256 257 (10)

256    METEOROLOGY FOR MARINERS

Distribution of Sea Ice at the Time of Greatest Extent

The Antarctic sea ice usually reaches its maximum extent in late September or early October when its northern limit extends as far north as 54°s in the Atlantic sector, to 56°s in the Indian Ocean sector and to 6o°s in the Pacific sector of the Southern Ocean (see Figurę 18.2). The width of the ice bclt surrounding the continent at this time, cxcluding the Drakę Passage ice belt, varies from about 1400 nautical miles in longitude 30°w, to about 500 nautical milcs in longitude 90°w. Almost all of this ice is pack ice drifting under the influence of wind and current but a smali proportion is fast to the coast.

Distribution of Fast Ice

The fast-icc belt around Antarctica is discontinuous and relatively narrow. Its cxact width is difficult to determine sińce its inner cdgc is oftcn fast to ice shelvcs which somctimes cxtend a considcrable distance off shore. The outer limit of the fast ice, howcvcr, approxiinates to the position of the 300-metre depth contour, within which vast numbers of iccbcrgs ground, thus forming anchoring points for the sea ice to becomc fast. The shapc of the fast-icc boundarics is thcreforc liablc to change in detail from time to time as the configuration of the ice shelves is altered by calving and as the distribution of groundcd bcrgs is altered as some bcrgs evcntually drift away.

Polynyas occur in places at the outer limit of the fast ice (or at the ice front, or coast whcrc there is no fast icc). They occur throughout the wintcr owing to the action of the off-shore winds which prevail around the coasts of Antarctica. Thesc off-shore winds are sometimes interrupted in places by northcrly winds associated with the deprcssions which frcqucntly affcct the pack-ice zonę. Where they occur, thesc northcrly winds tcmporarily prevent the formation of polynyas and closc thosc which havc already formcd. Thus at any one time there is no continuous ring of open water close in to the coast; there are instead a numbcr of unconnectcd polynyas surrounding the continent. The polynyas which have formed due to the off-shore winds become covercd ovcr with new and young icc whenevcr wind spccds become light. Subscąucnt strengthcning of the off-shore wind reopens the polynyas, rcsulting in the northward displace-mcnt of the pack ice, thus directly contributing to the northward advance of the outer ice cdge during the winter.

The Pattern of Breakup of the Sea Ice

The rcfreczing of the polynyas ceases when warming bcgins in the spring. Through the action of the off-shore winds and owing to the warming of the open water, the polynyas cxtend northwards and along the coasts, sometimes joining together to form vast, almost ice-free regions between the fast ice or coast and the off-lying pack icc. Meanwhilc melting is occurring at the outer edge of the pack ice as air and sea temperaturcs rise above freczing point.

The most spcctacular cxamplc of the Antarctic polynyas is that which forms off the Ross Sea ice barrier. During the 1969/70 season, which may not have bccn cxceptional, satellite pictures revealed that by late Novcmber a vast, almost icc-frce region existed between the meridians i6o°w and I70°E strctching from the ice barrier to 72°s on the 180° meridian and bcing narrower in the east than in the west. During Deccmber the polynya extcndcd northwards between longitudes 175°w and 175°E at the ratę of about 8 nautical milcs per day and by early January it had broken through to the open waters of the Southern Ocean.

257

DISTRIBUTION OF ICE AND ICEBERGS

A peculiarity of the breakup of the pack ice in the region east of the Weddell Sea is the mclting icc, leading to Iow concentrations of icc cover, which occurs over a large area centred around 65°s, io°e in early summer. This melting zonę later extends east-north-eastwards and west-south-westwards so that by late Dcccmbcr, in the ‘average’ year, it has rcached the open sea in about longitude 30°e and the Weddell Sea coast in about longitude 20°vv (see Figurę 18.2). The linc of this mclting zonę is morę or less coincident with the axis of a relatively warm sub-surface current which runs from the south Indian Ocean into the Weddell Sea. It is thought that this melting is due to the upwelling of the warmer sub-surface water which is possibly caused by the divcrgencc of the surface water associatcd with the depressions which rangę over the area. The tongue of ice which remains on the northern side of this melting zonę (see Figurę 18.2) is gradually croded, often bccoming detached into ice-ficlds before eventually disappearing.

A similar eflect takes place in the pack-ice region to the east of the Ross Sea. Lancs of broken icc conditions occur in spring and summer within a zonę centred roughly along 65°s, i25°vv to 75°s, i6o°w. At times these lanes may extend ovcr the whole length of this zonę whilc at other times they are discontinuous or even non-existent. In somc months the zonę may be displaced up to 100 nautical milcs north-west or south-cast of the linc dcscribed. In December 1971 the American Coast Guard Cutter Stalen Island entered the main pack-ice belt in the longitudcs of the Amundsen Sea and steamed south-west through a littlc over 1000 nautical miles of pack ice (mostly less than 5/10 concentration) to arrivc 6 days later in the open water of the Ross Sea polynya. The Stalen Island averaged 7 knots through the pack-ice belt.

Thus the greater part of the pack-ice zonę surrounding Antarctica is eroded at its northern and Southern boundaries and, in the area east of about 20°w, from within. The greater part of the melting occurs in November and December and then continues at a rcduced ratę until melting normally ceases in early March, by which time a very large proportion of the area affected by sea ice during the prcvious winter has become almost ice-frce.

ICEBERGS

Distribution of Arctic Icebergs

The icebergs of the Arctic region originatc almost entirely from the Greenland ice-cap which contains about 90% of the land icc of the northern hemisphere. Large numbers of icebergs are produced from the east coast glaciers, particularly in the region of Scoresby Sund and are carried south in the East Greenland Current. Most of thosc surviving this journey drift round Kap Farvel and mclt in the Davis Strait, but some follow south or south-east tracks from Kap Farvel, particularly in the winter half of the year, so that the maximum limit of icebergs (occurring in April in this region) lies over 400 nautical milcs south-east of Kap Farvel (see Figurę 18.1). Howevcr, a much larger crop of icebergs is derived from the glaciers which terminate in Baflin Bay. It has been estimated that morę than 40 000 bcrgs may be present in Baffin Bay at any one time. By far the greater number of bcrgs are locatcd close in to the Greenland coast between Disko Bugt and Mclville Bugt where most of the major parent glaciers are situated. This is clearly shown in Figurę 18.3 which displays the rcsults of an iceberg census taken in Baflin Bay during the autumn of 1970. Somc of this vast number of bcrgs become groundcd in the vicinity of their birthplace and never

M**