Reconstruction of major North Atlantic circulation system shows weakening

April 11, 2018

Rising levels of carbon dioxide in the atmosphere have affected one of the global ocean's major circulation systems, slowing the redistribution of heat in the North Atlantic Ocean. The resulting changes have been felt along the Northeast U.S. Shelf and in the Gulf of Maine, which has warmed 99 percent faster than the global ocean over the past ten years, impacting distributions of fish and other species and their prey.

The Atlantic Meridional Overturning Circulation (AMOC) is a large-scale system of ocean currents that circulates warm, salty water from the South Atlantic and tropics via the Gulf Stream to the colder North Atlantic. There, warm salty waters cool, release heat, and eventually sink to the deep ocean and move south. The AMOC plays a key role in the Earth's climate and is a major component of the Global Conveyor Belt.

In a study published online in Nature, researchers from Europe and the U.S. used computer model simulations to reconstruct changes in AMOC over time. Comparisons of these simulations with recent direct ocean measurements suggest the AMOC has slowed down or weakened by about 15 percent since the 1950s.

"Our findings show that in recent years the AMOC appears to have reached a new record low, consistent with the record low annual sea surface temperature in the subpolar North Atlantic since observations began in 1870 and reported by NOAA for 2015," the authors report. "The AMOC decline since the mid-20th century is a feature projected by climate models in response to rising carbon dioxide levels."

"We found a characteristic sea surface temperature fingerprint for an AMOC slowdown or weakening in both a high-resolution global climate model and in temperature trends observed since 1870," said Vincent Saba, a research fishery biologist at NOAA's Northeast Fisheries Science Center and a co-author of the study. Saba works with high-resolution global climate models at NOAA's Geophysical Fluid Dynamics Laboratory at Princeton University. His studies have focused on the impact of changing ocean conditions on fisheries, sea turtles, and other marine life.

"That fingerprint consists of a pattern of cooling in the North Atlantic Ocean's subpolar gyre and a warming in the Gulf Stream region due to reduced northward heat transport and an associated northward shift in the Gulf Stream," Saba said. "In other words, there is warming along the Northeast U.S. Shelf and Gulf Stream region, and at the same time a cooling in the North Atlantic subpolar gyre."

The researchers used NOAA's CM2.6 global climate model to identify the characteristic sea surface temperature (SST) fingerprint associated with an AMOC weakening in response to rising atmospheric carbon dioxide. The model results were compared to observed SST evolution since the late nineteenth century. The CM2.6 model provides very high resolution, which means the realism of the model is greater than many other models currently in use. For example, the ocean bottom is more accurately represented in the CM2.6 model compared to lower resolution models.

The study authors then used a group of global climate models known as CMIP5 to test and calibrate a revised AMOC index. The reconstruction of the evolution of the AMOC from 1870 to 2016 reveals the record low in the past few years and is consistent with direct measurements since 1995 from a number of AMOC studies using different methods.

NOAA's CM2.6 model is being used for a variety of fisheries studies on the impact of ocean temperatures on lobsters, scallops, various fish species, leatherback sea trutles, and other animals. The model's high spatial resolution enables researchers to look much more closely at ocean features in regions like the Gulf of Maine or along the Northeast U.S. Shelf than other models, which have a lower ocean resolution and can miss the finer details.

The rapid ocean warming observed along the Northeast U.S. Shelf may be associated with the Gulf Stream shifting northwards and closer to shore, a consequence of the AMOC slowdown. In NOAA's high-resolution climate model, enhanced warming of ocean bottom temperatures in the Northeast U.S. Shelf and in the Gulf of Maine is a result of both a poleward retreat of the Labrador Current and a northward shift of the Gulf Stream.

Continued warming is likely to further weaken the AMOC in the long term, through changes to the hydrological cycle, sea-ice loss, and accelerated melting of the Greenland Ice Sheet, all of which are causing the North Atlantic to become fresher and less dense. "If the AMOC continues to weaken," Saba said, "ocean temperature along the Northeast U.S. Shelf is expected to continue its trend of warming faster than the global ocean, which will further impact fisheries and living marine resources in the region."
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In addition to NOAA Fisheries, authors of the paper are affiliated with the Potsdam Institute for Climate Impact Research and the Institute of Physics and Astronomy at the University of Potsdam in Potsdam, Germany; the Complutense University of Madrid in Madrid, Spain; the Instituto de Geociencoas, CSIC-UCM in Madrid, Spain; and the National and Kapodistrian University of Athens in Athens, Greece.

NOAA Northeast Fisheries Science Center

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