244 245 (14)

244 METEOROLOGT FOR MARINERS

they reach an advanccd State of dccay, break into smali picces called brash ice, the last stage before mclting is complete. Wind, waves, and rising temperatures combine to elear the ice from areas which are affected by first-year icc. In other areas, mainly within the Arctic Basin, the summer melting probably accounts for a reduction in ice-floc thickncss of about t metre.

The breakup of fast ice by puddling sccms to be limited to the Arctic region; it has not been obscrvcd in the Antarctic wherc snów depths are usually greater (50 cm-i m). In the latter area, due to the prcscnce of the surrounding turbu-lent ocean, the fast ice is morc often broken up by the action of ocean swell, particularly after the pack ice has been rcmoved by the off-shore winds which prevail in thcsc regions. It is said that another factor of no little importance is the prcscnce of diatoms in the lowcr layers of the fast ice which, becausc of their dark colour, absorb solar radiation passing through any snow-free icc, leading to wcakening and melting from within.

Movement of Ice

Pack icc moves under the influence of wind and currcnt; fast ice stays immobile. The wind stress on the pack ice causes the floes to move in an appro.\imatcly downwind direction. The deflecting forcc due to the earth’s rotation (the Coriolis forcc) causes the floes to deviatc to the right of the surfacc wind direction in the northern hemisphere and to the left in the Southern hemi-spherc. Since the surface wind direction is normally backed relative to the gcostrophic wind direction by an amount roughly equal to the deflection (in the oppositc sensc) due to the Coriolis force, the ice movemcnt due to wind drift can be considered approximately parallcl to the isobars.

The speed of movcment, due to wind drift, varies not only with the wind speed but also with the concentration of the pack ice and the dcgrcc of ridging. In vcry open pack ice (1/10-3/10 cover) there is much morę freedom to respond to the wind than in close pack ice (7/10-8/10) where frec spacc is very limited. Also, if there is considcrable ridging, the wind will obviously havc morc cffcct in inducing motion than if the ice surface were smooth. The degree of ridging may be expressed in terms of a scalę of ten points. The ratio of icc drift to gcostrophic wind speed producing the drift is known as the ‘wind-drift factor’. Table 17.1 givcs approximate values of wind-drift factor for ccrtain concentra-tions and degrees of ridging.

Table 17.1. The ratio of icc drift to gcostrophic wind

Dcgrcc of Ridging	Concentration of icc
	2/10	5/10	8/10
	(Vcry Opcn Pack)	(Opcn Pack)	(Close Pack)
0/10	1/240	'/ 35°	1/480
3/10	i/55	1/ 80	i/i4°
6/10	i/3°	i/4«	1/70
>6/10	1/27	>/ 39	i/63

The total movemcnt of pack ice is the resultant of wind-drift component and current component. As regards the latter, sińce most of the ice is immersed in the sca it will move at the fuli currcnt ratę exccpt in narrow channels where it may form an ice jam. W hen the wind blows in the same direction as the

FORMA TION AND CHARACTER OF ICE

245

current, the latter will run at an inereased ratę and therefore the ice movement, under these conditions, due to wind and current, may be considerablc. This is particularly so in the Greenland Sea and to a lesser extent in the Barents Sea and oflf Labrador.

Anothcr cffect of the wind is that when it blows from the opcn sea on to the pack icc, it compacts the floes into higher conccntrations along the ice edge which now becomes well defincd. Converscly, a wind blowing oflf the icc moves the floes out into the opcn sea at varying rates, dependent on their size, roughncss and age, rcsulting in a diffuse icc edge.

Movement of Pack Ice within the Arctic Basin

Figurę 17.3 displays a schematic representation of surface currents within the Arctic Basin and adjacent Atlantic Ocean in so far as they are known. The main flow of ice occurs across the pole from the region of the East Siberian Sea towards the Greenland Sea. On the Eurasian side of this transpolar stream the pack ice movcs under the influence of the counter-clockwise current circulations within the pcriphcral seas and on the North American side the pack ice drifts in a eloekwise direction within the Beaufort Sea current gyre.

The bulk of the icc is carried out of the polar basin by the East Greenland Current although somc passes into Baffin Bay through Smith Sound. Icc formed in the East Siberian Sea takes from 3 to 5 years to drift across the polar basin and down to the coast of Greenland. Icc of this age, therefore, becomes pressed and hummocked to a degree unknown in icc formed in lower latitudcs.

Movement of Pack Ice in the Antarctic

The action of wind and current around the coast of Antarctica imparts a northerly component to the movcmcnt of pack ice so that it is cventually carried into the warmer waters of the Southern Ocean Currcnt where it mclts. The pack ice of this hcmispherc is therefore mainly first-ycar ice. It is only in a fcw areas, such as the Weddell and Bcllingshausen Seas, where the action of wind and current carries the icc on to the coasts, that second- or multi-ycar icc is commonly found.

ICEBERGS

Iccbcrgs are large masses of floating icc derived from floating glacicr tongues or from icc shclves. The specific gravity of icebcrg ice varics with the amount of imprisoncd air and the mean value has not been exactly determined, but it is assumed to be about 0*900 as compared with 0*916 for pure fresh-water icc, i.e. about 9/10 of the volume of an iccberg is submerged. The depth of a berg under water, compared with its height abovc the water, varies with diflferent typcs of bergs. Table 17.2 has been derived from actual measurcmcnts of bergs south of Ncwfoundland by the International Ice Patrol.

The colour of bergs is an opaąue flat white, with soft hues of green or bluc. Many show vcins of soil or debris; others have yellowish or brown stains in placcs, due probably to diatoms. Much air is imprisoncd in ice in the form of bubbles permeating its whole structure. The white appearancc is causcd by surface wcathering to a depth of 5 to 50 centimetrcs or morę and also to the eflfcct of the sun’s rays, which release innumerablc air bubbles.

Icebergs diminish in size in thrcc diflferent ways: by calving, melting, or