Antarctic sea ice. Photo: Antarctica New Zealand
Sea ice at both poles is a crucial component of the Earth system, regulating ocean and air temperatures, circulating ocean water, supporting animal habitats, and reflecting sunlight back into space. Arctic sea ice has been melting away for decades as the world warms, yet Antarctic sea ice hasn’t – why?
Sea ice around Antarctica has a huge seasonal cycle, growing 15 million square kilometres every winter, then melting away again in summer. On top of that, it’s exposed to the winds and storms of the Southern Ocean, and to climate variations from near and far. It’s difficult to untangle all this variability from climate change, but we really need to. Understanding the future of the sea-ice field is a key to understanding how the oceans, the atmosphere, the ice sheets and the climate system will respond to changing climate drivers.
Sea ice cycles and trends
In winter, seawater freezes into a layer of floating sea ice, which melts (or partially melts) in the summer months. The maximum winter extent can be up to five times the summer minimum (ranging from 4 million km2 in late summer, to 19 million km2 in late winter). In winter, this effectively doubles Antarctica’s ice-cover.
The timing and distribution of this annual cycle varies from year to year, and over longer time periods. Until 2014/15, the year-to-year variability was superimposed on a generally increasing trend in sea-ice cover (the extent of the sea-ice had been growing since satellite records began in 1979). That underlying trend now appears to have reversed. Since 2015, sea-ice extent in the Ross Sea region has declined so unexpectedly and sharply, that circum-Antarctic sea-ice loss in February over three years is equivalent to 30 years of sea-ice loss in the Arctic.
Sea ice extent around Antarctica grows 15 million square kilometres every winter to a peak extent as shown above (it’s all-time maximum in 2014), then melts away again in summer (Source: NASA's Scientific Visualization Studio/Cindy Starr) Click here to watch the video.
Sea ice influencers
Sea ice dynamics are influenced by climatic drivers and the interactions between the ice, ocean and atmosphere, but there’s no consensus on what’s driving the observed trends (or even if they are trends at all, rather than being random ups and downs). Factors affecting sea ice dynamics include atmospheric temperatures, changes in surface wind patterns around the continent, and oceanic factors such as the addition of cool fresh water from melting ice shelves or warming ocean currents.
Antarctic sea ice is greatly influenced by ocean and atmospheric circulation because, as it grows northwards, it’s unconstrained by continental boundaries. Sea ice covers a vast area of the vigorous Southern Ocean, spanning multiple oceanic and atmospheric zones, exposed to very strong winds and ocean currents.
Southern Ocean conditions and local weather events also influence sea ice. We’ve seen this near Scott Base recently. Unusually-late sea ice formation in the McMurdo Sound in winter 2022 has been attributed to a series of strong storms blowing freshly-formed sea ice out of McMurdo Sound before it could grow thick enough to withstand the next storm.
It seems logical that polar conditions would have a strong influence on sea-ice distribution, but those polar conditions themselves are affected by climate patterns far from Antarctica. The climate in the tropics can change the winds buffeting the Antarctic coast, which influences sea-ice formation and retreat. This link between weather phenomena and climate patterns at widely-separated locations on Earth is called teleconnections.
For example, numerous studies suggest a strong link between the tropical Pacific El Niño-Southern Oscillation (ENSO) phenomenon and Antarctic sea-ice variability. Changes in the tropics ripple across the Pacific Basin, causing large-scale atmospheric waves that tweak atmospheric circulation all the way to the Antarctic coast – ultimately shaping how sea ice behaves, especially on the West Antarctic coast.
Another key Southern Hemisphere climate pattern is the Southern Annular Mode (SAM). It controls how the westerly winds and bands of storms over the Southern Ocean move from week to week. Under a positive SAM, winds and storms move closer to the Antarctic coast. This brings unsettled conditions over the sea-ice field, but fine and dry weather to mid-latitude countries like New Zealand. The negative SAM, on the other hand, takes the storms and westerlies north over the mid-latitudes (including New Zealand) and brings more settled conditions to the Antarctic. Edmund Hillary’s expedition to the South Pole from Scott Base in 1957 got underway in a negative SAM period, “brilliantly fine” weather was reported at the time.
A positive SAM results in a colder sea surface and increased sea-ice production (due to affects of winds on currents in the upper circum-Antarctic ocean). SAM also affects the Amundsen Sea Low, a climatologically low pressure area over the Southern Ocean north of West Antarctica that shapes north-south wind fields, and is influential to the Ross Sea. These competing forces cause sea ice to grow in some places and to melt away in others. Better understanding of the effects of the SAM on sea ice and polynyas (areas of ice-free water) in the Ross Sea is an area of active research.
The future of sea ice
Over forty years of satellite observations of Antarctic sea-ice extent poses many puzzles. Where is the climate change trend? What exactly is driving regional changes? What explains the increasing sea ice cover from the early 1980s to 2014/15, and what explains the shrinkage since then? The way to tease out these questions is through careful observation and analysis, and by the use of models that can show how the sea-ice field responds to different influences.
The extreme natural variability in Antarctic sea-ice cover means it’s too soon to know if a significant sea-ice decline has begun – but it may have. Improved understanding of local and teleconnected drivers of sea-ice variability will help clarify the picture and help us better represent sea ice in climate and Earth System models, in turn improving near-term forecasting and long-term projections of Antarctic sea ice.
Article prepared by James Renwick.