Science Gazette

Ocean eddies may be to blame for the Antarctic sea-ice mystery

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The area of Antarctic sea ice has stayed virtually stable since 1979, despite global warming and Arctic sea-ice decline. In contrast to real data, present climate model-based predictions show dramatic sea-ice retreat. The water may reduce warming around Antarctica and slow sea-ice loss, according to researchers. Given that many models fail to adequately incorporate this element, as well as the function of ocean eddies, the research, which was just published in the journal Nature Communications, lays the groundwork for better simulations and projections of the Antarctic’s future growth.

Global warming is accelerating, with consequences that may be felt all across the globe. Climate change has had a particularly severe effect on the Arctic, where sea ice has reduced dramatically in response to increasing global temperatures since satellite observations began in 1979. According to the most recent projections, the Arctic might be ice-free in summer by 2050, and perhaps even before 2030 in certain years.

 

On the other hand, the sea ice in Antarctica seems to have escaped the global warming trend. Interannual variations have increased after 2010, compared to the prior period. The long-term mean sea-ice cover surrounding the Antarctic continent, however, has been steady since 1979, with the exception of a major negative excursion from 2016 to 2019. As a result, the observed reality differs with the bulk of scientific predictions, which indicate a dramatic decrease of sea ice during the same period. “For quite some time, the so-called Antarctic sea-ice enigma has engaged the scientific community,” explains first author Thomas Rackow of the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “Current models are unable to accurately represent the behavior of Antarctic sea ice; a fundamental component seems to be lacking. This also explains why the Intergovernmental Panel on Climate Change (IPCC) says that model-based forecasts of future Antarctic sea ice have a low degree of confidence.” In the Arctic, however, the models are already so trustworthy that the IPCC assigns a high degree of confidence to their estimates. “With this research, we now have a foundation on which to base future estimates for Antarctica.”

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During the research, the researchers used the AWI Climate Model (AWI-CM). Unlike previous climate models, the AWI-CM enables for significantly more detailed simulations of crucial areas like the Southern Ocean, or “high resolution” simulations. As a consequence, mixing processes in the ocean driven by smaller ocean eddies of 10 to 20 kilometers in diameter may be directly incorporated.

 

“For our simulations, we employed a variety of settings. Only models with a high-resolution representation of the Southern Ocean around the Antarctic generated delayed sea-ice retreat akin to what we are experiencing in reality “Rackow agrees. “Even in a severely unfavorable greenhouse-gas scenario, the Antarctic sea-ice cover stays relatively steady until mid-century, according to the model. After then, the sea ice vanishes quickly, just as the Arctic sea ice has been doing for decades.”

 

As a result, the AWI research provides a possible explanation for why Antarctic sea ice behavior does not follow the global warming trend. “The sea-ice cover’s paradoxical stability might be due to a variety of factors. It’s being debated if more melt water from the Antarctic helps to stabilize the water column and hence the ice by protecting the cold surface waters from the warmer deep waters. Another idea blames the westerlies blowing over the Antarctic, which have been intensifying as a result of climate change. These winds have the potential to stretch the ice out like thin pizza dough, allowing it to cover a larger region. The ice volume may be dropping in this scenario, but the ice-covered regions would provide the appearance of stability “Rackow elucidates.

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Ocean eddies are currently the focus of AWI’s scientific activities. These might be crucial in damping and therefore postponing the consequences of climate change in the Southern Ocean, enabling the ocean to transfer greater heat from the atmosphere north, toward the Equator. This northward heat movement is tightly related to the underlying overturning circulation in the top 1,000 meters of the ocean, which is affected by eddies and pushed by the wind in the Southern Ocean. While the northward component of the circulation is increasing due to stronger westerlies, low-resolution climate models’ simplified eddies often appear to overcompensate by adding a southward component toward Antarctica; the explicitly simulated eddies in the high-resolution model behave more neutrally. The high-resolution model shows a more dramatic northerly trend in heat transfer when taken combined. As a consequence, the water around Antarctica heats more slowly, allowing the ice cover to stay stable for longer. “Our research backs up the idea that climate models and estimates for Antarctic sea ice will be significantly more trustworthy once they can accurately simulate a high-resolution ocean with eddies,” Rackow adds. “Next-generation climate models should make this a normal process, thanks to the ever-increasing speed of parallel supercomputers and new, more efficient models.”

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