The Freshening Gyre

Melting land-based ice in the Arctic not only threatens to raise sea levels; it also threatens to disrupt a critical ocean circulation pattern that transports warm water from the tropics to the North Atlantic. This transport, part of the larger Atlantic Meridional Overturning Circulation (AMOC), is largely responsible for maintaining temperate conditions in Europe. Its weakening could drastically alter that continent’s climate, bringing extreme winter storms and summer heat waves—and cause extreme sea-level rise on the eastern seaboard of the United States. It all hinges on how salty North Atlantic seawater is.

High-salinity water brought about by evaporation in the subtropics is transported northward, where the excess salt, together with the cooling influence of surface winds, makes the water denser, causing it to sink. As the high-salinity water plunges lower, surface water is pulled in behind it (which then gets cooler and saltier and sinks as well), creating a flow. This is the driving force behind the AMOC, operating in the subpolar North Atlantic—so long as the water there is saline enough to sink.

“Much has been made of the potential for freshwater release from melting glaciers in Greenland to irreversibly disrupt the AMOC,” says Los Alamos climate scientist Wilbert Weijer, “and that’s certainly a concern. But there’s another source of freshwater up there—one that has increased its freshwater content by 40 percent in just the past two decades—and our research says it could freshen parts of the north Atlantic even more than meltwater from Greenland.”

The reservoir in question is known as the Beaufort Gyre, a circulating current in the Arctic Ocean directly north of Alaska, largely fed by freshwater rivers. If water flows out of the Gyre over a relatively short period of time due to changing wind conditions, as occurs periodically—most recently in the 1980s through early 90s—it could produce a seawater-freshening effect, greatly reducing the salinity wherever the Gyre’s water enters the Atlantic.

In a detailed computer simulation that retrospectively traced the freshwater outflows from the Gyre in the 80s–90s event, Weijer and Los Alamos colleague Milena Veneziani, together with collaborators at the National Oceanic and Atmospheric Administration and the University of Washington (including Jiaxu Zhang, who was a postdoctoral scientist at Los Alamos during the research), found that most of the freshwater emerges into the Atlantic via the Labrador Sea between Greenland and Canada. A smaller amount enters via the Nordic Seas, between Greenland and Norway. The AMOC mechanism operates in both regions.

“If a similar release were to occur now or in the near future,” says Weijer, “Beaufort Gyre water entering the Labrador Sea would easily eclipse its counterparts from Greenland and melting sea ice.”

Veneziani, a computational oceanographer, notes that while Los Alamos has extensive expertise in climate modeling studies like this one and regularly obtains important insights such as these, the earth is a complex system, and other effects may come into play.

“This study reliably assessed the movement of freshwater from the Beaufort Gyre,” Veneziani says. “But we did not specifically address the degree to which it would disrupt saline water sinking and therefore driving the AMOC.” Indeed, historical data has provided good reason to believe that winter conditions on the surface—a mild winter, or a harsh one—will significantly affect freshwater’s impact on the broader ocean current.

Yet the trend is firmly in one direction: more freshwater, more change. Short of a quick and dramatic end to global temperature rise, the AMOC is expected to slow down, possibly with global consequences. When will it happen? The Los Alamos experts exchange a troubled glance before summarizing the scientific consensus: maybe just a few decades, and almost certainly before this century is out.