252 253 (11)

252 METEO ROLOGY FOR MARINERS

It can bc scen that thcrc arc considcrablc latitudinal difTcrcnccs along this limit. It lies furthest nortli in the region around Svalbard and Zemlya Frantsa Iosifa duc to the rclatively warm currcnts of soutlicrly origin which prcvail in this region (see Figurę 17.3). The limit lies furthest south in the east Greenland scctor where pack icc is continuously brought out of the Arctic Basin in the cold Fast Greenland Current, and in the central parts of the Canadian Arctic Archipclago where the pack icc is drivcn southwards by the prevailing north-west winds of summer. Dcspite this Iattcr exception an almost ice-free roule exists through the North-Wcst Passage in early Septcmber in the averagc year.

The extremc minimum limit (7/10) lies a considcrablc distancc northwards of the mean minimum (4/10) just describcd. It should bc understood that this extremc is a composite limit based on the minimum ice conditions obscrved in any area in any year; the displayed extreme limit is unlikely to occur every-whcrc in any one year. During the period of the data, the only regions where the icc has ncver clcarcd the coasts arc in parts of the far nortli of the Canadian Arctic and Greenland.

The Advance of the Ice Edge in Winter

As winter becomes established over the Arctic regions the ice edge movcs southwards in all longitudes. The naturę of the icc edge advancc is determined to a large cxtcnt by the ocean current pattern. By latc Octobcr the Arctic Basin is usually complctcly covcred with sea icc while in Baflin Bay and off eastern Greenland the south-going currents (see Figurę 17.3) bring progressivcly coldcr water southwards. The atmospherc ovcr thcsc Iattcr regions is bccoming much colder because the prevailing winds blow from somc northerly point. Thcsc two factors result in the lowcring of the sea surfacc temperaturę which evcntually leads to the formation of ice. This eflect spreads a considcrablc distance southwards along the east coasts of Greenland and eastern Canada. By contrast, due to the prcvailing south-west winds and north-east-setting currents, the icc edge makes much slowcr southward progress in the Barents Sea.

A somewhat similar situation obtains in the Pacific sector where the icc edge advanccs further south to the west of longitudc i6o°e than it docs to the east of that meridian (see Figurę 18.1).

Distribution of Sea Ice in the Season when it is Most Extensive

At the time of its greatest cxtcnt which, in most regions and ycars, occurs in Fcbruary or March, the Barents Sca ice edge has spread south and east to cngulf all the coasts castwards of about longitudc 40°E. Svalbard is completely cncircled and the Greenland Sca ice edge has advanced southwards and south-westwards and cncloscs Jan \Iayen. It almost eloses the Dcnmark Strait and ultimately rounds Kap Farvel (see Figurę 18.1). Further west, the Baflin Bay ice edge has spread southwards along the coasts of Baflin Island and Labrador to reach Newfoundland, though the advancc in the east of Baflin Bay normally terminates at about the Arctic Circlc. The Canadian Arctic ice edge has spread south to covcr the wholc of Hudson Bay. Mcanwhilc, icc has formed over most of the Gulfof St Lawrcnce, the sea-way and the Great Lakcs. In the Furopcan sector the northern parts of the Baltic are affected by sea ice at this time as are the northern parts of the Black Sea.

In the Pacific scctor, by this time of greatest cxtcnt, the icc edge has advanced

DISTRIBUTION OF ICE AMD ICEBERGS    253

southwards through the Bering Strait to cover the northern portion of the Bering Sca (see Figurę 18.1). All but a smali south-eastern part of the Okhotsk Sea is ice-covcrcd and icc also affects parts of the Asian mainland coasts southwards to Po Hai. North of about latitude 46°3o'n, under avcrage condi-tions, all the Asian mainland coasts arc affected. South of this latitude, ice is largely rcstricted to the inner parts of the morę sheltercd bays and inlets, including Vladivostok, and parts of Po Hai as indicatcd in Figurę 18.1. The Gulf of Alaska remains ice-free except within the many inlets where ice is abundant in the inner parts.

The extreme maximum (1/10) lies well to the seaward of the mean maximum (4/10) in most places. In somc placcs, notably ofT Newfoundland, in parts of the Davis Strait, in the Barents Sea and near the Alcutian Islands the extreme limit (1/10) cxtcnds to some 250 to 300 n. mile bcyond the average (4/10) limit. In a severe winter some sheltercd parts of the eastern coast of USA are affected as far south as Cape Hatteras. The Baltic is another region where the extreme maximum limit lies a considerable distance from the mean maximuin (late winter) limit. In a severc season the whole Baltic region, including its approaches, is ice-covcrcd apart from an off-shore area in the Southern Baltic Sea (see Figurę 18.1). In such a season ice affects the Southern coasts ofNorway, the western coasts of Denmark and the coast of Holland. In fact, in a vcry severe season icc may form in sheltercd locations along the eastern and Southern coasts of Britain, along the north and west coasts of France and evcn in some northern parts of the Mediterranean Sca. The cxtrcmc limit shown in Figurę 18.1 is a composite derived from the worst conditions obscrvcd in any area in any year during the period of the data. As in the casc of the cxtrcme minimum limit (7/10), extreme maximum conditions are highly improbable in all areas in the same season.

Distribution of Fast Ice

During the winter some ice becomes firmly attachcd to the coast. The width of this fast-ice belt is greatly dependent upon the water dcpths. The outer limit often coincides with the 25-metrc contour so that, where the sea bottom shclves steeply, the fast-ice belt is vcry narrow or sometimes non-existent. Where the water is shallow for a considerable distance off shore the fast-ice belt may be extensivc as in the Laptcv Sea where it reaches from the Continental coast to a position just northwards of the Novosibirskiyc Ostrova.

The fast-ice limits are important as the recurring polynyas (see Chapter 17), which form thcrc duc to off-shore winds, have a considcrablc effect on the summer brcakup.

The Retreat of the Ice Edge in Summer

The crosion of the ice edge by mild southerly winds and the enlargcmcnt of recurring polynyas at the edge of the fast icc, together with the process of puddling, rcsult in the retreat of the icc in the summer months. Though this retreat occurs relatively slowly at first the ratę of melting is greatly accclcratcd in the warmer months of July and August. By about September the icc edge in the average year has retreated to the mean minimum position shown in Figurę 18.1.

M*