240 241 (13)

240 METE0R0L0GY FOR MARINERS

The next stage of devclopment, known as first-year ice, is sub-dividcd into thin, medium and thick. Medium first-ycar icc has a rangę of thickness from 70 to 120 centimetres. At the end of the winter thick first-year ice may attain a maximum thickness of alx)ut 2 metres. Should this ice survive the summer melting season, as it may wcll do within the Arctic Basin, it is designated second-year ice at the onset of the ncxt winter. Subseąuent persistence through summer melts warrants the dcscription multi-year ice which, after several years, attains a maximum thickness, where lcvel, of about 3-5 metres; this maximum thickness is attaincd when the accretion of icc in winter balanccs the loss due to melting in summer.

The buoyancy of level sea ice is such that approximatcly one scventh of the total thickness floats above the water.

Ice inereases in thickness from bclow as the sea water freczcs on the under surfacc of the ice. The ratę of incrcasc is determined by the severity of the frost and by its duration. The nced for a measure combining the efTects of air temperaturę and timc has given rise to the concept of ‘accumulatcd frost dcgrcc-days*—the total of daily mean air temperatures, below o^cc, summed ovcr the period of the frost. Figurę 17.2 shows the relationship between accumulatcd frost degree-days and ice thickness. (This is bascd on observations from a few sites within the Arctic and therefore is not necessarily applicablc to all areas.)

It will be scen that as the ice becomes thickcr the ratę of incrcasc in thickness diminishes due to the insulating effect of the ice (and its overlying snów covcr) in rcducing the upward transport of hcat from the sea to the vcry cold air above. Under extreme conditions, when the air temperaturę may suddcnly fali to as Iow as -30 to -4O⁰c, it is possiblc that a laycr of icc can form and grow in thickness to about 10 centimetres in 24 hours and to a total of about 18 centimetres in 48 hours.

Two other factors which contribute to the growth of sea ice are morę applicablc to the Antarctic region, than to the Arctic. The first concerns the effcct of snów cover. Where this is rclativcly deep, say upwards of 50 centimetres, the sheer wcight of the snów may depress the original ice layer bclow sea lcvcl so that the snów becomes water-loggcd. In winter the wet snów gradually freczes, thus inereasing the depth of the ice laycr.

The other effcct is due to the supercooling of water as it flows under the deep icc shelvcs which are a typical feature of the Antarctic coastlinc. The super-coolcd water is preventcd from freczing by the pressure at this depth. Observa-tions have shown that the flow of water under the ice shelves is often vigorous, the consequent turbulcncc rcsulting in somc of the supercooled water rising towards the surface as it lcaves the vicinity of the ice shclf. The consequent rcduction of pressure may lead to the rapid formation of frazil ice in the ncar-surface water. The same mechanism can also result in the accumulation of a rclativcly deep laycr of porous ice beneath an original ice layer. In this way recently broken fast icc (see page 242 for a definition) ovcr 4 metres thick, encountered on the approachcs to Enderby Land in the Southern autumn (March) was observed to consist of only 30 centimetres of solid ice and 4 metres of porous ice, the wholc laycr offering little rcsistance to forward progress. This effect is almost cntircly confincd to the fast-ice zohe. (For distribution of fast ice see Chapter 18.)

At the first stage of its devclopment sea icc is formed of purc water and contains no salt. The downward growth of icc crystals from the under surface

FORMA TION AND CU A RAC TER OF ICE

241

Figurę 17.*. The relationship between ice growth and accumulatcd frost dcgree-days (bclow o C) for various initial ice thicknesses

of the ice results in a network of crystals and smali pockcts of sea water. Even-tually these pockets become cut off from the underlying water and, with furthcr cooling, they shrink in sizc as somc of the water in thcsc pockcts freezes out. The rcsidual solution (brinc) now has a higher salt content, its salinity being highly dependent on temperaturę. Since there exists, at least in winter, a substantial positive temperaturę gradient downwards through the ice, it follows that the temperaturę at the top of a pockct of brinc is lowcr than at its base. This leads to freezing at the top of the brine pocket and melting at the base, resulting in a slow downward migration of the brine through the ice. Thus brine is drained from the icc at a vcry slow ratę.

As cooling continues the salt content is gradually crystallized out of solution. There are certain critical temperatures in this process, namely -8°c, the temperaturę at which the first crystallinc deposit (sodium sulphate) occurs, and — 23°c, the temperaturę at which common salt begins to be deposited. As the