For years, the dramatic shrinkage of Antarctic sea ice, once thought to be resistant to climate change, has puzzled scientists. Recent research now points to intensified winds as a primary driver, churning warmer, deeper ocean waters to the surface and breaching the protective upper layers that previously shielded the ice from melting.
The contrast with the Arctic, where sea ice has diminished by approximately 40% over four decades, was striking. Until recently, Antarctic sea ice had shown a slight expansion, a trend that defied most prevailing climate models. However, following 2015, this pattern reversed abruptly, with ice extent plummeting from a record high to numerous record lows, resulting in the loss of an area comparable to that of Greenland.
Earlier hypotheses speculated that atmospheric warming, evidenced by unusually high air temperatures prompting Antarctic researchers to pose for swimwear photos, was the main culprit. Yet, two new studies strongly suggest that ocean warming played a more significant role in this notable “regime shift.”
Simon Josey of the National Oceanography Centre in Southampton, UK, who was not involved in the new research, commented on the prevailing view. “Plenty of people will say… that it was atmospheric warming which melted the sea ice from above,” he noted. “Now these scientists have done the thorough analysis and have got a plausible chain of events, which says that the ocean is the key player in that 2016 melt. Nobody’s put that argument together before.”
The Role of Circumpolar Deep Water
As an integral part of global ocean circulation, a mass of warm, salty water known as circumpolar deep water originates from tropical regions and encircles Antarctica at depths exceeding 200 meters. Evidence from two decades of temperature and salinity measurements, collected by hundreds of drifting buoys, indicates that this water is increasingly surfacing, where it can directly melt sea ice.
Shifting Storm Tracks and Wind Patterns
Precipitation’s Initial Protective Layer
Antarctica is historically characterized by a belt of intense winds and storms situated in the “roaring forties,” “furious fifties,” and “screaming sixties” latitudes. A study by Earle Wilson at Stanford University and his colleagues indicates that climate change has caused these storm tracks to shift southward. This shift has led to increased precipitation over the sea ice zone. Initially, this influx of fresh water formed a surface layer that effectively insulated the underside of the sea ice from the warmer deep ocean water, contributing to its expansion to a record extent by 2014.
Wind-Driven Upwelling Takes Precedence
However, the southward displacement of the storm track also introduced stronger winds. These winds propel surface water and ice towards the poles. Due to the Earth’s rotation, this movement causes water to deflect approximately 90 degrees to the left of the wind’s direction, creating spiraling currents such as the Weddell Sea gyre. As surface water is pushed towards the gyre’s periphery, deeper ocean water rises to fill the vacated space at the center.
The Tug-of-War Between Precipitation and Upwelling
Between 2014 and 2016, this wind-driven upwelling began to overpower the insulating effect of the increased precipitation. This dynamic shift led to the observed melting of sea ice in the Weddell Sea. When the researchers incorporated the measured changes in temperature and salinity into a simplified computer model, it accurately projected the observed pattern of sea ice expansion followed by contraction.
“Most signs point to a persistent and sustained decline in sea ice, because even with the precipitation potentially suppressing the deep ocean heat… the heat is still there,” Wilson stated. “All it would take is a sudden reversal of conditions for that heat to come back up.”
The Role of Global Warming in Setting the Stage
A second study, conducted by Theo Spira at the Alfred Wegener Institute in Bremerhaven, Germany, and his team, posits that a series of intensified wind storms initiated this reversal. Even prior to the recent increase in precipitation, circumpolar deep water was largely prevented from reaching the surface by a layer of “winter water.” This layer, cold and saline, forms as sea ice crystallizes in winter, expelling salt ions.
Thinning of the Winter Water Layer
Global warming has led to a gradual heating of the circumpolar deep water. As warmer water expands, this deeper, warmer layer has encroached upon the space previously occupied by the winter water, causing it to thin. During 2015 and 2016, winds stronger than average facilitated the upward movement of more deep water, breaching the winter water barrier. The researchers noted that this stratification has not since recovered.
Global Warming’s Underlying Influence
This finding suggests that even if the unusually strong winds were a naturally occurring fluctuation, global warming had already prepared the conditions for such a scenario. “It’s the wind that pushes [sea ice] over into these rapid declines, but it’s the ocean that really keeps it low,” Spira explained. “There’s definitely evidence that we’re in a new regime.”
Potential Ecosystem and Circulation Impacts
While the melting of sea ice does not directly contribute to sea-level rise, it carries significant implications for Antarctic ecosystems. Species such as krill and penguins, which rely on sea ice for parts of their life cycles, could be negatively impacted. Furthermore, a reduction in sea ice formation near crucial ice shelves, where salt rejection plays a role in the formation of dense Antarctic bottom water, could affect global ocean currents. This includes the Atlantic Meridional Overturning Circulation (AMOC), which is instrumental in regulating Europe’s climate.
“If you were to reduce sea ice production in those regions… you’ll have less bottom water and potentially a slowdown of the meridional overturning circulation,” Wilson noted. He also pointed out that meltwater from glaciers has a more substantial influence on bottom water formation than sea ice melt.