'Unprecedented' marine heat wave hits Canada's East Coast this summer

DFO scientist reviewed sea surface temperatures in satellite data dating back 4 decades

The Atlantic Ocean off Canada's East Coast experienced an "unprecedented" marine heat wave this summer.

Surface temperatures reached record highs throughout the region — including a huge weeklong spike off Newfoundland that averaged 6.7 C above normal.

Fisheries and Oceans climate scientist Peter Galbraith compiled the data for the World Meteorological Organization, a United Nations agency that monitors extreme weather events around the planet.

Environment and Climate Change Canada submitted the data to the agency in September.

Galbraith compared sea surface temperatures that have been measured by satellite since 1982.

"In terms of what we have recorded, it's unprecedented," Galbraith told CBC News. "We just don't have anything close to that in our data record."

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What exactly is a marine heat wave?

Marine heat waves are periods of unusually warm temperatures in the ocean. They are generally defined as five or more consecutive days when surface waters are in the uppermost 10 per cent of historical temperatures.

The month-long Atlantic event took place during the last three weeks of July and the first week of August.

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NOAA: A map from the U.S. National Oceanic and Atmospheric Administration shows sea surface temperature anamolies in the North Atlantic as of Oct. 3, 2023. (National Oceanic and Atmospheric Administration)

A map from the U.S. National Oceanic and Atmospheric Administration shows sea surface temperature anamolies in the North Atlantic as of Oct. 3, 2023. (National Oceanic and Atmospheric Administration)

Galbraith averaged the temperatures from the Scotian Shelf, Gulf of St. Lawrence, the Newfoundland Shelf and the Labrador Shelf.

In July, the ocean surface was two to four degrees above normal — the warmest in the 41 years that satellite data has been collected.

'It's a huge difference'

Galbraith said that off Newfoundland's Flemish Cap, where the surface temperature is normally 12 C, it was 18.6 C.

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"Anything over like 1 degree Celsius is already off normal. So 6.5 degrees is the largest single weekly anomaly for a fairly large area in the whole satellite record for our Eastern Canadian waters," he said. "It's a huge difference"

Galbraith has been monitoring ocean conditions in the Gulf of St. Lawrence and the Atlantic Ocean for many years.

"I was surprised by the spatial extent of it. Usually part of the Atlantic zone is warm, but not all of it. And this was spreading across the whole zone," he said.

He was also shocked by how long it lasted.

"Often you'll see a peak, [when] the sea surface temperatures could be above normal and one week and just a few weeks later would be below normal, just with weather systems going through," he said.

"To have that at record levels over three whole weeks and then remaining really high for a fourth. It was actually my first time seeing that on the scale of the Atlantic zone for such a long time."

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B.C. coast affected too

DFO scientists also reported to the World Meteorological Organization that there is an ongoing marine heat wave off British Columbia.

When it peaked in early August, it covered 90 per cent of Canada's 200-mile exclusive economic zone in the Pacific.

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The maximum anomaly was 3.5 C above normal in the Pacific, where marine heat waves have become more common.

In submitting the data to the World Meteorological Organization, DFO scientists described the Pacific event as "unusual" while they classified the Atlantic one as "unprecedented."

Impact on the ecosystem

Marine heat waves have been linked to devastating effects like algae blooms, coral bleaching and local fish loss.

However a paper published in the science journal Nature on Aug. 30 contained some good news.

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It looked at data from scientific bottom-trawl surveys in North America and Europe from 1993 to 2019, which included 238 marine heat waves.

Bottom dwelling fish were surprisingly resilient. Populations were generally within normal variability after a marine heat wave.

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It was an encouraging result. But the authors still said it's important to keep global warming to 1.5 C or less, because there's no certainty that resilience would continue if warming intensifies.

But the recent warming trend is continuing in the Atlantic Ocean off Canada's East Coast.

In 2021, temperature records were broken there that had been set only nine years earlier. And 2022 was the warmest year ever, with highs recorded throughout the water column.

This summer's marine heat wave was significant enough that it may drive up the overall average for 2023, Galbraith said.

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This article, written by Paul Withers, was originally published for CBC News.