A warmer ocean is driving a ‘regime shift’ in Antarctic sea ice

Scientists are debating why Antarctic sea ice, which once seemed resilient to climate change, has shrunk dramatically in the past decade. Now, research suggests that stronger winds have stirred up warming water from the ocean depths, breaking through the upper layers of water that protected the ice from melting.

While Arctic sea ice has shrunk by about 40 percent over four decades, until recently sea ice around Antarctica has been increasing slightly, confounding most climate models. Then after 2015, the ice extent dropped from a record high to several record lows, losing one flat the size of Greenland.

Some research suggested that sea ice may be melting in large part because of air temperatures that have been so high in recent years that Antarctic researchers assumed photo in a swimsuit. Two new studies show that ocean warming played a larger role in this “regime shift.”

“A lot of people will say … that it was the warming of the atmosphere that melted the sea ice from above,” he says Simon Josey at the National Oceanographic Center in Southampton, UK, who was not involved in the research. “Now these scientists have done a thorough analysis and obtained a plausible chain of events that says the ocean is a key player in this 2016 melt. No one has put this argument together before.”

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

Antarctica is surrounded by a belt of strong winds and storms in the latitudes of the “Roaring Forties”, “Furious Fifties” and “Screaming Sixties”. Climate change has shifted this storm track south, bringing more precipitation to the sea ice zone, study says Earl Wilson at Stanford University and his colleagues. Initially, the 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 record extent in 2014.

But the southward-shifted storm track also brings stronger winds that drive surface water and ice forward. Because of the Earth’s rotation, the water moves 90 degrees to the left of the wind direction, forming spirals like the Weddell Sea gyre. As surface water flows to the edges, deep water wells up from below to fill the void in the middle.

Between 2014 and 2016, this wind-driven upwelling began to win a “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, it was assumed that the sea ice would expand and then contract as it did in the real world.

“Most signs point to a persistent and permanent decline in sea ice, because even though the precipitation is potentially suppressing the heat in the deep ocean … the heat is still there,” Wilson says. “All it would take is a sudden reversal of conditions to bring that warmth back.”

According to the second study, this reversal began with a series of wind storms Theo Spira at the Alfred Wegener Institute in Bremerhaven, Germany, and his colleagues.

Even before the additional precipitation in recent years, the warm circumpolar deep water was held back from the surface layer by winter water, a layer of cold, salty water that forms when sea ice forms in winter, shedding salt ions from its new crystalline structure.

But the deep water is getting warmer because of global warming. Because water expands when it’s warmer, the deep water takes up more space and dilutes the winter water. In 2015 and 2016, stronger than average winds brought more deep water over the winter water barrier. The layering has not resumed since.

The finding suggests that while strong winds were a natural fluctuation, the stage was set by global warming.

“It is the wind that pushes. [sea ice] into these rapid declines, but it’s the ocean that’s really holding it down,” says Spira. “There’s certainly evidence that we’re in a new regime.”

Although melting sea ice does not raise sea levels, it could harm species that spend part of their lives on this ice, such as krill or penguins. And if sea ice recedes near key ice shelves, where its rejection of salt helps create dense groundwater in Antarctica, it could affect global ocean currents, including the Atlantic Southern Overturning Circulation, which keeps Europe warm.

“If you were to reduce sea ice production in these areas … you would have less groundwater and potentially a slowing of the meridional overturning circulation,” Wilson says, although he notes that freshwater from melting glaciers has a greater effect on groundwater.

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