Arctic heatwaves building in intensity

Ocean heatwaves in the Arctic are increasing in power and frequency, quite probably at a greater rate than in other oceans, according to new research.

Marine heatwaves are events in which water temperatures rise well above normal and remain there for days, weeks, or even months. At lower latitudes, they can damage corals and force fish and marine mammals to temporarily move away in pursuit of food and cooler water. It’s an effect that has been compared to that of decades of climate change impact, but all at once.

In the Arctic, the effect isn’t as well documented, but with climate change proceeding more dramatically there than at lower latitudes, a team led by Boyin Huang of the US National Oceanic and Atmospheric Administration’s National Centers for Environmental Information in Asheville, North Carolina, examined nearly four decades of US, European and Japanese satellite data to find out how climate change is altering the strength and frequency of these heatwaves.

Huang presented his study this week at the 2022 Ocean Sciences Meeting, which for the first time was conducted entirely virtually. (Organisers had already said two years ago that they wanted to include a virtual component this year, in order to save resources and spare the climate, even before COVID-19 forced them to do so.)

The study examined sea-surface temperatures,  air temperatures, ice coverage and the extent of cloud coverage, all of which can increase the likelihood of heatwaves. 

On average, Huang said, the Arctic sees one to two such heatwaves per summer. But over his study period, from 1982 to 2020, they were getting longer, stronger and extending ever deeper into autumn.

In the early years, Huang says, the heatwaves were confined to a short season from late July to early August. Since then, however, that season has dramatically expanded, ending in mid-August or even through to the end of September.

Other scientists are studying different types of changes in the Arctic that may (or may not) be related. For example, Michael Karcher, a senior researcher at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, says scientists realised 25 years ago that plumes of iodine-129 (a rare, radioactive isotope of iodine) from nuclear fuel reprocessing plants in France and the UK can be used as a tracer of the flow of North Atlantic seawater into and around the Artic Ocean.

What this has revealed, Karcher told the Ocean Sciences Meeting, is that there has been a change in the circulation pattern in the Arctic Ocean that has allowed relatively warm Atlantic Ocean water to penetrate further toward Alaska, reducing the spread of cooler Pacific water (which is not contaminated with iodine-129, and is therefore easy to distinguish).

Karcher and Huang were unable to say whether this was contributing to Arctic heatwaves. In theory, the warmer water from the Atlantic should have some impact as it travels beneath the pack ice.

“But how much it interacts with ice is the open question,” says Huang. “Our speculation is that atmospheric forcing [ie top-down heat] is the more important contributor.”

The bottom line, however, is clear. Arctic waters are changing, and quickly.

Huang’s research was published late last year in Geophysical Research Letters.