Adding to concerns about potential interruptions to the North Atlantic Current is the possibility that a subcomponent of that crucial network of currents, the Northern Subpolar Gyre, has been losing stability since the 1950s, indicating that it may also be approaching its own tipping point. While the consequences of an interruption in the Gyre’s circulation wouldn’t be as drastic as those from a shutdown of the larger system, the impacts would still be far-reaching, with climate effects that would be felt on both sides of the Atlantic.
Comprising the northern section of the Atlantic Meridional Overturning Circulation (AMOC), the Northern Subpolar Gyre (NSG) is a counter-clockwise-rotating series of currents defined by the eastward-flowing North Atlantic Current in the south; the northward Irminger Current to the east; and the Labrador Current flowing back south along the North American coast.
While the NSG doesn’t represent the entirety of the AMOC—itself playing a major role in regulating the climates on either side of the Atlantic and a key component in the circulation of global ocean currents—its connection with the North Atlantic Current could spell trouble for the transport of heat energy from the Caribbean across the ocean to northern Europe if it were to weaken, and there are signs that the NSG is in danger of doing just that.
This destabilization of the NSG’s currents was uncovered by a study led by researchers with the U.K’s University of Exeter by reconstructing a history of the Gyre’s currents through an analysis of the effect of its climate-altering effects on the shells of clams. By studying the growth rates and the isotopic composition of the bivalves’ shells, the team discovered that the NSG suffered no less than two major destabilization events over the past 150 years: one that preceded a steep decline in NSG currents in the 1920s; and the second, while seeing a continuous increase in current strength that started in the 1950s, has also displayed an decrease in the stability of that flow.
“It’s highly worrying,” remarked study lead Beatriz Arellano Nava, a University of Exeter postdoctoral research fellow. “The subpolar gyre was recently acknowledged as a tipping element. We still need to understand more of the impacts of a subpolar gyre abrupt weakening. But what we know so far with the few studies that have been published is that it would bring more extreme weather events, particularly in Europe… and also changes in global precipitation patterns.”
While the climate effects from a disruption of the NSG mightn’t prove to be as drastic as a slowdown of the AMOC itself, Arellano Nava pointed out that “even if the consequences are not as catastrophic as for an AMOC collapse, a subpolar gyre weakening can bring substantial climate impacts.”
Like the AMOC, the robustness of the NSG’s currents is extremely thermohaline sensitive, meaning that both the temperature and the salinity of their waters affect how well they flow. For instance, when the warm water of the North Atlantic Current cools as it meets the Arctic currents off the coast of northern Europe, the water sinks to lower depths, where it meets another flow that travels south along the seafloor that eventually reconnects with the currents circling Antarctica.
But in recent decades the increase in the flow of fresh water from melting glaciers in Greenland mixing with the North Atlantic Current has been hindering that mechanism: since the less-saline glacier water is less dense, and thus more buoyant, than the saltier flow coming from the south, it impedes the water from sinking as it normally would, preventing it from returning to the global circulation.
The NSG operates using the same mechanism, with the waters of the Irminger Current, branching northwest off of the North Atlantic Current, sinking as it cools as it reaches the waters between Greenland and Labrador, meaning it is just as vulnerable to an influx of fresh water as the larger system. However, Arellano Nava points out that, while an interruption of the AMOC would negatively impact the NSG, a collapse of the NSG wouldn’t necessarily doom the AMOC itself.
“The subpolar gyre can weaken abruptly without the AMOC collapsing,” she explained. “That’s what happened during the transition into the Little Ice Age, which happened in the 13th and 14th centuries,” a period between the 13th and 19th centuries that saw a decrease in average temperatures across Europe and North America.
While ironic, a global warming-driven collapse of the AMOC, NSG—or both—could lead to a similar plunge in temperatures in the Northern Hemisphere, a potentially catastrophic situation explored by Whitley and co-author Art Bell in their 1999 book The Coming Global Superstorm.