Melting glaciers often brings to mind sea level rise—coastal cities inundated with several meters of water or popular tourist spots and landmarks poking out of waves. But another, less vivid effect of climate change on the world’s oceans is coming to light: a slowing of the global ocean circulation that brings warm water and nutrients to the poles and cold water to the equator.
Scientists, with help from an armada of elephant seals wearing scientific instruments, found that fresh water leaching from melting glaciers disrupts the formation of the Antarctic Bottom Water (AABW), a layer of deep, cold ocean water—a process that drives the southern half of the global conveyor belt.
“This suppression is likely to be already underway in the other key AABW regions around Antarctica,” said Guy Williams, an observational polar oceanographer at the University of Tasmania in Australia. He and his colleagues published a paper about their findings on 23 August in Nature Communications.
The Global Conveyor Belt
The ocean isn’t just waves on top of a static block of water—distinct layers, stacked on top of each other, swirl around the Earth, distinguishable by their temperature or saltiness. These different layers drive the thermohaline circulation—the global conveyor belt of water that moves heat and nutrients around the globe, brings oxygen down to the lower layers of the ocean, and helps the ocean absorb heat caused by excess carbon dioxide.
In several locations along Antarctica’s coastline, strong winds whip away sea ice, exposing areas of bare ocean called polynyas. These areas are important to the AABW, which drives the southern arm of the global ocean circulation. When sea ice forms in the ephemeral polynyas, it ejects salt, creating a heavy, briny water that sinks to the ocean’s floor.
“Polynyas can generate sufficient sea ice growth to produce dense shelf waters around Antarctica,” Williams said.
The researchers honed in on one less understood area: Cape Darnley Polynya in East Antarctica, which is thought to be an important source of this dense shelf water. The researchers also wanted to better understand its relationship with the nearby Prydz Bay, which hosts its own set of wind-swept polynyas.
Armada of Seals
There’s only so much modeling and theorizing that a scientist can do, Williams said, and frigid polar waters are often too hostile for direct observations from research cruises or field work. In the Arctic, thermometer-wearing narwhals have helped scientists detect rising sea temperatures in Baffin Bay, between northern Canada and Greenland.
For this new research, it was time to call in aid from some of the Southern Ocean’s natural-born swimmers—elephant seals.
Armed with small instruments glued to their heads, elephant seals have helped scientists profile much of the Southern Ocean since the early 2000s. Like living, breathing Argo floats, these 4000-kilogram blubbery creatures dive down for food, sometimes 1000 meters deep and into the AABW, Williams said.
The seals’ dives provide a complete profile of the water column, including temperature and salinity, through key masses of water like the deep shelf water that drives the AABW. The animals dive throughout autumn, winter, and spring, allowing researchers to see what happens to the water column during this time.
The researchers looked closely at Prydz Bay and Cape Darnley, using a subset of elephant seal data from the Marine Mammals Exploring the Oceans Pole to Pole data set from 2011–2013. The seals’ instruments detected an ominous phenomenon along the continental shelf: fresh water flushing in from glacier melt.
Because this water contains less salt and thus is less dense, it dilutes and subtly suppresses the formation of the Prydz Bay dense water, which in turn affects the Cape Darnley bottom water. In particular, fresh water was coming from the Amery glacier and the West Antarctic Ice Sheet.
“Dense shelf water is still forming in this location, the polynyas are still doing their job,” Williams noted. “It is just now more obvious when we see the big picture, courtesy of the amazing coverage of the seal data in space and time, that this formation is being ‘held back,’ so to speak, by the local ice shelves.”
Scientists have observed this effect in the Arctic as well, said Stefan Rahmstorf, an oceanographer and climatologist at the Potsdam Institute for Climate Impact Research in Germany, who wasn’t involved in the new paper. Last year, Rahmstorf and his colleagues used a reconstruction of North Atlantic sea surface temperatures and found that fresh water from glacier melt triggered a slowdown of the overturning circulation in the 1970s. Ice loss from the Greenland Ice Sheet may be enough to dilute the salty deep water enough to weaken the circulation.
“The new study reaffirms such concerns also for the Antarctic ocean,” Rahmstorf said. It “significantly improves our understanding of the details of bottom water production around Antarctica.”
—JoAnna Wendel, Staff Writer