind systems – both of which influence the strength of the AMOC. Internal forcings are generated within the AMOC itself and are triggered by the chaotic nature of the climate system. Climate models suggest that the AMOC responds to increasing concentrations of greenhouse gases in the atmosphere but also has a wide range of internal variability; in the real world it is very difficult to distinguish between these two factors.

But why are we interested in AMOC variability at all? Due to the inflow of warm water and the outflow of cold water in the sub-polar region, the AMOC is responsible for up to 90 per cent of the heat transport northwards in the Atlantic Ocean and plays a major role, for example, in the Northern Hemisphere being on average 1-2 degrees warmer than the Southern Hemisphere. In fact, paleoclimatic data (climate data from the past) suggest that there were times in Earth’s history when the AMOC was much weaker than it is today, and temperatures around the North Atlantic were therefore a few degrees colder on average.


ocean and clouds

But even smaller fluctuations in AMOC strength can have an impact on our lives. Through the massive exchange of warm and cold water, the AMOC system transports up to about a petawatt (1 with 15 zeros) of heat northwards in the Atlantic. For comparison, this is equivalent to the energy production of about one million nuclear power plants. Since part of that petawatt of heat energy transported northwards by the AMOC is released into the atmosphere and transported to Europe by westerly winds across the North Atlantic, the AMOC exerts a major influence on the climate in Europe. A weakening of the AMOC can lead to a cooling of the subpolar North Atlantic, which in turn can trigger heat waves. For example, the 2015 heatwave in Europe was linked to anomalously cold sea surface temperatures south of Greenland. The connection is that low subpolar sea surface temperatures favour an air pressure distribution that directs warm air northwards towards Europe. But the AMOC does not only influence temperatures.

crowded beach
Cold seas in the subpolar North Atlantic region can bring beach weather to Europe

In addition to the enormous amount of thermal energy, the AMOC also moves nutrients and carbon across the Atlantic and transports the latter to deeper ocean layers. Changes in AMOC strength therefore also affect the marine ecosystem and the oceanic carbon sink (i.e., the ocean’s ability to absorb carbon dioxide from the atmosphere). Other areas affected by AMOC strength include tropical rainfall patterns (i.e., the African and Asian monsoon systems), coastal sea levels on both sides of the Atlantic, and the likelihood of storms in northwest Europe.

Due to this significant impact that AMOC variations have on our lives, scientists are making great efforts to better understand both natural AMOC fluctuations and the AMOC response to anthropogenic influences.


Spotlight article by Levke Cesar