Waters off Scotian Shelf are cooling, but scientists can't say for how long

Lower temperatures have scientists wondering if a decade-long warming trend is over.

The latest survey of Atlantic Ocean conditions off Nova Scotia show after a decade of warming, temperatures on the Scotian Shelf are cooling.

The slight climate reversal has scientists asking if this is the beginning of a return to previous norms — or a blip.

"What remains unknown is whether this is a longer term trend or just short-term variability in our region," said Lindsay Beazley, a Fisheries and Oceans biologist and operational lead for the Maritimes Region Atlantic Zone Monitoring Program.

DFO has been tracking ocean conditions on the East Coast of Canada for decades.

Beazley is just back from the annual spring hydrographic sampling survey from the Gulf of Maine to the Cabot Strait. The mission measures a variety of physical, chemical and biological conditions.

'It's getting cooler'

In recent years, warming temperatures have grabbed headlines, with record highs being set throughout the region. Recently, on the Scotian shelf, it has moved in the other direction.

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"It is really interesting," Beazley said in a wharfside interview at the Bedford Institute of Oceanography.

"We did see a continuation of the trend that we observed in 2023, which was the temperatures are actually returning to normal or even below normal conditions in some areas. It's getting cooler."

Since 2012, ocean temperatures off Nova Scotia at depth have been consistently warmer — by about two degrees above normal.

For example, near-bottom warming anomalies were detected in the Cabot Strait between Cape Breton and Newfoundland, Misaine Bank off Cape Breton, Emerald Basin on the Eastern Shore and Lurcher Shoal, Georges Basin and Georges Bank off southern Nova Scotia.

The large and abrupt warming was enough to constitute a regime shift, said Dave Hebert, an ocean climate scientist who has run the Maritimes survey twice annually for many years.

Regime shift indicates a persistent change in the structure and function of the ocean ecosystem.

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Scientists Lindsay Beazley, right, and Dave Hebert, both involved in the Maritimes Region Atlantic Zone Monitoring Program, say it's too early to determine if cooling temperatures will become a long-term trend. (Robert Short/CBC)

Half a degree below norm

However, in 2023, bottom temperatures were about a half degree below the average between 1990 and 2021, said Hebert.

Measurements taken in the last month on the so-called Halifax line stretching from the coast to the shelf showed temperatures throughout the water column back down to normal or below normal.

One explanation for the reversal is that colder, oxygen-rich water is making its way farther south again.

"In the past, less Labrador current water was coming around the tail of the Grand Banks and the Gulf Stream water was moving onshore and producing warmer water," Hebert said.

"It appeared last year that the Labrador current was returning to normal. We're hoping that it's not being cut off at the tail of the Grand Banks like it has been in the past, and we'll get back to cooler temperatures, fresher water at depth," he said.

The answer won't be known for several years.

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"It's going to take another two or three years of continuous measurements to address that," said Hebert.

One measure scientists will be looking for is the number of marine heat waves in the Atlantic. Hebert said they have increased in recent years while their cold spell equivalents have decreased.

"The question is whether or not that will flip back to how it was in the past where there would be more cold pulses of water."

As for this year, the science survey results will come as no surprise to fishermen in southwestern Nova Scotia.

They saw landings drop as cooler temperatures at bottom — particularly farther from shore — immobilized lobsters, making them harder to catch.

This article was written for the CBC by Paul Withers. Header image: Crew aboard Canadian Coast Guard Ship Teleost took part in the spring Atlantic Zone Monitoring Program surveying Atlantic Ocean conditions off Nova Scotia. (Department of Fisheries and Oceans via the CBC)