HOW SEA ICE FORMS

Ice packs fascinate me when I watch them in videos. Many a time I have wondered how they are formed. That curiosity led to this post with its main input from a paper by Peter Wadhams, Professor of Ocean Physics, Scott Polar Research Institute, University of Cambridge, UK. I hope you will learn a thing or two from this post.

Sea ice occupies about 7% of the area of the world ocean, and is of enormous importance climatically because it reflects most of the solar radiation that falls on it, affecting the average albedo of the earth, and also because it interposes a solid layer between the ocean and the atmosphere which reduces the free transfer of heat and moisture between the two. Observational evidence at the moment tells us that the sea ice in the Arctic (although not in the Antarctic) is retreating and thinning, and computer models predict that by the 2080s the ice cover will completely disappear in summer, so it is important for us to understand the mechanisms by which sea ice forms and decays.

The Arctic ice pack is the ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall and winter. Summer ice cover in the Arctic is about 50% of winter cover. Some of the ice survives from one year to the next. Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal ice: up to 3–4 meters (9.8–13.1 ft.) thick over large areas, with ridges up to 20 meters (65.6 ft.) thick. As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades

WHY ICE FLOATS

We first have to account for the fact that ice floats on water at all, since ice is one of very few substances where the solid is less dense than its molten form. This is because the structure of normal ice, called ice I, is an open hexagonal structure. Each oxygen atom is at the centre of a tetrahedron with four other O atoms at the apices. The O atoms are concentrated close to a series of parallel planes that are known as the basal planes. The principal axis, or c-axis, of the crystal unit cell lies perpendicular to the basal plane. The whole structure looks much like a beehive, composed of layers of slightly crumpled hexagons. The net of O atoms is held together by hydrogen bonds. The H atoms lie along these bonds. It is the length of the hydrogen bond that creates the open structure of ice; when ice melts, some of the bonds are broken, causing a disordered structure with a higher density. But even in liquid water some short-range order remains, with a few water molecules retaining the crystal-like bonded structure until this is destroyed by thermal motion; this causes a curious density behaviour in fresh water, where there is a maximum density at 4°C.

COOLING DOWN WATER

Consider a fresh water body being cooled from above, for instance a lake at the end of summer experiencing subzero air temperatures. As the water cools the density increases so the surface water sinks, to be replaced by warmer water from below, which is in its turn cooled. This creates a pattern of convection through which the whole water body gradually cools. When the temperature reaches 4°C, the lake reaches its maximum density. Further cooling results in the colder water becoming less dense and staying at the surface. This thin cold layer can then be rapidly cooled down to the freezing point, and ice can form on the surface even though the temperature of the underlying water may still be close to 4°C. Thus a lake can experience ice formation while considerable heat still remains in the deeper parts.

This does not apply to sea water. The addition of salt to the water lowers the temperature of maximum density, and once the salinity exceeds 24.7 parts per thousand (most Arctic surface water is 30-35), the temperature of maximum density disappears. Cooling of the ocean surface by a cold atmosphere will therefore always make the surface water more dense and will continue to cause convection right down to the freezing point - which itself is depressed by the addition of salt to about -1.8°C for typical sea water. It may seem, then, that the whole water column in an ocean has to be cooled to the freezing point before freezing can begin at the surface, but in fact the Arctic Ocean is composed of layers of water with different properties, and at the base of the surface layer there is a big jump in density (known as a pycnocline), so convection only involves the surface layer down to that level (about 100-150 metres). Even so, it takes some time to cool a heated summer water mass down to the freezing point, and so new sea ice forms on a sea surface later in the autumn than does lake ice in similar climatic conditions.

FORMATION OF ICE IN CALM WATER

In quiet conditions the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than 2-3 mm. Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent, it is called nilas. When only a few centimetres thick this is transparent (dark nilas) but as the ice grows thicker the nilas takes on a grey and finally a white appearance. Once nilas has formed, a quite different growth process occurs, in which water molecules freeze on to the bottom of the existing ice sheet, a process called congelation growth. This growth process yields first-year ice, which in a single season in the Arctic reaches a thickness of 1.5-2 m.

FORMATION OF ICE IN ROUGH WATER

If the initial ice formation occurs in rough water, for instance at the extreme ice edge in rough seas such as the Greenland or Bering Seas, then the high energy and turbulence in the wave field maintains the new ice as a dense suspension of frazil, rather than forming nilas. This suspension undergoes cyclic compression because of the particle orbits in the wave field, and during the compression phase the crystals can freeze together to form small coherent cakes of slush which grow larger by accretion from the frazil ice and more solid through continued freezing between the crystals. This becomes known as pancake ice because collisions between the cakes pump frazil ice suspension onto the edges of the cakes, then the water drains away to leave a raised rim of ice which gives each cake the appearance of a pancake. At the ice edge the pancakes are only a few cm in diameter, but they gradually grow in diameter and thickness with increasing distance from the ice edge, until they may reach 3-5 m diameter and 50-70 cm thickness. The surrounding frazil continues to grow and supply material to the growing pancakes.

At greater distances inside the ice edge, where the wave field is calmed, the pancakes may begin to freeze together in groups and eventually coalesce to form first large floes, then finally a continuous sheet of first-year ice known as consolidated pancake ice. Such ice has a different bottom morphology from normal sea ice. The pancakes at the time of consolidation are jumbled together and rafted over one another, and freeze together in this way with the frazil acting as "glue". The result is a very rough, jagged bottom, with rafted cakes doubling or tripling the normal ice thickness, and with the edges of pancakes protruding upwards to give a surface topography resembling a "stony field". The rough bottom is an excellent substrate for algal growth and a refuge for krill. The thin ice permits much light to penetrate, and the result is a fertile winter ice ecosystem.

GROWTH OF THE ICE

Once a continuous sheet of nilas has formed, the individual crystals which are in contact with the ice-water interface grow downwards by freezing of water molecules onto the crystal face. This freezing process is easier for crystals with horizontal c-axes than for those with c-axes vertical. The crystals with c-axis horizontal grow at the expense of the others, and as the ice sheet grows thicker crowd them out in a form of crystalline Darwinism . Thus the crystals near the top of a first-year ice sheet are small and randomly oriented, then there is a transition to a fabric composed of long vertical columnar crystals with horizontal c-axes. This columnar structure is a key identifier of congelation ice (i.e., ice which has grown thermodynamically by freezing onto an existing ice bottom), and is a striking feature of first-year ice even when viewed by the naked eye. The ions of the salts in sea water cannot enter the crystal structure despite its open nature. One might expect all salt to be rejected, therefore, leading to a sea ice cover composed of pure ice. Such is not the case, however. If you suck on a piece of first-year sea ice it will taste distinctly salty. The water from young sea ice may have a salinity of about 10 parts per thousand, dropping to 1-3 in old ice. How does this salt get into the ice?

The answer lies in the way that the ice sheet grows. The ice-water interface advances in the form of parallel rows of cellular projections called dendrites. Brine rejected from the growing ice sheet accumulates in the grooves between rows of dendrites. As the dendrites advance, ice bridges develop across the narrow grooves that contain the rejected brine, leaving the brine trapped and isolated. The walls of the "prison" close in through freezing, until the salt is contained in a very small cell of highly concentrated brine, concentrated enough to lower the freezing point to a level where the surrounding walls can close in no further. The cell then remains, a tiny inclusion. They eventually drain out of the ice, by way of a network of brine drainage channels which they create, and as the ice sheet ages the brine concentration drops.  

 

Source:

http://en.wikipedia.org/wiki/Artic_ice_pack

http://www.artic.noaa.gov/essay_wadhams.html