Why Did Antarctic Sea Ice Suddenly Decline After 2015?
Between 2015 and 2017, the extent of Antarctic sea ice went from record highs to historically low levels. This abrupt transformation is explained by a combination of oceanic and atmospheric factors that had been building up over more than a decade.
For ten years before 2015, a layer of cold water called Winter Water, which acts as a natural barrier between the surface and the deeper, warmer waters, had been gradually thinning. This layer plays a key role in maintaining ocean stability and limiting the rise of heat from the depths. Meanwhile, the deeper waters, which are saltier and warmer, moved closer to the surface. In 2015, exceptionally strong winds mixed the ocean layers, allowing this warm water to rise and reduce the stratification of the upper ocean. This mixing brought heat to the surface, accelerating ice melt.
Before 2015, Antarctic sea ice had reached record levels due to powerful winds pushing the ice northward and complex interactions between ice, ocean, and ice shelves. But in August 2015, an early peak in ice thickness was followed by a rapid and unexpected decline. Observations show that the temperature of the deep waters increased by 0.6°C between 1981 and 2011, while the surface became cooler and fresher. These changes weakened the separation between ocean layers, facilitating heat exchange.
Winter Water, formed in winter in polar regions, normally overlies the warmer circumpolar deep waters. Its gradual thinning reduced its insulating effect. In 2015, intense winds enhanced vertical mixing, causing warm, salty water to rise and disrupt the usual stratification. This phenomenon prevented the formation of new ice and triggered a lasting loss of sea ice.
Since 2016, the extent of sea ice has remained low, with record melting observed in 2016, 2022, and 2023. Data reveal that the structure of the upper ocean has changed: the salinity of Winter Water and the surface layer has increased, while that of the deep waters has decreased. These changes have reduced the density difference between layers, further weakening stratification and promoting heat transfer to the surface.
Winds played a decisive role in 2015. Their intensity increased turbulence in the ocean’s upper layer, allowing heat from the depths to reach the surface. Without the Winter Water barrier, upward heat fluxes increased sixfold, accelerating melting and preventing ice recovery.
This shift suggests a new regime for Antarctic sea ice, marked by greater vulnerability to oceanic and atmospheric variations. Understanding these mechanisms is essential for anticipating impacts on the global climate, ocean currents, and polar ecosystems. Observations confirm that the ocean set the stage for this transition, while the 2015 winds triggered the change. The persistence of these conditions could indicate a lasting transformation of Antarctica.
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Cited Study
DOI: https://doi.org/10.1038/s41558-026-02601-4
Title: Wind-triggered Antarctic sea-ice decline preconditioned by thinning Winter Water
Journal: Nature Climate Change
Publisher: Springer Science and Business Media LLC
Authors: Theo Spira; Marcel du Plessis; F. Alexander Haumann; Isabelle Giddy; Aditya Narayanan; Alessandro Silvano; Sebastiaan Swart