Past and future of the Atlantic Meridional Overturning Circulation | Polarjournal
The North Atlantic ocean around South Greenland is one of the few regions where oxygen-rich surface water sinks down to the deep ocean due to severe cooling, thus driving the Atlantic Meridional Overturning Circulation. (Photo: Julia Hager)

The Atlantic Meridional Overturning Circulation plays a crucial role for the global climate. But since the early 2000s, it has been weakening, and there is growing concern about a possible collapse of the circulation system. Climate change is considered at least partly responsible for the weakening. But aftereffects of several consecutive very cold winters around South Greenland in the 1990s may also play a partial role, according to a new study.

The Atlantic Meridional Overturning Circulation (AMOC) distributes the heat generated by solar radiation worldwide, and Europe gets a good portion of it in the form of the Gulf Stream, which is part of the huge current system. Thanks to the Gulf Stream, Europe has a comparatively mild climate – in Alaska, for example, temperatures are significantly lower than in Scandinavia, even though both regions lie on the same latitude.

Since 2004, researchers have been able to directly measure the evolution of the AMOC, and since then a weakening of the circulation has been observed, which could be related to the input of huge amounts of meltwater from Greenland’s ice sheet. A research team from the German GEOMAR Helmholtz Centre for Ocean Research in Kiel has now investigated further causes for the weakening of the circulation in a new modelling study. The results were published recently in the journal Nature Communications.

Circulation of the subpolar North Atlantic: The image shows a snapshot of the surface velocity in the high-resolution VIKING20X model, showing the meandering flow of the North Atlantic Current and the narrow boundary current that develops south of the Denmark Strait along the eastern continental shelf of Greenland. Shaded in grey is the area where convection exceeded 1800 m depth during the winters of 1990-1994. (Figure: GEOMAR)

In their elaborate computer simulations, which included the past 60 years, the researchers focused on the Labrador Sea between southwest Greenland and Canada’s Labrador Peninsula. They note that variations there can have strong effects on deep water formation – the sinking of cold, dense surface water to depth – in the Irminger Sea between southeast Greenland and Iceland.

“We oceanographers have long had our eyes on the Labrador Sea between Canada and Greenland,” says Professor Dr Claus Böning, who led the study. “Winter storms with icy air cool the ocean temperatures to such an extent that the surface water becomes heavier than the water below. The result is deep winter mixing of the water column, whereby the volume and density of the resulting water mass can vary greatly from year to year.”

Particularly the years 1990 to 1994, when very cold, harsh winters had an especially strong cooling effect on the Labrador Sea, stood out from the rest. In subsequent years, the researchers noted a marked increase in the sinking of water masses in the Irminger Sea, which resulted in a more than 20 percent increase in Atlantic overturning transport in their model calculations. Towards the end of the 1990s, the transport had reached its peak.

Based on these results, the research team suspects that the weakening of the AMOC measured from 2004 onwards could be at least partly an after-effect of this transport maximum, which was then in its decay phase.

In a press release from GEOMAR, Professor Dr. Arne Biastoch, head of the Ocean Dynamics research unit at GEOMAR and co-author of the study, emphasises that all climate models predict a weakening of the Atlantic overturning circulation as a result of human-induced climate change as “very likely” in the future, even though the results so far cannot currently point to a longer-term weakening.

The AMOC is part of the global thermohaline ocean circulation, which is driven by density differences in seawater. In the Atlantic, oxygen-rich water flows at the surface from Antarctica through the Gulf of Mexico , where it takes up a lot of heat, into the North Atlantic (red). There it cools down dramatically and sinks into the depths in the so-called deep water formation zones (yellow dots) and flows back in the deep sea along the two American sub-continents towards Antarctica (blue). (Figure: Rahmstorf 2002)

If the AMOC was not only to weaken, but to collapse again, as it last did 12,000 years ago, this could have far-reaching consequences for the entire planet. A study published only a few days earlier, also in Nature Communications, warns that this could possibly occur very soon – between 2025 and 2095. Even if other scientists regard an imminent collapse as questionable, there is general consensus about the possible consequences:

  • The Northern Hemisphere would be affected by a drastic cooling of air temperatures, especially over the Arctic Ocean and the North Atlantic south of Greenland.
  • Europe in particular would face colder times with more winter storms and a drop in temperature of 3°C to 8°C.
  • Temperatures in the tropics would rise even further.
  • In the Sahel, even less rain would fall, with devastating consequences for the crops.
  • The summer monsoon in Asia would weaken.
  • Sea levels would rise even faster.
  • Minor changes in Atlantic currents could have serious impacts on marine organisms.

Even if science is still divided on how close the system really is to the AMOC tipping point, there’s certainty that it will come – with potentially massive impacts worldwide. Many experts take the global extreme events of recent weeks and months as an example. These have surprised even the climate scientists, who had not yet expected such strong effects.

Julia Hager, PolarJournal

Links to the studies:

Böning, C.W., Wagner, P., Handmann, P. et al. Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection. Nat Commun 14, 4635 (2023).

Ditlevsen, P., Ditlevsen, S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nat Commun 14, 4254 (2023).

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