2.8                                   Some aspects of Antarctic ice

The relevance of sea ice to weather forecasting may not always be entirely obvious. An example of the need to consider sea ice formation occurs when, in light wind conditions, moist air from open–water areas moves over the sea ice resulting in low stratus and fog (see, for example, the discussion in Section 6.6.4.1). This can prevent helicopter reconnaissance and limit satellite interpretation of sea ice conditions and thus disrupt operations. Another example of the need to consider the evolution of sea ice as it effects weather forecasting is the mid to late–summer formation of lower–level cloud around Casey Station after the sea ice dispersion has allowed increased levels of low–level moisture to occur (see the discussion on clouds in Section 7.10.1).

For an excellent overview on the observing of Antarctic sea ice the reader is referred to Worby (1999) that is a compact disk (CD) based practical guide for conducting sea ice observations from vessels operating in the Antarctic ice pack. Worby (1999) also touches on the role of sea ice in climate and weather process. (Armstrong et al., 1973) and World Meteorological Organization (WMO) (2000) are also other very relevant sources. The latter, while summarising information relating to sea ice, focuses on globally available sea–ice information services. Since, generally speaking, weather forecasters do not have extensive background in this subject, this section covers some of the basics of the subject of Antarctic ice, with particular emphasis on sea ice.

2.8.1                                Ice terminology and classification

The primary division of ice is into sea ice and freshwater ice. Sea ice may be classified by age or thickness as follows:

Ice less than 30 cm thick

·                         New ice is a general term for recently formed sea ice. These types of ice (including frazil, grease, slush and shuga) are composed of ice crystals that are only weakly frozen together and have a definite form only while they are afloat.

·                         Frazil ice is comprised of fine spicules or plates of ice suspended in water and is the first indication of sea ice formation and gives the sea an oily appearance (see Figure 2.8.1.1).

·                         Grease ice is at a later stage of freezing when the frazil ice crystals have coagulated to form a soupy layer on the surface. It reflects little light, giving the sea a matt appearance (see Figure 2.8.1.2).

·                         Shuga is an accumulation of spongy white ice lumps, a few centimetres across; they are formed from frazil, grease ice, or slush under the influence of wind and wave action (see Figure 2.8.1.3).

·                         Nilas is a thin (up to 10 cm) elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking fingers (see Figure 2.8.1.4). Ice rind may also be referred to as nilas but is formed from sea ice of low salinity and tends to be thin and brittle.

·                         Pancake ice is floating ice predominantly circular from 30 cm to 3 m in diameter and forms in the boundary between two water layers of different salinity (see Figure 2.8.1.5).

·                         Young ice forms when ice rind, nilas or pancake ice thicken into grey ice (10–15 cm thick), which is less elastic than nilas and breaks on swell, or into grey–white ice which, being 15–30 cm thick, is more likely to ridge than to raft when under pressure. Young ice is less elastic than nilas, breaks under the influence of swell, and usually rafts under pressure (see Figure 2.8.1.6).

Ice 30 cm – 2 m thick

·                         First–year ice is of not more than one winter's growth, developing from young ice; thickness 30 cm – 2 m. May be subdivided into thin first–year/white ice (30 – 70 cm), medium first–year ice (70 – 120 cm) and thick first–year ice (over 120 cm) (see Figure 2.8.1.7).

Ice 1.2 m to 5 m thick

·                         Old ice is sea ice that has survived at least one summer's melt. Because it is thicker and less dense than first–year ice, it stands higher out of the water. Summer melting produces a regular pattern of numerous small puddles that develop into smooth hammocks. Bare patches and puddles are usually greenish–blue (see Figure 2.8.1.8).

Other relevant terminology also includes:

·                         Fast ice is sea ice that forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during sea level changes. Fast ice may be formed in situ from seawater or freezing of pack ice of any age to the shore, and it may extend a few metres or several hundred kilometres from the coast. Fast ice may be more than one year old. If it is thicker than about 2 m and above sea level it is called an ice shelf (see Figure 2.8.1.9).

