Antarctic sea ice

Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer, getting closer and closer to the coastline every year due to sea ice melting.[1] Sea ice is frozen seawater that is usually less than a few meters thick. This is the opposite of ice shelves, which are formed by glaciers, they float in the sea, and are up to a kilometer thick. There are two subdivisions of sea ice: fast ice, which are attached to land; and ice floes, which are not.

Earth image on September 21, 2005 with the full Antarctic region visible

Sea ice that comes from the Southern Ocean melts from the bottom instead of the surface like Arctic ice because it is covered in snow on top. As a result, melt ponds are rarely observed. On average, Antarctic sea ice is younger, thinner, warmer, saltier, and more mobile than Arctic sea ice.[2] Since sea ice is not studied very well in comparison to Arctic ice, it is less accessible.

Measurements of sea ice
Main article: Measurement of sea ice

Extent

The Antarctic sea ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. It peaks (~18 × 10^6 km^2) during September, which marks the end of austral winter, and retreats to a minimum (~3 × 10^6 km^2) in February.[2][3] Consequently, most Antarctic sea ice is first year ice, a few meters thick, but the exact thickness is not known. The area of 18 million km^2 of ice is 18 trillion square meters, so for each meter of thickness, given that the density of ice is about 0.88 teratonnes/million km^3, the mass of the top meter of Antarctic sea ice is roughly 16 teratonnes (trillion metric tons) in late winter.

Since the ocean off the Antarctic coast usually is much warmer than the air above it, the extent of the sea ice is largely controlled by the winds and currents that push it northwards. If it is pushed quickly, the ice can travel much further north before it melts. Most ice is formed along the coast, as the northward-moving ice leaves areas of open water (coastal latent heat polynyas), which rapidly freeze.

Thickness
Main article: Sea ice thickness

Because Antarctic ice is mainly first-year ice, which is not as thick as multiyear ice, it is generally less than a few meters thick. Snowfall and flooding of the ice can thicken it substantially, and the layer structure of Antarctic ice is often quite complex.

Further information: Climate of Antarctica § Climate change in Antarctica

Recent changes in wind patterns, which are connected to regional changes in the number of extratropical cyclones and anticyclones,[4] around Antarctica have advected the sea ice farther north in some areas and not as far north in others (see images). The net change is a slight increase in the area of sea ice in the Antarctic seas (unlike the Arctic Ocean, which is showing a much stronger decrease in the area of sea ice).[5][6] Increased sea ice extent does not indicate that the Southern Ocean is cooling, since the Southern Ocean is warming.[7]

  • Antarctic sea ice cover grows in autumn and winter, and shrinks again each spring and summer. In 2013 (black line) and 2012 (red line), the ice reached the highest extents ever recorded, but it was only slightly above the historical average (blue line). Light blue regions show the range of natural variability.
  • The (then-record) 2012 Antarctic sea ice extent; compare with the yellow outline, which shows the median sea ice extent in September from 1979 to 2000. Sea ice coverage in the Arctic has shrunk at a much faster rate than it has expanded in the Southern Ocean.
  • Antarctic sea ice cover shrinks to its minimum extent each year in February or March; the ice cover then grows until reaching its maximum extent in September or October. The graph above shows the maximum extent for each September since 1979, in millions of square kilometers. There is variability from year to year, though the overall trend shows growth of about 1.5 percent per decade.

The IPCC AR5 report concluded that "it is very likely" that annual mean Antarctic sea ice extent increased 1.2 to 1.8% per decade, which is 0.13 to 0.20 million km2 per decade, during the period 1979 to 2012.[8] IPCC AR5 also concluded that the lack of data precludes determining the trend in total volume or mass of the sea ice. The increase in sea ice area probably has a number of causes.[9] These are tied to changes in the southern hemispheric westerly winds, which are a combination of natural variability and forced change from greenhouse gases and the ozone hole. Another possible driver is ice-shelves melting, which increases freshwater input to the ocean; this increases the weakly stratified ocean surface layer and so reduces the ability of warm subsurface water to reach the surface. A 2015 study found this effect in climate models run to simulate future climate change, resulting in an increase of sea ice in the winter months.[10]

Atmospheric and oceanic drivers likely have contributed to the formation of regionally varying trends in Antarctic sea-ice extent. For example, temperatures in the atmosphere and Southern Ocean have increased during the period 1979–2004. However, sea ice grows faster than it melts, because of a weakly stratified Ocean. Thus, this oceanic mechanism is, among others, contributing to an increase in the net ice production, potentially resulting in more sea ice.[11] Although thickness observations are limited, modelling suggests that observed ice-drift toward the coastal regions makes an additional contribution for dynamical sea-ice thickening during autumn and winter.[12] Observed autumn and spring trends in the number of extratropical cyclones, anticyclones and blocks, which have a strong thermodynamic control through temperature advection, and a strong dynamic control through ice-drift, on sea-ice extent during the same and also during following seasons are almost everywhere around Antarctica in agreement with the observed, regionally varying, trends in sea-ice extent.[4] Consequently, the near-surface winds steered around weather systems are thought to explain large parts of the inhomogeneous Antarctica sea-ice trends.

