Separately, the research teams usually go out on longer voyages every 18 months, to remove and replace sensors from three or four moorings on the eastern side of the Bahamas. Their UK counterparts do the same job on the eastern side of the ocean and along the Atlantic Ridge.
Other groups have set up arrays of moorings across different parts of the Atlantic to better understand how varying components work, how tightly the system is connected, and whether changes in one part are rippling throughout.
Susan Lozier, an oceanographer at the Georgia Institute of Technology, leads an international effort known as OSNAP, which began in 2014. It has anchored cables across the Labrador Sea and from the southeastern edge of Greenland to the coast of Scotland.
The hope of the international research effort was to go to the sources of the deep-water sinking, which is largely responsible for propelling the currents in the Atlantic, to “try to get a much better understanding of the mechanisms driving change in the AMOC,” Lozier says.
So far, what the monitoring programs have largely found is that the Atlantic circulation is more variable than previously believed, she says.
Its strength and speed fluctuate dramatically from month to month, year to year, and region to region. Most of the deep-water sinking in the North Atlantic seems to be occurring not in the Labrador Sea, as long believed, but rather in the basins to the east of Greenland. The northward- and southward-flowing limbs operate more independently than previously understood. Local wind patterns seem to exercise a more influential role than expected. And some findings are just befuddling.
It’s very likely that the Atlantic circulation has weakened. Studies by Rahmstorf of the Potsdam Institute and others have concluded it’s about 15% slower than during the mid-20th century and may be at its weakest in more than 1,000 years. Both findings are based, in part, on long-term reconstructions of its behavior using records like Atlantic Ocean temperatures and the size of grains on the ocean floor, which can reflect changes in deep-sea currents.
There’s also “strong agreement” in models that the currents will continue to weaken this century if greenhouse-gas emissions continue.