The Gulf of St. Lawrence has warmed and lost oxygen faster than almost anywhere else in the global oceans, impacting oxygen levels in the St. Lawrence Seaway, according to new research.

The new study led by the University of Washington, Rapid coastal deoxygenation due to ocean circulation shift in the northwest Atlantic, looks at the causes of this rapid deoxygenation and links it to two of the ocean’s most powerful currents: the Gulf Stream and the Labrador Current. Published September 17th in Nature Climate Change, the research explains how large-scale climate change is causing oxygen levels to drop in the deeper parts of this waterway.

“The area south of Newfoundland is one of the best-sampled regions in the ocean,” said first author Mariona Claret, a research associate at the UW’s Joint Institute for the Study of the Atmosphere and Ocean. “It’s also a very interesting area because it’s at the crossroads where two big, larger-scale currents interact.”

Fisheries and Oceans Canada has tracked rising salinity and temperature in the St. Lawrence region since 1920. Oxygen has only been monitored since 1960, and the declining trend is causing concern.

“The oxygen decline in this region was already reported, but what was not explored before was the underlying cause,” said Claret, who conducted the research while at McGill University.

The Gulf Stream and Labrador Current both split near the Laurentian Channel, a deep channel within the Gulf of St. Lawrence fed by both currents. The Gulf Stream in turn is sensitive to changes in the Atlantic Meridional Overturning Circulation. Credit: Mariona Claret/University of Washington.

The research confirms a recent study showing that, as carbon dioxide levels rose over the past century due to human emissions, the Gulf Stream has shifted northward and the Labrador Current has weakened. Claret et al.’s work finds that this causes more of the Gulf Stream’s warm, salty, and oxygen-poor water to enter the St. Lawrence Seaway.

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The new study uses output from NOAA’s Geophysical Fluid Dynamics Laboratory model, a high-resolution computer model that simulates the world’s oceans with a data point every 8 kilometers. This simulation took nine months to run using 10,000 computational nodes, which is huge, even by the standards of global climate models.

With this precision, eddies and details of the coastline that can influence ocean circulation begin to appear. Model output combined with the historical observations show that as the carbon dioxide levels go up, Gulf Stream water replaces Labrador Sea water in the deeper parts of the St. Lawrence gulf.

“We relate a change in oxygen on the coast to a change in large-scale currents in the open ocean,” Claret said.

In the model, the shift in the large-scale ocean circulation causing warming and deoxygenation in the Gulf of Saint Lawrence also corresponds with a decline in the Atlantic Meridional Overturning Circulation, an ocean circulation pattern known to strongly influence climate in the Northern Hemisphere.

“Being able to potentially link the coastal changes with the Atlantic Meridional Overturning Current is pretty exciting,” Claret added.

Analysis shows that half the drop in oxygen observed deep in the St. Lawrence River is due to the warmer water, which can’t hold as much oxygen. The other half is likely due to other factors, such as biological activity in the two currents and inside the channel. What will happen next is unknown, Claret said. The oxygen levels in the St. Lawrence will depend on much larger questions, like how much carbon dioxide humans will emit into the atmosphere in the coming decades and how large-scale ocean currents will respond, she said.

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The research was funded by the European Research Council, the Spanish Ministry of Economy and Competitiveness, the Canada Foundation for Innovation, and NOAA. Co-authors are Eric Galbraith at the Autonomous University of Barcelona; Jaime Palter at the University of Rhode Island; Daniele Bianchi at the University of California, Los Angeles; Katja Fennel at Dalhousie University in Nova Scotia; Denis Gilbert at Fisheries and Oceans Canada; and John Dunne at NOAA’s Geophysical Fluid Dynamics Laboratory.

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