For decades, warmer waters seeping into the Arctic Ocean have increasingly threatened Arctic sea ice, and scientists predict that the ice sheet could disappear completely in summer starting in the middle of next decade.
Researchers have now uncovered one mechanism for this catastrophe by determining how “heat bombs” of warm salt water from the Pacific Ocean enter the cold Arctic Ocean, heating the ice below for months or even years.
“The rate of accelerating sea ice melt in the Arctic has been difficult to accurately predict, in part because of all the complex local feedbacks between the ice, the ocean, and the atmosphere,” says physical oceanographer Jennifer McKinnon of the Scripps Institution of Oceanography at the University of California, San Diego.
“This work demonstrates the large role in warming that ocean water plays as part of these feedbacks.”
These vortex heat bombs can exist for months to years, moving far north beneath the main ice sheet near the north pole, destabilizing that ice as the heat in them gradually but steadily spreads upward.
One of the project’s goals was to learn more about how warmer water flows from the Pacific Ocean into the Arctic Ocean through the Bering Strait, bringing with it “unprecedented amounts of heat” that spread hundreds of kilometers into Beaufort Current, a giant ocean current north of the Alaskan coast and Canada.
“This water of Pacific origin brings both heat and unique biogeochemical properties, contributing to changes in the Arctic ecosystem,” the researchers explain in their new study.
“However, our ability to understand or predict the role of this incoming water mass has been hampered by a lack of understanding of the physical processes that control the subduction and evolution of this warm water.”
Thanks to the SODA expedition and an array of scientific measurements, including analysis of satellite imagery and a number of underwater sensor and vehicle readings, the physical processes have become much less mysterious.
According to the team’s new observations, the salty, denser summer water of the Pacific Ocean (PSW), which the team compares to a “warm stream in a cold ocean,” slides under the colder, fresh water of the Arctic Ocean at the surface, through a process of subduction.
As the meandering jet flows into the stream, pockets of warmer water – dubbed “heat bombs” – are wedged under the fresher waters above, vertically compressed and broken up into smaller, rotating eddies.
“Although some heat is lost to the atmosphere, the vast majority subducts to escape direct contact with the atmosphere and then mixes and spreads below the surface toward the central basin,” the authors write.
In addition to the long-term melting effects provided by these heated eddies to the sea ice above, the intrusion of Pacific waters also introduces a mixture of organic and chemical substances into the Arctic environment, the effects of which are not yet definitively known.
While this new information will help us develop better models to more accurately predict the physics underlying future changes in Arctic sea ice, simply put, the immediate prediction is already determined.
“As PSW heat content increases, the combination of PSW subduction, lateral mixing, and upward vertical mixing should lead to accelerated sea-ice melt spreading from the Pacific tributary, as has been observed in recent decades,” the team concludes.
The findings are reported in the journal Nature Communications.