What is Sea Ice?

Each winter the surface of the Southern Ocean freezes over, forming a cover of sea ice that surrounds much of Antarctica. We differentiate zonal regions by their physical characteristics. The majority of sea ice occurs as "pack ice", which drifts under the forcing of ocean currents, waves, tides and wind stress. To the north the sea-ice zone merges with the so-called blue ocean in a transition zone, the so-called "marginal ice zone". It is characterized by wave incursion, bands of sea ice interlaced with bands of open water and generally small sea-ice floes. At the southern end of the Antarctic sea-ice region a near-continuous often level band of sea ice is encountered, the so-called "(land)fast ice". It is pinned to the coast, glacial ice tongues/shelves, islands or icebergs, and may fringe coastal polynyas. Fast ice is considered to be stationary, although transient fast ice has been observed.

The Antarctic pack ice is a region of highly variable sea-ice conditions, including individual sea-ice floes of a range of sizes, ages and thicknesses, in varying concentrations (proportions of sea-ice covered sea-surface area). Depending on the time of year and thus air and upper ocean temperature, there may be open water (break or lead) between floes, and it is common to observe sea ice at various stages of development mixed within an area. This is due to the dynamic nature of Antarctic pack ice, with the thickness of sea-ice floes increasing through rafting or ridging, and new open water being created through divergent events allowing new sea ice to form.

Sea ice provides habitat for a range of plant and animal species. Algal communities reside within or under the ice, and algal blooms occur in the stable freshwater "lens" that remains as the outer sea-ice edge retreats during spring or summer. Krill feed on algae and other plankton, and in turn krill are a stable diet for larger marine animals such as fish, whales, seals, penguins and flying seabirds. Sea ice also offers resting and breeding platforms to seals and penguins, as well as refuge from predators. In short, sea ice affords critical ecosystem functions in the Southern Ocean.

Extent and Variability of Sea Ice

Antarctic sea ice undergoes one of the greatest seasonal cycles on the surface of the Earth. It expands from a minimum extent of about 4 million square kilometres in February to a maximum of about 19 million square kilometres in September. At maximum annual extent, the area of sea ice is about 1.6 times the area of Antarctica itself. This comparison allows us to appreciate the scale of sea-ice processes, and thus their potential to influence global climate.

There is also marked variability on short time scales. The position of the ice edge may vary by tens of kilometres a day driven by passaging synoptic systems and associated changes in wind stress or direction. At the other end of the time scale, there are cycles of interannual variability, and long-term change.

Sea ice also varies with location. In the Ross Sea and Weddell Sea embayments, clockwise ocean currents influence the drift and distribution of sea ice. The Weddell Sea pack extends further north than anywhere else around Antarctica, and may be up to 2200 km from the coast. In contrast, East Antarctic pack may extend only a few hundred kilometres from the coast. This is primarily because the East Antarctic coastline is further north.

Ice Formation

The first stage of development is ice crystals in the surface layer of the ocean. These crystals, known as frazil, form in open water when water temperature drops below -1.86°C.

Frazil ice gives the water an oily appearance. With further freezing the crystals coagulate to form a soupy layer at the surface known as grease ice. How the sea ice develops then depends on whether the surface is calm or disturbed.

In calm conditions, frazil and grease ice may consolidate into continuous flexible sheets called nilas. Nilas may be up to 10 cm thick, and rafts easily, thus thickening exponentially. Sea ice 10-30 centimetres thick is termed young ice, and can be grey or grey-white. With further rafting, it develops into first-year ice (>30 cm).

Under rough conditions, common process of development is the "pancake cycle". With wind and waves, frazil crystals coagulate, eventually consolidating into small circular discs called pancakes. Pancakes have raised rims due to collisions with other pancakes, and grow at the edges by fusing with frazil in surrounding water. Rafting and bonding together, pancakes rapidly increase to a few metres in diameter and up to 40 cm thick, and eventually freeze together to form larger floes and more consolidated ice cover.

Although new ice forms most rapidly in open water, sea ice also grows on the underside of floes as ocean heat is conducted through the floe. The 'congelation ice' characteristic is columnar crystals, distinct from small randomly-oriented crystals of frazil.

Snow ice forms when snow overlaying sea ice is flooded and then freezes. When the weight of snow is sufficient, sea ice is depressed below sea level and in the presence of any cracks or percolation sea water saturates lower layers of snow. Similarly, floes can be wave-washed and snow and sea water contained within the rims of pancakes freezes to form snow ice.

Ice Drift and Deformation

Ice drift is one of the important features of the Antarctic pack. The sea ice is highly mobile because pack ice is unconstrained by land. Speed and direction of drift are determined mainly by wind.

Sea-ice thickness distribution is a description of the surface of the pack in terms of concentration (fraction of surface area) of different ice-thickness categories. Ice drift determines thickness distribution of sea ice. Net drift of pack ice is divergent, but frequent periods of convergence cause floe deformation.

Southerly winds cause the ice to be advected northward. This creates open water between floes, allowing rapid ice growth (frazil). Northerly winds cause the pack to converge, increasing concentration and thickening by rafting and ridging.

Low-pressure systems cause the pack to undergo cyclical periods of convergence and divergence due to fluctuating wind directions. The importance of ice drift to the sea ice thickness distribution is evident in the ice core analysis: Frazil in the East Antarctic pack is almost half of the ice mass. This is in stark contrast with the Arctic, where frazil ice comprises only about 5% of the total ice mass.

