For those who wonder what the Bering Sea will be like decades from now, last year may have been a glimpse of the future. It brought something many did not expect: the disappearance of the undersea “cold pool,” which is the nursery for Alaska’s pollock fishery.
When it was discovered by oceanographers, the giant, cold pool of water at the bottom of the Bering Sea shelf off of Alaska was seen as a huge wall that would keep that sea cold, even as the rest of the North Pacific Ocean warmed up. The expectations were that it would remain a refuge for the region’s lucrative pollock fishery.
The strongest advocate of that idea was Phyllis Stabeno, a physical oceanographer with the National Oceanic and Atmospheric Administration, or NOAA. Then the cold pool began to fluctuate. Two years ago, Stabeno changed her mind and predicted that it would disappear. The cold pool did get smaller, but no one expected it to disappear quickly. Then the south wind arrived.
“As our colleague Nick Bond likes to say, ‘wind rules,'” says Tom Van Pelt, a scientist with the Joint Institute for the Study of the Atmosphere and Ocean at Washington University. “Everything does go back to the wind.”
Van Pelt presented an analysis of the cold pool to the Alaska Marine Science Symposium for Phyllis Stabeno and her colleagues at NOAA, because they were prevented from attending by the government shutdown.
2017 was a warm summer. The press was talking about a warm blob of water which headed north, leaving a trail of dead seabirds. Fall arrived, a time when the wind normally blows out of the north. That fall it came out of the south and stayed, pushing warm water northwards and keeping the ice from coming down and forming. Van Pelt says that the wind kept blowing from the south into the winter, its warm temperatures keeping ice from growing until deep into the season when the winds finally shifted.
“Ice is beginning to form, but behind schedule and reduced. Winds out of the northeast, well they’re probably more typical. But air temperatures are much, much warmer than average, even above freezing at times. And this is in the depth of winter,” said Van Pelt.
The winter of 2017-18 had the least ice that the region has seen in historic memory, and it wasn’t there very long. The south wind came again early last spring, diminishing the ice and pushing what was left of it into the north.
“Stepping into February and March: ice is already retreating in February, winds continue out of the south, air temperature continues to be warmer than average,” said Van Pelt.
Van Pelt says that the south wind also did something else: it pushed enough warm water into the region to mix with the formerly colder and saltier water at the bottom, making it less salty. Scientists noticed it last spring. After April, bottom temperatures begin to warm in parallel with solar warming.
“That’s something you usually only see in the surface layer,” he said. “They just kept increasing, even getting past that 2-degree threshold, what we think of as the cold pool. So why is that? We think salinity has a lot to do with it.”
The result last summer was a tiny, “micro” cold pool, and the pollock swam further north, looking for colder water. This fall and winter, the north wind and the ice returned. Most scientists expect the cold pool, along with the pollock, to be back on the Bering Sea shelf this summer. But the future will be more like last summer, with the south wind ruling.
“There’s a prediction that winds out of the south will increase over this century,” says Van Pelt. “So winters like we experienced in the region last year are predicted to be more common.”
Modelers say that it will take around 40 years to shift to that “new normal,” but they’ve been wrong before. The arctic is warming up faster than anyone predicted.