• Ocean eddies could explain Antarctic sea

    From ScienceDaily@1:317/3 to All on Wed Feb 2 21:30:42 2022
    Ocean eddies could explain Antarctic sea-ice paradox
    Basis for reliable projections of the impacts of climate change in the Antarctic

    Date:
    February 2, 2022
    Source:
    Alfred Wegener Institute, Helmholtz Centre for Polar and Marine
    Research
    Summary:
    Despite global warming and the sea-ice loss in the Arctic, the
    Antarctic sea-ice extent has remained largely unchanged since
    1979. However, existing climate model-based simulations indicate
    significant sea-ice loss, contrary to actual observations. As
    experts have now shown, the ocean may weaken warming around
    Antarctica and delay sea-ice retreat.



    FULL STORY ========================================================================== Despite global warming and the sea-ice loss in the Arctic, the Antarctic
    sea- ice extent has remained largely unchanged since 1979. However,
    existing climate model-based simulations indicate significant sea-ice
    loss, contrary to actual observations. As experts from the Alfred Wegener Institute have now shown, the ocean may weaken warming around Antarctica
    and delay sea-ice retreat. Given that many models are not capable of
    accurately reflecting this factor and the role of ocean eddies, the
    study, which was just published in the journal Nature Communications,
    provides the basis for improved simulations and forecasts of the future development of the Antarctic.


    ========================================================================== Global warming is progressing rapidly, producing effects that can be
    felt around the world. The impacts of climate change are especially
    dramatic in the Arctic: since the beginning of satellite observation in
    1979, the sea ice has declined massively in the face of rising global temperatures. According to the latest simulations, the Arctic could
    be consistently ice-free in summer before 2050, and in some years even
    before 2030.

    Yet on the other side of the planet in Antarctica, the sea ice seems
    to have evaded the global warming trend. Since 2010, there have been
    more interannual fluctuations than in the previous period. However,
    apart from a significant negative excursion in the years 2016 to 2019,
    the long-term mean sea-ice cover around the Antarctic continent has
    remained stable since 1979. As such, the observable reality does not
    match the majority of scientific simulations, which show a significant
    sea-ice loss over the same timeframe. "This so-called Antarctic sea-ice
    paradox has preoccupied the scientific community for some time now," says
    first author Thomas Rackow from the Alfred Wegener Institute, Helmholtz
    Centre for Polar and Marine Research (AWI). "The current models cannot
    yet correctly describe the behaviour of the Antarctic sea ice; some key
    element seems to be missing. This also explains why the Intergovernmental
    Panel on Climate Change, IPCC, concludes that the confidence level for model-based projections of future Antarctic sea ice is low." In contrast,
    the models are already so reliable in the Arctic that the IPCC ascribes
    a high confidence level to their projections. "With our study, we now
    provide a basis that could make future projections for Antarctica much
    more reliable." In the course of the study, the team applied the AWI
    Climate Model (AWI-CM).

    Unlike other climate models, the AWI-CM allows certain key regions like
    the Southern Ocean to be simulated in far more detail -- or in other
    words, in "high resolution." As a result, mixing processes in the ocean,
    caused by smaller ocean eddies with diameters of 10 to 20 kilometres,
    can also be directly included.

    "We used a broad range of configurations for our simulations. In
    the process, it became clear that only those simulations with
    a high-resolution description of the Southern Ocean encircling the
    Antarctic produced delayed sea-ice loss similar to what we are seeing in reality," says Rackow. "When we then extended the model into the future,
    even under a highly unfavourable greenhouse-gas scenario the Antarctic
    sea-ice cover remains largely stable until mid-century.

    After that point the sea ice retreats rather rapidly, just as the Arctic
    sea ice has been doing for decades." As such, the AWI study offers a
    potential explanation for why the behaviour of the Antarctic sea ice
    does not follow the global warming trend. "There could be a number of
    reasons for the paradoxical stability of the sea-ice cover. The theory
    that additional melt water from the Antarctic stabilises the water
    column and thus also the ice by shielding the cool surface waters from
    the warmer deep waters is being discussed. According to another theory,
    the prime suspects are the westerlies blowing around the Antarctic,
    which have been strengthening under climate change. These winds could essentially spread out the ice like a thin pizza dough, so that it
    covers a greater area. In this scenario, the ice volume could already
    be declining, while the ice-covered areas would give the illusion of stability," Rackow explains.

    AWI's research efforts now bring ocean eddies into the focus. These
    could play a decisive part in dampening and thus delaying the effects
    of climate change in the Southern Ocean, allowing the ocean to
    transport additional heat taken up from the atmosphere north, toward
    the Equator. This northward heat transport is closely linked to the
    underlying overturning circulation in the upper about 1,000 metres of
    the ocean, which in the Southern Ocean is driven by the wind on the one
    hand but is also influenced by eddies. While the northward component of
    the circulation is growing due to stronger westerlies, the simplified
    eddies in low-resolution climate models often seem to overcompensate for
    this factor by a southward component toward Antarctica; the explicitly simulated eddies in the high-resolution model display a more neutral
    behaviour. Taken together, a more pronounced northerly change in heat
    transport can be seen in the high- resolution model. As a result,
    the ocean surrounding the Antarctic warms more slowly and the ice cover
    remains stable for longer. "Our study supports the hypothesis that climate models and projections of the Antarctic sea ice will be far more reliable
    as soon as they are capable of realistically simulating a high-resolution ocean, complete with eddies," says Rackow. "Thanks to the ever-
    increasing performance of parallel supercomputers and new, more efficient models, next-generation climate models should make this a routine task." ========================================================================== Story Source: Materials provided by Alfred_Wegener_Institute,_Helmholtz_Centre_for_Polar_and
    Marine_Research. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Thomas Rackow, Sergey Danilov, Helge F. Goessling, Hartmut
    H. Hellmer,
    Dmitry V. Sein, Tido Semmler, Dmitry Sidorenko, Thomas Jung. Delayed
    Antarctic sea-ice decline in high-resolution climate change
    simulations.

    Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-28259-y ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/02/220202080319.htm

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