Sea ice encircles the Antarctic continent, variously melting and expanding according to the season and geographic location. A recent article in Reviews of Geophysics presented a synthesis of what we know about seasonal cycles of ice extent variability and the drivers in the context of longer term change. Here, some of the authors give an overview of what we know about the regional, seasonal, and interannual variability of Antarctic sea ice, and suggest where additional research is needed.
How has the overall extent of Antarctic sea ice changed over the past few decades?
For nearly 40 years, satellites have made a nearly complete and consistent record of daily sea ice concentration and extent. Over this time, there has been a modest increase in Antarctic sea ice extent. This small increasing trend is made up of the sum of much larger opposing regional trends. The last few years of the record have stood out: there were record increases in total Antarctic sea ice extent from 2012 to 2014, followed by record lows through 2018.
How does sea ice extent vary through the year?
Sea ice extent is at a minimum in February when it fringes the Antarctic continent (at approximately 75 degrees south). It grows to a maximum in September when it is limited at approximately 55 degrees south by the Antarctic Circumpolar Current that separates cold, fresh polar waters from warmer subtropical waters. There is a consistent pattern in the seasonal cycle, characterized by a slow growth period and rapid melt period. This consistency is remarkable, considering the substantial regional, seasonal, and interannual variability of Antarctic sea ice observed over the 40-year satellite period.
Are there geographic variations around the continent?
There are significant regional variations from the overall pattern. For example, sea ice advance in the western Antarctic Peninsula region is generally either shorter than or equivalent to the period of sea ice retreat. This is opposite to the pattern for Antarctica as a whole. Such regional variations have climatic, biological, and biogeochemical consequences at the local scale and warrant further investigation.
How long does it take each season for the sea ice to grow and melt?
Each year Antarctic sea ice takes seven months to grow and five months to melt, with most melt occurring in just a couple of months. The maximum melt rate is approximately twice the maximum growth rate. This asymmetrical pattern is in stark contrast to the nearly symmetrical seasonal cycle seen in the Arctic.
Why is there a longer growth period than melt period?
Winds are thought to play a significant role in this. The tracks of individual storms make up a band of low pressure (trough) that circles Antarctica. This trough separates westerly winds to the north and easterly winds to the south. Twice a year, the trough deepens and contracts towards the continent.
At the start of the growing season, the sea ice edge is south of the trough and easterly winds work against the advancing ice edge to hamper its progress. As the ice advances and crosses the trough, its intensity weakens.
At the start of the melting season, the ice edge is north of the trough and westerly winds speed up its retreat until the ice edge again returns to the south of the trough. The warming ocean increases the rate of melting, but it is unclear whether this contributes to the asymmetry in the seasonal cycle.
How accurately do climate models capture the Antarctic seasonal cycle?
On the whole, climate models do a reasonable job of capturing the seasonal cycle of Antarctic sea ice.
The mean of all models is very similar to the observed cycle of slow growth and rapid melt (see right).
Over 60 per cent of climate models grow ice for seven months, 20 per cent have longer growth seasons (eight months), and 20 per cent have equal growth and melt seasons.
However, none of the simulations has a melt season that is longer than the growth season. This implies that the mechanisms that drive the seasonal cycle are inherent in the climate models.
The models don’t always get the maximum and minimum extents correct, and there is a large spread across the individual models. The majority of the models melt too much ice during summer, so there is not enough ice during February. Many models do not grow enough ice, and so the winter maximum is less than two-thirds of the observed.
What are some of the unresolved questions where additional research, data or modeling is needed?
The role of the westerly winds to the north of the trough has not been quantified. Until we fully understand the mechanisms that drive the seasonal cycle, it is difficult to put the long-term variability into context.
Regional differences are likely to be of major importance for local ecosystems but the mechanisms driving the seasonal cycle at the regional scale have not yet been investigated.
Finally, changes in the sea ice cover only provide part of the story. Satellite technology does not yet adequately capture changes in the sea ice volume but this is an active area of research. An understanding of the changes in the total mass of the sea ice cover over time remains a significant gap in Southern Ocean observations.
We need a better understanding of how sea ice changes because it is a key climate indicator: its presence dramatically increases the amount of sunlight reflected by the surface, affects interactions between the atmosphere and the ocean, and is of major importance for ecosystems.
—Clare Eayrs ([email protected]; 0000-0003-3129-7604), David Holland ( 0000-0002-5768-0866), Diana Francis ( 0000-0002-7587-0006) and Rajesh Kumar, Center for Global Sea Level Change, New York University Abu Dhabi, United Arab Emirates; Till Wagner ( 0000-0003-4572-1285), Department of Physics and Physical Oceanography, University of North Carolina, USA; and Xichen Li, Institute of Atmospheric Physics, China