Over the last few decades, the extent of Antarctic sea ice diminished by about 40 percent, whereas the ice surrounding Antarctica, until recently, had paradoxically seen a slight expansion. This trend went against the predictions of many climate models. However, the situation drastically shifted after 2015: the ice coverage swung from record highs to several record lows, losing an area equivalent to the size of Greenland. New studies published in PNAS and Nature Climate Change reveal that ocean warming played a more significant role in this reversal than just rising air temperatures.
Previously, some researchers mainly attributed the melting to atmospheric warming. Yet, as Simon Josey from the National Oceanography Centre in Southampton noted, “Scientists have now conducted thorough analysis and built a plausible sequence of events showing the ocean was instrumental in the 2016 ice melt.” This relates to a warm, saline mass of water known as Circumpolar Deep Water. Originating from the tropics, it flows southward and envelops Antarctica at depths below 200 meters. Temperature and salinity data gathered over two decades by hundreds of drifting buoys indicate that this water is increasingly being brought up toward the surface, where it can accelerate sea ice melting.
Antarctica is encircled by a band of intense winds and storms in latitudes referred to as the “Roaring Forties,” “Furious Fifties,” and “Screaming Sixties.” Research by Earle Wilson of Stanford University demonstrated that climate change has shifted storm tracks further south, resulting in greater precipitation within the sea ice zone. Initially, this created a layer of fresh water at the surface, which effectively insulated the base of the ice from the warmer deep water below. According to the study, this insulation contributed to the sea ice expanding to its record extent in 2014.
Nevertheless, the same shift in storm paths intensified winds that began to push the surface water and ice onward. Due to the Earth’s rotation, water is deflected to the left of the wind direction, causing deep water to upwell from below in regions like the Weddell Sea. Between 2014 and 2016, this upwelling process became stronger than the protective freshwater layer, leading to a subsequent contraction of sea ice in the Weddell Sea. When researchers input the actual temperature and salinity changes into a simple computer model, it mirrored the observed pattern: initial expansion followed by reduction, reports Planet Today.
According to Wilson, “Most indicators point towards a sustained and prolonged decrease in sea ice because even if precipitation temporarily suppresses the deep heat, that heat does not disappear.” A second paper by Theo Strewe of the Alfred Wegener Institute suggests that prior to the increase in precipitation, the warm deep water was separated from the surface by a layer of cold, salty winter water that forms as ice freezes. However, as the general warming progresses, the deep waters heat up and expand, and this winter water layer becomes less dense. In 2015 and 2016, unusually strong winds mixed more deep water through this barrier, and the former stratification has not recovered since then.
Strewe emphasized, “It is the wind that causes such rapid sea ice reduction, but it is the ocean that keeps it low.” He added there are signs of a new regime. The melting of sea ice itself does not raise sea levels, but it can negatively impact ice-dependent species, including krill and penguins. Furthermore, if the ice retreats near significant ice shelves, it could affect the formation of dense bottom waters around Antarctica and impact global currents, including the Atlantic Meridional Overturning Circulation (AMOC). As Wilson noted, “Reducing sea ice formation in these areas will decrease the amount of bottom water produced, and circulation could slow down,” although he clarified that freshwater from melting glaciers has a stronger influence on bottom waters.
About the Author
