NOAA: Major North Atlantic Circulation System Shows Weakening

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 image shows the sea surface temperature trends in the North Atlantic using the NOAA CM2.6 climate model. The image at right shows the observed trends during the period 1870-2016. Regions showing cooling or below-average warming are shown in blue; regions that show above-average warming are shown in red. Image credit: L. Caesar et al. 2018

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.

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.”