·                         Pack ice is a term used in a wide sense to include any areas of sea ice, other than fast ice, no matter what form it takes or how it is disposed. The concentration is generally expressed in terms of the areal density of ice in a given area. An average thickness of Antarctic pack ice is about 1.0 m.

·                         Slush is snow that is saturated and mixed with water on land or ice surfaces, or as a viscous floating mass in water after a heavy snowfall in near–freezing conditions.

·                         Brash comprises accumulations of floating ice made up of fragments not more than 2 m across (the wreckage of other forms of ice) (see Figure 2.8.1.10).

·                         A lead in the ice is any fracture or passageway through the sea ice that is navigable by surface vessels.

·                         A polynya is any non–linear shaped opening enclosed in the ice. Polynyas may contain brash ice and/or be covered with new ice, nilas or young ice; submariners refer to these as "skylights". Sometimes the polynya is limited on one side by the coast and is called "shore or coastal polynya" or by fast ice and is called "flaw polynya". If it recurs in the same position every year, it is called a "recurring polynya". Polynyas can occur throughout the year around the coast due to persistent offshore winds. During periods of light winds, the sea surface may be covered over with new and young ice or, for onshore winds, packing occurs that closes the polynyas.

·                         Frost smoke is a fog–like cloud due to contact of cold air with relatively warm water, which can appear over openings in the ice.

·                         For ice of land origin:

–Ice Cap: 95% of the continent is covered by ice sheets that range in typical thickness from 100 m at the coast to over 40 km over parts of the plateau. The average thickness of the ice sheets is about 2,500 m: this weight of ice has caused an ice static depression that has resulted in parts of the continental ground surface being 600 m below sea level. The pressure of ice also results in a constant flow towards the coast, the rate of movement varying between 1 to 6 m per day.

–A glacier is a mass of snow and ice continuously moving from higher to lower ground or, if afloat, continuously spreading.

–Glaciology is the study of frozen water in any of its forms or locations. Sometimes the term is used in the restricted sense for the study of glaciers.

–An iceberg is a massive piece of ice of greatly varying shape, more than 5 m above sea–level, which has broken away from a glacier, and that may be afloat or aground. A tubular berg is flat–topped usually formed by calving from an ice shelf and with horizontal banding.

–A "bergy bit" is a large piece of floating glacier ice, generally showing less than 5 m above sea–level but more than 1 m and normally about 100–300 m² in area.

–Growlers are smaller pieces of thin bergy bits, often transparent but appearing green or almost black in colour, extending less than 1 m above sea–level and normally occupying an area of about 20 m².

–Surface snow layer: The formation of the surface snow layer is a complex process involving both the existence of a glazed surface and the deposition–erosion process depending on snow surface conditions such as micro–relief, undulations, texture and structure and meteorological conditions. Snow deposition and erosion are primarily dependent on the roughness of the surface, so occur least when the surface is wind–packed smooth or glazed. In light winds, snow particles settle behind obstacles taking forms, such as barchans, tails and dunes.

–Sastrugi are an erosional form of the surface layer caused by katabatic winds carving pre–existing dunes (dunes are orientated at an acute angle of about 30º). Cyclonic winds usually last for a short time so sastrugi are not modified by them. Sastrugi generally occur in elongated forms parallel to the persistent wind.

-Firn is old snow that has crystallised into a dense material. Unlike snow, the particles are to some extent joined together; but unlike ice, the air spaces in it still connect with each other.

–The firn line: On the lower steeper slopes up to 500 m (the firn line) the surface is usually exposed ice. There is surface leveling in summer with an ice crust; such a flat surface is called a summer surface.

–The dry snow line: The area of summer melting extends up to 700 to 1,000 m (the dry snow line) depending on the local geography.

–Above 1,800 m are found the areas with a glazed surface and they are usually well inland, so away from most cyclonic influences.

–Snow deposition is most common during the "winter", generally from February to September.

