The warming atmosphere is causing an arm of the powerful Gulf Stream to weaken, some scientists fear.
Mar 3, 2021 | Atlanta, GA
Re-posted from the Full Article in the New York Times:
Recent studies suggests this northern portion of the Gulf Stream and the deep ocean currents it’s connected to may be slowing. Pushing the bounds of oceanography, scientists have slung necklace-like sensor arrays across the Atlantic to better understand the complex network of currents that the Gulf Stream belongs to, not only at the surface, but hundreds of feet deep.
Susan Lozier, a physical oceanographer and dean at the College of Sciences at Georgia Tech, has her doubts about whether the AMOC is currently slowing. At issue, she says, is how scientists infer changes in the AMOC. We can directly measure many aspects of the ocean, such as temperature (it’s warming), oxygen levels (they’re declining), even how stratified it has become (more so). “There are very strong signals in the ocean of climate change,” she said.
But most studies on the AMOC don’t measure the “conveyor belt” directly. Instead, they use proxies to infer that the overturning has changed.
Such inference can be problematic when considering changes that occur over short time frames, says Dr. Lozier, because the changes observed could have other causes. Consider that cold blob in the North Atlantic, she said. Dr. Rahmstorf and others see it as evidence of a weakening Gulf Stream, but Dr. Lozier notes that shifts in wind patterns or how storms move over the ocean could also underlie the phenomenon. “There are other ways to explain it,” she said. “A lot of our conceptual understanding of AMOC is in isolation of other things going on in the ocean.”
Direct measurement of the AMOC only began relatively recently. A line of sensors between the Bahamas and the Canary Islands, called Rapid, was installed in 2004. A second sensor array, spanning the North Atlantic from Canada to Greenland to Scotland and called Osnap, went live in 2014. (Dr. Lozier is the international project lead for Osnap.)
Neither project has operated long enough to produce clear trends, in Dr. Lozier’s view. What they have shown, though, is lots of natural variability. In 2009 and 2010, for example, the AMOC weakened — “people were like, ‘Oh my God, this is happening,’” she said — only to pick right back up again over the following years.
They’ve also revealed a system of currents that’s far more complex than once envisioned.
Dr. Broecker’s old schematics of the AMOC posit a neat warm current flowing north along the western edge of the Atlantic and an equally neat cold current flowing back south below it. In fact, says Dr. Lozier, that deeper current is not confined to the western edge of the Atlantic, but rather flows southward via a number of “rivers” that are filled with eddies. The network of deep ocean currents is much more complicated than once envisioned, in other words, and figuring out how buoyant meltwater from Greenland might affect the formation of cold deepwater has become more complicated as well.
This is the place scientists currently find themselves in. They suspect the AMOC can work like a climate switch. They’re watching it closely. Some argue that it’s already changing, others that it’s too soon to tell.
“There’s no consensus on whether it has slowed to date, or if it’s currently slowing,” said Dr. Lozier. “But there is a consensus that if we continue to warm the atmosphere, it will slow.”