Warmer ocean is driving the Antarctic sea ice ‘regime shift’

Scientists are wondering why Antarctica’s sea ice, which once seemed impervious to climate change, has declined significantly over the past decade. Research now suggests that stronger winds lifted warm water from the depths of the ocean, breaking through the upper layers of water that protected the ice from melting.

While Arctic sea ice has shrunk by about 40% in four decades, until recently sea ice around Antarctica was expanding slightly, confounding most climate models. Then, after 2015, ice extent went from record high to multiple record lows, losing an area the size of Greenland.

Some research suggests that sea ice may be melting largely because of air temperatures, which have been so high in recent years that Antarctic researchers have posed for photos in swimsuits. Two new studies show that ocean warming played a larger role in this “regime shift.”

“A lot of people will say… that it was atmospheric warming that melted the sea ice,” says Simon Josey of the National Oceanography Center in Southampton, UK, who was not involved in the research. “Now these scientists have done a thorough analysis and have a plausible chain of events that points to the ocean as the key player in the 2016 melt. No one has made that argument yet.”

As part of global ocean circulation, a mass of warm, salty water called circumpolar deep water flows south from the tropics and circles Antarctica at depths less than 200 meters. But it is increasingly rising to the surface where it can melt sea ice, suggest two decades of temperature and salinity measurements taken by several hundred drifting buoys.

Antarctica is surrounded by a belt of strong winds and storms at the latitudes of the “Roaring 40s”, “Furious 50s” and “Screaming 60s”. Climate change has shifted the track of this storm south, bringing more precipitation into the sea ice zone, according to a study by Earle Wilson of Stanford University and colleagues. Initially, precipitation created a layer of fresh surface water that better insulated the sea ice floor from warm, deep water, allowing it to expand to its 2014 record extent.

But the storm’s track, shifted south, also produces stronger winds that push surface water and ice forward. Due to the Earth’s rotation, water moves 90 degrees to the left of the wind direction, generating spirals like the Weddell Sea Gyre. As surface water is thrown toward the edges, deep water surges from below to fill the void in the center.

Between 2014 and 2016, this wind-driven upwelling began to win the “tug of war” against the protective layer of increased precipitation, and sea ice began to melt in the Weddell Sea. When the researchers incorporated the observed changes in temperature and salinity into a simple computer model, they predicted that the sea ice would expand and then contract, as happened in the real world.

“Most signs point to a persistent and sustained decline in sea ice, because even with precipitation potentially removing heat from the deep ocean… the heat is still there,” Wilson says. “It would only take a sudden reversal of conditions for this heat to reappear. »

This reversal began with a series of windstorms, according to the second study by Theo Spira of the Alfred Wegener Institute in Bremerhaven, Germany, and colleagues.

Even before the additional precipitation in recent years, warm circumpolar deep waters were pushed away from the surface layer by winter water, a layer of cold, salty water created when sea ice forms in winter, shedding salt ions from its new crystal structure.

But deep waters have become warmer because of global warming. Because water expands when it’s warmer, deep water takes up more space, diluting the water in winter. In 2015 and 2016, stronger than average winds lifted deeper waters beyond the winter water barrier. Stratification has not recovered since.

The results suggest that although the strong winds were a natural fluctuation, the stage was set by global warming.

“It’s the wind that pushes [sea ice] “There is clear evidence that we are in a new regime,” Spira says.

If melting sea ice does not cause sea levels to rise, it could harm species that spend part of their lives on this ice, such as krill or penguins. And if sea ice retreats near major ice shelves where its salt release helps form dense Antarctic deep waters, it could impact global ocean currents, including the Atlantic meridional overturning circulation that keeps Europe warm.

“If you were to reduce sea ice production in these regions…you would have less bottom water and potentially a slowing of the meridional overturning circulation,” Wilson says, while noting that fresh water from melting glaciers has a greater effect on bottom water.