After gradual increases in sea ice as referenced above, southern hemisphere spring (i.e. September, October and November) 2016 saw a rapid decline in Antarctic sea ice. [13]

Implications

Monitoring changes in sea ice is important as this impacts the psychrophiles that live here.[14]

Changes in Antarctic sea ice are also important because of implications for atmospheric and oceanic circulation.[15] When sea ice forms, it rejects salt (ocean water is saline but sea ice is largely fresh) so dense salty water is formed which sinks and plays a key role in formation of Antarctic Bottom Water.

Effects on Navigation
[T]he greatest part of this southern continent (supposing there is one), must lie within the polar circle, where the sea is so pestered with ice, that the land is thereby inaccessible.

Captain James Cook. A Voyage Towards the South Pole and Round the World, etc.[16]

The force of moving ice is considerable; it can crush ships that are caught in the ice pack, and severely limits the areas where ships can reach the land, even in summer. Icebreakers, iceports and ice piers are used to land supplies.

See also

References
  1. https://nsidc.org/arcticseaicenews/. Missing or empty |title= (help)
  2. "Archived copy" (PDF). Archived from the original (PDF) on 2014-11-11. Retrieved 2014-11-11.CS1 maint: archived copy as title (link)
  3. NASA (2009-05-22). "Antarctic Sea Ice".
  4. Schemm, S. (2018). "Regional Trends in Weather Systems Help Explain Antarctic Sea Ice Trends". Geophysical Research Letters. 45 (14): 7165–7175. Bibcode:2018GeoRL..45.7165S. doi:10.1029/2018GL079109. hdl:20.500.11850/286394.CS1 maint: uses authors parameter (link)
  5. "NASA - What's Holding Antarctic Sea Ice Back from Melting?".
  6. "Arctic Sea Ice News and Analysis | Sea ice data updated daily with one-day lag".
  7. "Ocean warming in Southern Hemisphere underestimated, scientists suggest".
  8. IPCC AR5 WG1 (2013). "The Physical Science Basis" (PDF): 7. Archived from the original (PDF) on March 8, 2014. Cite journal requires |journal= (help)
  9. "Q&A with NASA's Joey Comiso: What is Happening with Antarctic Sea Ice?". Oct 7, 2014.
  10. R. Bintanja, G. J. Van Oldenborgh, C. A. Katsman (2015). "The effect of increased fresh water from Antarctic ice shelves on future trends in Antarctic sea ice" (PDF). Annals of Glaciology. 56 (69): 120–126. Bibcode:2015AnGla..56..120B. doi:10.3189/2015AoG69A001.CS1 maint: uses authors parameter (link)
  11. Zhang, Jinlun (2007). "Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions" (PDF). Journal of Climate. 20 (11): 2515–2529. Bibcode:2007JCli...20.2515Z. doi:10.1175/JCLI4136.1.Abstract
  12. Holland, Paul R., Nicolas Bruneau, Clare Enright, Martin Losch, Nathan T. Kurtz, Ron Kwok (January 17, 2014). "Modeled Trends in Antarctic Sea Ice Thickness" (PDF). Journal of Climate. 27 (10): 3784–3801. Bibcode:2014JCli...27.3784H. doi:10.1175/JCLI-D-13-00301.1.CS1 maint: uses authors parameter (link)
  13. Sharon Stammerjohn and Ted Scambos. Jessica Blunden; Derek S. Arndt (eds.). "State of the Climate in 2016: Antarctica" (PDF). Bulletin of the American Meteorological Association (98, Si–S280).CS1 maint: uses authors parameter (link)
  14. Andrew Martin, Andrew McMinn (2018). "Sea ice, extremophiles and life on extra-terrestrial ocean worlds". International Journal of Astrobiology. 17 (1): 1–16. Bibcode:2018IJAsB..17....1M. doi:10.1017/S1473550416000483.CS1 maint: uses authors parameter (link)
  15. "Sea Ice and Global Climate". NSIDC. Retrieved 11 Jul 2018. NSIDC
  16. Cook, James. (1777). A Voyage Towards the South Pole, and Round the World. Performed in His Majesty's Ships the Resolution and Adventure, In the Years 1772, 1773, 1774, and 1775. In which is included, Captain Furneaux's Narrative of his Proceedings in the Adventure during the Separation of the Ships. Volume II. London: Printed for W. Strahan and T. Cadell. (Relevant fragment)
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