2.8.2                                Freezing of sea water

If an aqueous salt solution is frozen extremely slowly, the foreign ions remain in the melt and perfectly pure ice is formed. The freezing rate in nature is usually too rapid for the rejection process to approach completion and the growing ice crystals trap a certain amount of brine. Over time the brine may slowly drain through the ice under the influence of gravity or migrate along the direction of higher temperatures (usually down). If a block of ice is removed from contact with the sea, as by being pushed up on the shore, it loses salt very rapidly during the warmer months of spring and summer. Much Antarctic ice is polar (that is continental) ice that is several years old and when without a snow cover is pale blue in colour, in contrast to the greyish–white of annual sea ice. Polar ice has a low salinity.

The concentration of salts in seawater is sufficiently uniform to be described by a single parameter, the salinity S, which is defined as the total amount of solid material contained in unit mass of seawater. It is usually quoted as a ratio in g per kg of seawater, that is, in parts per thousand (ppt). A value of S=35 ppt is typical of the oceans.

Pure water freezes at 0⁰C at normal atmospheric surface pressures but expands by about 9% on freezing. It has a maximum density at 4ºC. For a salinity of S, the freezing point is –0.053 × S (i.e. ~ –1.9ºC for a typical salinity of 35 ppt). In still pure water, as the surface layer cools, it becomes denser and sinks setting up a vertical circulation until the water reaches a uniform 4ºC. Further cooling to 0ºC results in ice forming on the surface. In seawater this circulation is not set up and the entire body of water must cool to its freezing point before ice can form, hence sea ice only forms at high latitudes. Ice is usually described as a visco–elastic solid, that is, it can be subjected to deformations and it can flow.

2.8.3                                Formation and dissipation of sea ice

As implied in Section 2.2.2 the melting of the Antarctic floating ice occurs from the latter part of October through to early March. Solar heat absorption is more intensive in ice of high salinity i.e. first year ice. There are accumulations of diatoms on the underside of the ice and there is a vigorous absorption of radiant energy and melting is initially greatest from below. In November the numbers of polynyas and leads begin to increase, accelerating the southward retreat of the ice edge. Refreezing occurs quickly in early summer where snow laden and brashy sea water forms a favourable aggregate in calm conditions and temperatures as high as –3ºC. The final disintegration of the sea ice is due to swell and wind–induced break-up. The polynyas act as solar heat accumulators, on account of the relatively low albedo present on the dark water surface and young ice. Consequently, the surface waters are warmed more with an increased rate of ice melting.

Figure 2.8.1.1       Frazil ice near an ice edge. (From Worby (1999).)

Figure 2.8.1.2     Grease ice forming in turbulent conditions. (From Worby (1999).)

Figure 2.8.1.3     Shuga may line up along the wind direction, and form characteristic bands, as a result of the interaction between surface wind and waves. (From Worby (1999).)

Figure 2.8.1.4     “Finger rafting” is evident in this example of Nilas. (From Worby (1999).)

Figure 2.8.1.5     Large, loose pancakes; up to 1.5 m in diameter. (From Worby (1999).)

Figure 2.8.1.6     Young ice may be grey or grey–white in appearance: the above example is mostly of the grey type. (From Worby (1999).)

Figure 2.8.1.7     An example of first–year ice. (From Worby (1999).)

Figure 2.8.1.8     Ridged multi–year ice trapped near the coast off East Antarctica. (From Worby (1999).)

Figure 2.8.1.9     An example of fast ice pinned by grounded icebergs (aerial photograph from approximately 5,000 ft. (From Worby (1999).)

Figure 2.8.1.10     An example of brash ice. (From Worby (1999).)

Steady ice formation starts approximately in mid–March, first in narrow bays and at the southern shores of marginal seas, such as Ross Sea. As a rule of thumb it requires calm days of –12ºC. New ice could possibly appear in other regions, especially in the presence of residual ice and grounded icebergs, as well as along the eastern shore of ice shelves that interfere with the general ice drift from east to west in the coastal zone.

2.8.4                                Ice drift

In the Antarctic beyond the shore–fast ice lies the pack ice that is a mixture of sea ice, fragments broken off from the shore–fast ice, and pieces of disintegrated land ice. The averaged wind flow around the coast is easterly and there is an easterly weak current causing the pack ice to drift to the west. This ice drift is a diverging flow since the ice is being moved towards an ever–increasing area of ocean. Consequently the pack ice is open with wide leads and channels between the floes. If it were not for this fortunate fact, marine navigation would be extraordinarily difficult. When the drifting ice reaches 60º S, or further north, it encounters the region of the westerlies and of the Antarctic Convergence zone.

