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The computer model is detailed enough to solve the large eddies for ocean circulation. The triangular island of Newfoundland, in the center, lies on the eastern edge of the study area, at the mouth of the Gulf of St. Lawrence. This graph shows the oxygen at the surface, where the red shows more oxygen. Credit: Mariona Claret / University of Washington
The Gulf of St. Lawrence has warmed up and lost oxygen faster than almost everywhere else in the oceans. The vast, biologically rich waterway of eastern Canada drains the Great Lakes of North America and is popular with fishing boats, whales and tourists alike.
A new study by the University of Washington is looking into the causes of this rapid deoxygenation and links it to two of the most powerful currents in the ocean: the Gulf Stream and the Labrador Current. The study, published on 17 September Nature Climate Change, explains how large – scale climate change is already leading to a drop in oxygen levels in the deepest parts of this waterway.
"The region south of Newfoundland is one of the best sampled regions of the ocean," said the first author, Mariona Claret, Research Associate at the Joint Institute for the University of Newfoundland. 39, study of the atmosphere and oceans. "It's also a very interesting area because it's at the crossroads where two large, large-scale streams interact."
The Fisheries Agency of Canada monitors the increase in salinity and temperature in the St. Lawrence since 1920. Oxygen has only been monitored since 1960 and the trend downward is worrying.
"The observations in the very interior of the Gulf of St. Lawrence show a dramatic decline in oxygen, which reaches hypoxic conditions, which means that it can not fully support marine life," he said. Claret.
The Gulf Stream and the Labrador Current have both separated near the Laurentian Channel, a deep channel in the Gulf of St. Lawrence fed by both currents. The Gulf Stream, meanwhile, is sensitive to changes in the southern circulation of spills in the Atlantic. Credit: Mariona Claret / University of Washington
It has been observed that the decline of oxygen is affecting Atlantic Wolffish, said Claret, and is threatening cod, snow crab and Greenland halibut all of which live in the depths.
"The decline of oxygen in this region has already been reported, but what has not been explored before was the underlying cause," said Claret, who did the work at the same time. McGill University.
The research confirms a recent study showing that as carbon dioxide emissions increased over the last century as a result of human emissions, the Gulf Stream shifted northward and the Labrador Current became unstable. is weakened. The new article reveals that this leads to greater penetration of hot, salty and oxygen-poor Gulf Stream water into the St. Lawrence Seaway.
The new study uses NOAA's geophysical fluid dynamics laboratory model results, a high-resolution computer model that simulates the world's oceans with a data point every 8 kilometers (5 miles). It took nine months to run this simulation using 10,000 compute nodes, which is huge, even by the standards of global climate models.
With this precision, whirlpools and coast details that can influence ocean circulation begin to appear. The model results combined with historical observations show that as carbon dioxide levels increase, Gulf Stream water replaces Labrador Sea water in the deepest parts of the Gulf. St. Lawrence.
The waters carried by the Labrador Current have been driven by storms in the Labrador Sea, so that the air absorbed at the surface mixes well below the surface. The Gulf Stream, however, is more stratified in stable horizontal layers; the upper layer contains oxygen from the air above, but the marine life has consumed oxygen from the lower layers. In addition, the warmer Gulf Stream is also denser to a greater depth, so that the Gulf Stream's deeper and oxygen-free layers follow the same density trajectory of the near-surface water rich in the Gulf Stream. oxygen from the Labrador Current.
"We report a change of oxygen on the coast to a change in large-scale currents at sea," Claret said.
In the model, the movement of large-scale ocean circulation causing warming and deoxygenation in the Gulf of St. Lawrence also corresponds to a decline in the southern Atlantic circulation, an oceanic circulation known for its considerable influence on the climate of the Atlantic Ocean. 'northern hemisphere.
"Being able to potentially connect the coastal changes to the current reversal of the Atlantic is very exciting," added Claret.
The analysis shows that half of the oxygen drop observed deep in the St. Lawrence River is simply due to the presence of warmer water, which can not contain as much oxygen . The other half is probably due to other factors, such as biological activity in both currents and in the interior of the channel. What will happen next is unknown, Claret said. Oxygen levels in the St. Lawrence will depend on much larger questions, such as the amount of carbon dioxide that humans will emit into the atmosphere over the next few decades, and how large-scale ocean currents will react. .
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More information:
Mariona Claret et al, rapid deoxygenation of the coast due to the displacement of ocean circulation in the Northwest Atlantic, Nature Climate Change (2018). DOI: 10.1038 / s41558-018-0263-1
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