Seasonal sea ice in the Southern Ocean melts twice as fast as it grows

David Attenborough described the freezing and melting of sea ice as the largest seasonal cycle on the planet. Nearly 16 million square kilometres of sea ice grows and subsequently melts each year in the seas surrounding Antarctica. So the winter sea ice cover is about 13 times larger than South Africa! This growth in sea ice cover effectively doubles the size of the Antarctic continent each winter, but despite its vast extent, Antarctic sea ice forms only a thin veneer on the ocean surface, less than 1 m thick on average.

Sea ice is frozen seawater that forms on the ocean surface in winter when seawater cools below approximately -1.8 degrees Celcius. It forms at both poles during their respective winter seasons; the maximum extent is during March in the Arctic and during September in the Southern Ocean. If temperatures remain cool, then the ice does not melt (multi-year ice); otherwise, the sea ice melts in spring/summer to grow again in winter (seasonal ice). Antarctic sea ice is mostly seasonal.

Sea ice cover is at a minimum in February (left) and peaks in September (right). Approximately 15 million square kilometres of ice melt and freeze during the annual cycle. These images were downloaded from the following FTP location: ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/ and are based on data from the National Snow and Ice Data Center Sea Ice Index, Version 3 (Fetterer et al., 2017)

Sea ice is a critical component of the climate system, affecting global climate dynamics through its interplay with planetary albedo, atmospheric circulation, ocean productivity and the thermohaline circulation. It plays a crucial role in mediating exchange between the atmosphere and the ocean and therefore understanding its seasonal cycle and being able to model this phenomenon is of broad interest.

Since the late 1970s, satellites have regularly monitored sea ice properties, and we now have a 40-year record of changes in sea ice concentration and extent. Sea ice concentration is the fraction (or percentage) of sea ice coverage. Sea ice extent is the total area covered by some amount of sea ice (usually 15 %) at a given time, including open water between floes.  Compared to the Arctic, where there has been a well-publicised, massive decrease in sea ice extent, there has been a modest increase in Antarctic sea ice cover over the satellite record (1.5 % per decade, 1981–2010). However, this small overall change masks substantial interannual and regional variability and, in recent years, record maximum (2012–2014) and record minimum (2016–2018) extents have been observed. The global trend in sea ice is decreasing and it is hard to imagine in a warming climate that Antarctic sea ice will not decrease in the long-term.

These line graphs plot monthly deviations and overall trends in polar sea ice from 1979 to 2017 as measured by satellites. The top line shows the Arctic; the middle shows Antarctica; and the third shows the global, combined total. The graphs depict how much the sea ice concentration moved above or below the long-term average. (They do not plot total sea ice concentration.) Arctic and global sea ice totals have moved consistently downward over 38 years. Antarctic trends are more muddled, but they do not offset the great losses in the Arctic. Credits: Joshua Stevens/NASA Earth Observatory The figure is taken from this article. See further information at NASA’s Earth Observatory

The overall increasing trend in Antarctic sea ice extent is made up of substantial regional changes, and different drivers are dominant for different regions and seasons. There is still no consensus on the main drivers of trends in Antarctic sea ice, but surface winds play a key role, and seasonal timescales and regional scale processes are important. One of the major difficulties is that observations are sparse and the variability is strong. Climate models do not currently capture the observed trends and, for now, remain limited tools to examine the importance of different drivers of sea ice change.

The seasonal cycle of Antarctic sea ice extent for the duration of the satellite record. The blue line shows the 1981-2010 mean sea ice extent, as provided by NSIDC (Fetterer et al., 2017). The grey dots show the data for all years from 1979 to 2018. The yellow line shows the sea ice extent for 2018.

The Antarctic cycle is characterised by slow growth (7 months) and rapid melt (5 months) and this pattern is seen consistently throughout the satellite record, which is remarkable considering the high regional, seasonal and interannual variability of Antarctic sea ice observed over this period. For all years in the satellite record, there are more “growth days” than “melt days”, even though the interannual variation in mean sea ice extent varies by nearly 3 million square kilometres and the timing of the ice edge advance and its subsequent retreat can vary significantly. Most of the sea ice melts in about 60 days between mid-November and mid-January.

The ability of climate models to capture this regular seasonal cycle is a telling metric on how well a model can represent the system. Climate models do a poor job of representing Antarctic sea ice, do not currently capture the observed trends and, for now, remain limited tools to examine the importance of different drivers of sea ice change. However, on the whole, they tend to capture the pattern and timing, but not the magnitude, of the seasonal cycle.

 

Fetterer, F., K. Knowles, W. Meier, M. Savoie, and A. K. Windnagel. 2017, updated daily. Sea Ice Index, Version 3. [Antarctica, sea ice concentration hires images]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: https://doi.org/10.7265/N5K072F8. [24 December 2018].

 

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