An iceberg will move under the same forces but the effect of the current is the dominant one since 90% of its mass is below the surface. It is sometimes observed that icebergs will move through the pack ice in a separate general direction reflecting the effect of sub–surface currents. Some of the Antarctic icebergs are over 500 m thick and tens of kilometres in width. Since one of these vast icebergs may take up to 10 years to melt, it can travel great distances and there are recorded cases of icebergs being sighted in the tropical zone.

Typically at the time of maximum sea ice extent the percentage of open water within the sea ice zone appears to decrease from about 50% near the edge to about 10% near the fast ice that is generally within about 50 km of the coast of Antarctica. The ice concentration or the amount of open water is highly variable in both space and time and is the dominant factor in the exchange of heat from ocean to atmosphere.

Similarly the average sea ice thickness is highly variable, and dynamic as the winds and currents move the ice together to form thick pressure ice regions, or apart, to form leads and open water regions. A large part of the Antarctic sea ice zone is young ice with an average ice thickness in the range of 0.5 to 1 m. This relatively low thickness is partly a feature of the general movement and divergence of the sea ice. The drift of the ocean and icebergs generally provides an indication of the general drift motion of the sea ice. In the short term there is a strong correlation between the wind and the movements of the pack ice (order cm s^–1) with the sea ice moving at about 2% of the wind speed for close pack but with much higher drift rates being possible in open ice. A force is also exerted on drift ice by currents in the upper layers of the sea. It is usually very difficult to differentiate between the wind and current effects on sea ice drift, the resultant movement being, of course, the vector sum of the effects of the two forces. Wind stress normally predominates the short–term movements, particularly in offshore areas, whereas the average long–term transport is dominated by the prevailing surface currents (WMO, 2000).

Figures 2.8.4.1 and 2.8.4.2 show, respectively, USA National Oceanographic and Aeronautical Administration (NOAA)–12 visible band images for 29 and 30 October 1992. On these days strong to gale–force southeasterly katabatic winds were believed to be noticeably moving the sea ice in Vincennes Bay southwest of Casey Station. Note also the movement of the ice floe located northeast of Casey between the two images that are almost 24 hours apart.

 Figure 2.8.4.1     NOAA–12 visible image at 2242 UTC 29 October 1992.

 Figure 2.8.4.2     NOAA–12 visible image at 2220 UTC 30 October 1992.

2.8.5                                Icebreakers

The icebreaker is the brute–force approach to making a channel in unwanted ice. Typically these ships have two screws aft capable of driving the vessel at 8 m s^–1 (~16 kt). The bow is narrow and sharply raked to provide a good cutting edge. The hull below the water line is very rounded so that if the ship is trapped in a converging ice area, the pressure will tend to lift the vessel without punching the sides. To resist ice damage, the part of the hull that may come in contact with ice is made of 4 cm steel plates.

A peculiar feature is a system of heeling tanks. These are symmetrically placed sets of tanks down either side of the vessel that allow water to be rapidly pumped from one side of the vessel to the other. If this heeling action is carried out with the engines running full astern, it is extremely effective. As a last resort the ship is sometimes backed slowly into the ice barrier so that the propeller may chop the ice to pieces. However, this technique is rather hard on the propeller blades.

2.8.6                                Sea ice information services  

Since the establishment of satellite imagery, it has been possible to regularly monitor sea ice extent on a large-scale. WMO (2000) provides an overview of the sea ice information services that are available worldwide. The most well known service, which has operated from 1973 to the present, is the USA Navy Fleet Weather Facility's Ice Forecasting Group, now called the National Ice Center (NIC). NIC provides analyses of the Antarctic sea ice extent and concentrations during the southern summer and occasionally the location of the larger icebergs.

Allison (personal communication) has summarised some web–based data sources related to sea ice as shown in Table 2.8.6.1.

