Experts: Evident shift in Atlantic circulation
A growing body of evidence suggests that a massive change is underway in the sensitive circulation system of the Atlantic Ocean, a group of scientists said Thursday.
The strength of the Atlantic meridional overturning circulation (AMOC), a system of currents that includes the Florida Current and the Gulf Stream, is now “in its weakest state in over a millennium,” these experts say. This has implications for everything from the climate of Europe to the rates of sea-level rise along the U.S. East Coast.
Although evidence of the system’s weakening has been published before, the new research cites 11 sources of “proxy” evidence of the circulation’s strength, including clues hidden in seafloor mud as well as patterns of ocean temperatures. The enormous flow has been directly measured only since 2004, too short a period to definitively establish a trend, which makes these indirect measures critical for understanding its behavior.
Prior research had suggested a roughly 15 percent weakening since about 1950. But when it comes to the latest evidence, “I think it just makes this conclusion considerably stronger,” said Stefan Rahmstorf, an author of the research and an oceanographer with the Potsdam Institute for Climate Impact Research in Germany.
The study was published in Nature Geosciences by scientists from the Potsdam Institute, Ireland’s Maynooth University and University College London.
The AMOC is driven by two vital components of ocean water: temperature and salt. In the North Atlantic, warm salty water flows northward off the U.S. coastline, carrying heat from the tropics. But as it reaches the middle latitudes, it cools, and, around Greenland, the cooling and the saltiness create enough density that the water begins to sink deep beneath the surface.
The water then swings back southward and travels all the way to the Southern Hemisphere, submerged, where it makes its way to the Antarctic. The system is referred to as the global ocean conveyor belt, because it redistributes heat worldwide.
In the North Atlantic, most important is the transport of heat northward, which has a moderating effect on Europe’s climate in particular. But the circulation can be weakened by making northern water more fresh and less salty, and therefore less dense.
Scientists note that even the modest slowing of 15 percent has been accompanied by odd temperature patterns in the ocean and the significant upending of certain key fisheries, such as lobster and cod off the coast of New England.
In particular, a recurrent “cold blob” has been observed in the ocean to the south of Greenland. Scientists think this is evidence that less warm water is reaching this region than previously, and that it may also be a result of runoff from the melting ice sheet.
At the same time, warm water has lingered instead off the coast of the northeastern United States, where the Gulf of Maine is showing some of the fastest-warming ocean water anywhere in the world.
Michael Mann, a climate scientist at Penn State who co-wrote a major 2015 study with Rahmstorf showing a slowing AMOC, said the cold blob has been “remarkable” in its staying power.
Mann said that the cold blob’s presence since 2015 has convinced him that the ocean current’s slowdown is a robust finding. “The remarkable persistence of the cold blob since the publication of our article has convinced me, ‘Yeah, it’s real.’ ”