Peggy’s data were a bit of a shock.
From an anchored vantage point in an expanse of the southeastern Bering Sea west of Alaska, Peggy, or mooring M2, had monitored conditions in the water for 25 years. A line of sensors extended down more than 70 meters to where Peggy was tethered to the seafloor, collecting information on temperature, salinity and other properties of the water.
Most years, the waxing and waning of floating sea ice follows a consistent seasonal pattern that is reflected in Peggy’s data. By November, sea ice migrates in through the Bering Strait or forms in some parts of the Bering Sea. As a by-product of the sea ice formation, a large mass of cold, salty water begins to pool near the seafloor. In the spring, phytoplankton bloom, and by early summer, the sea ice begins to melt away. The cold pool, however, lingers through the summer.
With an average temperature just below zero degrees Celsius — a few degrees colder than the surrounding water — that deep, cold pool is central to the Bering Sea ecosystem. The cold pool is where Arctic cod take refuge, hiding from predators such as Pacific cod and pollock, which are less tolerant of the cold. The Arctic cod get fat on large, shrimp-like copepods and spawn their young. In turn, the fish keep polar bears and seals well-fed.
But in the winter of 2017–2018, the sea ice never appeared. And Peggy’s data, along with that of other moorings, revealed that the cold pool was AWOL too. Alarm trickled through the ocean science community, researchers who study everything from the physics of the Bering Sea to the small creatures that live on the seafloor and the larger marine mammals at the top of the food chain. In December in Washington, D.C., at the American Geophysical Union’s annual meeting, these researchers gathered to present their data, trade stories and ponder what it all means.
Were these findings a fluke? “We don’t yet have enough data” to say whether the Bering Sea is increasingly likely to be ice-free, says Jacqueline Grebmeier, a biological oceanographer at the University of Maryland’s Center for Environmental Science in Solomons. But Grebmeier, who has studied seafloor life in the Arctic for more than 30 years, has “a gut feeling,” she says, that it’s not a one-off incident. “I think it’s the beginning of change.”
If last year’s events represent a new normal for the Bering Sea (and the very low sea ice extent as of February this year signals they might), then a cascade of changes are in store for the complicated ecosystem that has long thrived in those waters — and for the fishing and tourism industries that rely on the area’s bounty.
At their closest point, Alaska and Russia are separated by the 82-kilometer-wide Bering Strait. To the north of the strait lies the Chukchi Sea, on the edge of the Arctic Ocean; to the south is the Bering Sea, extending down to Alaska’s outflung arm of islands, the Aleutians.
In the summer, the Bering Sea is largely ice-free, but in winter, ice forms in the northern Bering Sea, or migrates southward through the strait from the Chukchi. The waters reach “freeze-up” when there is at least 20 percent ice cover, scientists say.
There were early signs that conditions in 2017 and 2018 were going to be different. By November 2017, the sea ice was already late. The air above the waves wasn’t especially warm. In fact, the air temperature was typical for that time of year, Phyllis Stabeno, a physical oceanographer at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle, reported at the December meeting. But an unusually persistent wind was blowing from the south, she said, preventing the ice from drifting down from the Chukchi Sea as it would normally.
The wind tapered off by December and January, but by then air temperatures were higher than normal. The Chukchi Sea, normally at least 80 percent covered by thick, tough, icebreaker-testing pack ice by January, still had large open swaths of water. That meant less ice was available to migrate southward through the Bering Strait.
Mooring M8, about 800 kilometers northwest of Peggy, had never recorded so little ice in the winter. M8, taking measurements since 2008, registered temperatures just above the seafloor that were more than 3 degrees C above normal. And Peggy, down by the Aleutians, had never recorded higher summertime water temperatures near the seafloor. That summer, the water never dropped below freezing.
Then, in February, the strong southerly winds began again, and the unusual wind direction persisted through March; scientists suspect those winds kept the Chukchi Sea unusually warm, by pushing warmer waters from the Bering Sea northward. The warmer waters also prevented the formation of sea ice. The ice that did form in the Chukchi and Bering seas was thin and easily pushed back northward by the prevailing winds.
Sea ice, whether migrating in or forming in place, is an anchoring part of the Bering Sea ecosystem. The ice helps determine when and where food becomes available to creatures living in the water or on the seafloor. As the migrating sea ice travels south, it melts. That meltwater is relatively fresh and less dense than the surrounding water. As a result, the waters become stratified, with the layer of fresher water staying on top. That freshwater, full of nutrients, helps give rise to the southern Bering Sea’s springtime phytoplankton blooms, which in turn feed copepods and other small floating creatures. When the phytoplankton eventually die and sink to the seafloor, they provide an important food source for creatures living on the bottom.
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