Table 2.8.6.1     Some web–based sources of data related to sea ice.

Type of data	Source	Web URL	Comment
AVHRR	AMC*, Casey	http://www.bom.gov.au/weather/tas/inside/amc/satindex.shtml	GIF images
SSM/I	NIC	http://www.natice.noaa.gov/science/products.html	Gridded (coarse), but ~1 day behind.
	NSIDC	http://nsidc.org/data/seaice/current.html	Ungridded and ~1 day behind.
Radar scatterometer	SeaWinds QuikSCAT	http://manati.orbit.nesdis.noaa.gov/quikscat/	25 km res., 1800 km swath. Ungridded on this website.
	NIC	http://www.natice.noaa.gov/science/products.html	Semi-gridded, but ~1 day behind in time.
Chart analysis		http://www.natice.noaa.gov/pub/antarctica/	Updated each two weeks for the Antarctic.
*Antarctic Meteorological Centre

2.8.6.1       Egg code

At http://www.natice.noaa.gov/egg_code/index.html a brief description of the WMO's system for sea ice symbolism is given. The key symbol used is known as the "Egg code" due its the oval shape. Referring to Figure 2.8.6.1.1 the following explanation of the “Egg code” is to be found at the above web address.

Total Concentration

The total concentration (C) is reported in tenths and is the uppermost group. Concentration may be expressed as a single number or as a range, not to exceed two tenths (i.e., 3-5, 5-7).

Partial Concentrations

Partial concentration (Ca, Cb, Cc) are also reported in tenths, but must be reported as a single digit. These are reported in order of decreasing thickness, that is, Ca is the concentration of the thickness ice and Cc is the concentration of the thinnest ice.

Stages of Development

Stages of development (Sa, Sb, Sc, So, Sd) are listed using the following code in decreasing order of thickness. These codes are directly correlated with the partial concentrations above. Ca is the concentration of stage Sa, Cb is the concentration of stage Sb, and Cc is the concentration of Sc. "So" is used to report a development with the greatest remaining concentration that will not fit into the egg. If all particle concentrations equal the total concentration and there is a Sd, Sd is considered to be present in a trace amount. The codes that are used to denote stages of development for sea ice are given in Table 2.8.6.1.1.

Forms of Sea Ice

Forms of sea ice (Fa, Fb, Fc) indicate the floe size corresponding to the stages identified in Sa, Sb, and Sc respectively. The codes used to denote forms of sea ice are shown in Table 2.8.6.1.2.

Table 2.8.6.1.1     Egg code figures for stages of development of sea ice.

Stage of Development	Code Figure
New Ice-Frazil, Grease, Slush, Shuga (0-10 cm)	1
Nilas, Ice Rind (0-10 cm)	2
Young (10-30 cm)	3
Grey (10-15 cm)	4
Grey-White (15-30 cm)	5
First Year (30-120 cm)	6
First Year Thin (30-70 cm)	7
First Year Thin- First Stage (30-70 cm)	8
First Year Thin- Second Stage (30-70 cm)	9
Med First Year (70-120 cm)	1.
Thick First Year (>120 cm) Old-Survived at least one seasons melt (>2 m)	4.
Old-Survived at least one seasons melt (>2 m)	7.
Second Year (>2 m)	8.
Multi-Year (>2 m)	9.
Ice of Land Origin

Table 2.8.6.1.2     Egg code figures for forms of sea ice.

Form of sea ice	Code Figure
New Ice (0 cm - 10 cm)	X
Pancake Ice (30 cm - 3 m)	0
Brash Ice (less than 2 m)	1
Ice Cake (3 m - 20 m)	2
Small Ice Floe (20 m - 100 m)	3
Medium Ice Floe (100 m - 500 m)	4
Big Ice Floe (500 m - 2 km)	5
Vast Ice Floe (2 km - 10 km)	6
Giant Ice Floe (greater than 10 km)	7
Fast Ice	8
Ice of Land Origin	9
Undetermined or Unknown          (Iceberg, Growlers, Bergy Bits) (Used for Fa, Fb, Fc, only)	/