A New Low for the Atlantic Meridional Overturning Circulation

In a recent paper published in Nature, a team of researchers from Canada, the United States and the United Kingdom collaborated across the Atlantic to break ground on a new study that the Atlantic Meridional Overturning Circulation (AMOC) is at its lowest in the past 150 years.

Instrumental in regulating global climate, the AMOC transports heat from the tropics and southern hemisphere toward the North Atlantic and the atmosphere. The strength of the AMOC varies periodically on a 60 to 80 year timescale, which is known as the Atlantic Meridional Oscillation. Currently in its negative phase, a weakening of the AMOC is often associated with the cooling of the Northern Hemisphere. However, it has other widespread impacts on American, African and Asian monsoons.

While other studies have shown that the AMOC has been weakening in recent years, the team demonstrated that the strength of the AMOC is at its lowest in 1,500 years, a finding that is cause for great concern. The researchers also suggested that the magnitude of decline has been severely underestimated due to the lack of sensitivity toward climate (freshwater) forcing. As meltwater is added into the oceans, changes in salinity and water density occur, which disrupt oceanic circulation patterns.

The Global Thermohaline Circulation System, with the AMOC boxed in red (Source: Authorship.me / Twitter).

One such example is the weakening of the circulation in the Labrador Sea as suggested by paleo-oceanographic evidence. This was determined by sorting the size of sediment grains deposited on the ocean bed, which showed a decrease in velocity of the Labrador Sea deep western boundary current.

The deep Labrador Sea density is always used as a predictor for changes in AMOC strength. As the Labrador Sea convection weakens, the velocity of the deep western boundary current decreases, which in turn decreases AMOC strength. This is attributed to freshwater inputs as Arctic and Nordic sea ice, ice sheets, and glaciers started to melt, which is usually thought to have been the outcome of the end of the Little Ice Age in the mid-nineteenth century.

Meanwhile, various methods and models were also used to reconstruct the surface temperatures of regions affected by the AMOC. In an interview following the publication of the study, lead author David Thornalley, a senior lecturer at University College London. stated that the “AMOC has weakened over the past 150 years by approximately 15 to 20 percent.”

Yet, the researchers results indicate the onset of AMOC weakening during the late Little Ice Age, coinciding with the industrial era. At that time, increased activity of exporting Arctic and Nordic sea ice occurred, coupled with melting circum-Arctic ice shelves.

In the same interview, Thornally had asserted that “warming and melting are predicted to continue in the future due to continued carbon dioxide emissions.” This comes as no surprise, with many studies already proving that global temperatures have been steadily increasing since the Industrial Revolution.

As the AMOC represents a dynamic coupled ocean-atmosphere system, it can be viewed as both a driver or a responder to climate change. In this case, the study believes that the AMOC has largely responded to the recent century and a half of climate change. Nonetheless, as stated in their report the AMOC may have “modified northward ocean heat transport, as well as atmospheric warming by altering ocean–atmosphere heat transfer, underscoring the need for continued investigation of the role of the AMOC in climate change.”

Thankfully, the AMOC is unlikely to completely shutdown. In an interview with GlacierHub, Wallace Smith Broecker, a well-known geoscience professor in Columbia University’s department of Earth and Environmental Sciences, said, “For what it’s worth, there are dozens of ‘water hosing’ experiments” that simulated freshwater input of higher magnitudes coming from Greenland. “Still they failed to shutdown the AMOC,” Broecker added.

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