Persistent katabatic winds from Antarctica also maintain open water in the form of coastal polynyas and advect new ice northwards. Polynyas act as "ice factories" and make a disproportionately large contribution to total mass of sea ice.

Rafting and Ridging

Dynamic processes (rafting and ridging) have a major role in the development of sea ice. Ice-core analysis shows that deformation, rather than basal freezing, is the dominant mechanism for sea ice thickening in Antarctica.

Rafting of nilas and pancakes results in rapid thickening of sea ice to about 0.5m thick. More than 0.5 m thick, convergence is likely to result in ridges.

Convergence results in an increase in local ice thickness whilst at the same time opening leads where new crystals form. The net effect is increased ice production and an increase in the total mass of ice within the pack.

Ridged areas contain a disproportionately large proportion of ice. As total ice volume is the key parameter for determining the amount of salt rejected from the ice during the growth season, and the amount of fresh water released during the melt season, any estimate of ice thickness needs to incorporate the ridges.

Seasonal Development

The greatest seasonal changes in the thickness distribution of East Antarctic pack ice are in the categories of open water and thin ice.

The amount of open water decreases from almost 60% in December to little more than 10% in August, and thin ice (0 - 0.2 m) increases by 30% from December to March.

In March, at the beginning of the growth season, approximately 25% is open water and an additional 60% is less than 0.4 m thick. This is indicative of rapid new ice growth over large areas of the Southern Ocean as the air temperatures begin to cool.

As winter progresses the amount of open water within the pack decreases and new ice thickens quite rapidly due to storms and the cold air temperatures. This leads to a decrease in thinner ice and an associated increase in thicker ice. In August, the pack is quite consolidated, and the open water fraction averages 12%. There is only a small percentage of ice less than 0.4 m, and the ice between 0.4-0.8 m thick is of greatest concentration.

Antarctic vs Arctic

There are several fundamental differences between Arctic and Antarctic sea ice due to geography.

Antarctic sea ice is unbounded at its northern extent, thus highly dynamic. Influenced by winds and ocean currents, pack ice has cyclical periods of convergence and divergence. North of the Antarctic Divergence (~65°S latitude), pack ice generally moves from west to east in the Antarctic Circumpolar Current, but with a net northward component of drift. The unconstrained nature of the Antarctic pack results in large seasonal variability in maximum extent, and very little sea ice surviving more than one season.

In contrast, the Arctic basin is predominantly land-locked, with only a small fraction of ice advected out of the area, mostly via Fram Strait between Greenland and Svalbard. Thus Arctic sea ice exhibits less annual variability and retains a greater proportion of multiyear ice. Because of its constrained nature and greater age, Arctic sea ice attains a greater thickness than sea ice in the Antarctic. Historically, multiyear ice in the Arctic was up to several decades old and up to several metres thick, even a few tens of metres thick in ridged areas. Undeformed first-year sea ice in the Antarctic rarely reaches thicknesses greater than two metres, and only then in fast-ice regions close to the coast.

Within the Antarctic pack, frazil crystals form in open water and make a major contribution (50%) to total ice mass. Dynamic processes of rafting and ridging are the main mechanisms by which sea ice thickens. In the Arctic Basin, sea ice is not as mobile; there is less open water. Frazil ice is a smaller component (5%) of total ice mass. Thickening in wave action by rafting of floes and snow ice is less, although large pressure ridges are common.

Another major difference between Arctic and Antarctic sea ice is during the melt season. In the Arctic, sea ice begins to melt at the surface. Melt pools form and decrease the albedo of the surface, allowing more solar radiation to be absorbed, further enhancing the melt process. In the Antarctic, melt occurs mainly from the bottom and sides of the floes, where they are in contact with the ocean. Divergence of the pack creates open water where solar radiation is absorbed by the ocean. Thus the surface layer of the ocean warms, increasing the rate of melting. Closer to the coast, the low relative humidity of air flowing from Antarctica causes surface ablation via direct sublimation of the ice to water vapour.

Sea Ice and Climate

With its' expansive maximum extent and large seasonal and interannual variability, sea ice has a major influence on the global climate system. Sea ice affects ocean and atmosphere by modifying heat, mass and momentum, thus influencing the circulation of both atmosphere and ocean.

Sea ice acts as a physical barrier to the exchange of gases, and as an insulating blanket between the relatively warm ocean and colder atmosphere. During winter, there are sharp temperature gradients between atmosphere and ocean, because ocean temperature never falls below -1.9°C. Heat loss from ocean can be two orders of magnitude less over sea ice than over open water.

With its high albedo (fraction of incident solar radiation reflected by the surface), sea ice and its' snow cover reduces the amount of solar radiation absorbed by the ocean. Much of the sun's energy is reflected back to space.

Sea ice influences ocean currents in several ways. The transfer of momentum from atmosphere (wind) to ocean (currents) is modified by the presence of sea ice. Sea ice is considerably less salty than sea water, and salt rejected from ice during formation increases the salinity and density of underlying water. This may induce deep vertical convection that contributes to the upwelling of nutrients and the overall thermohaline circulation (water movement driven by salinity and temperature gradients). Conversely, when sea ice melts in spring it releases fresher water, forming a stable low-salinity surface.

The way sea ice affects the atmosphere and ocean depends on ice extent and thickness distribution. Thickness distribution, in turn, is determined by the atmosphere and ocean. Thus ocean, sea ice and atmosphere form a complex interactive system.