From Scientific American by Andrea Thompson:
April should be prime walrus hunting season for the native villages that dot Alaska’s remote western coast. In years past the winter sea ice where the animals rest would still be abundant, providing prime targets for subsistence hunters. But this year sea-ice coverage as of late April was more like what would be expected for mid-June, well into the melt season. These conditions are the continuation of a winter-long scarcity of sea ice in the Bering Sea—a decline so stark it has stunned researchers who have spent years watching Arctic sea ice dwindle due to climate change.
Sea ice expands outward from the central Arctic Ocean each autumn as the sun dips low in the sky and temperatures drop. In the Chukchi and Bering seas off Alaska, freeze-up used to begin in October. Ice would edge southward and build up throughout the winter until peaking in March when the sun climbs high again, and the ice would then start melting back. But autumn freeze-up in the region has begun steadily later as Arctic temperatures have risen at twice the global rate, fueling a self-perpetuating cycle of ice loss: As it melts it leaves more open water to absorb the sun’s rays in summer, and this further warms the ocean causing more ice to melt, thereby delaying the autumn freeze. In recent years that freeze had moved into November but this year temperatures were so warm the Chukchi Sea still had open ocean in December.
The unusual warmth continued throughout this winter, in part because of an atmospheric pattern that kept warm air and storms periodically sweeping up from the south. One such event in February helped push the monthly temperature over the Bering and Chukchi seas some 18 to 21.5 degrees Fahrenheit (10 to 12 degrees Celsius) above normal. Consequently, the Bering Sea lost half its ice extent at a time when ice should still have been growing. The storms also pushed back against the normal southward flow of ice from the Chukchi Sea into the Bering. Accompanying winds stirred up waves that kept new ice from forming, and broke up what thin ice there was.
Such atmospheric conditions have long been a limiting factor to sea-ice growth in the Bering Sea. But until recently the water there was reliably cold enough in autumn that when winds did blow from the north, sea ice would still spread. The last few years have seen unusually warm ocean waters in the Bering. Research meteorologist Nick Bond and others think this is “a lingering hangover” of a larger marine heat wave—dubbed “The Blob”—that lay off the west coast of the U.S. and Canadian mainland from 2014 to 2016. Bond, who works for NOAA’s Pacific Marine Environmental Laboratory, thinks some of those warm waters followed ocean currents up into the Bering and left a deep reservoir of warmth that impeded ice formation, although he has not yet formally studied this.