254 255 (11)

254

METEOROLOGY FOR MARINERS SEA ICE IN THE SOUTHERN HEMISPHERE

Though sca icc extends a considerablc distancc from thc Antarctic coastline in winter to cover a widc bclt of thc Southern Ocean, most of this ice melts during the following summer, so that many parts of thc coastline are accessible to shipping from Decembcr to March in an average year.

Figurę 18.2 displays the mean limits of sca icc (about 5/10 concentration) at the timc of least extent (usually Fcbruary/March) and greatest extent (usually Scptcmbcr/October). It also includcs thc icc cdge for 12 December 1969, as bcing rcprcscntativc of the ‘mid-season’ conditions, in order to help thc reader to understand thc complex naturę of the breakup of sca icc which is latcr described. Of these limits, the mean at thc timc of least extent, and the actual

limit for 12 Decembcr 1969, are based largcly on satcllite pictures for four ycars betwecn 1969 and 1973. The mean at thc timc of greatest extent is based on a longer period of carlier data. It should be noted that the mean limit at the time of least extent is appreciably difTcrcnt from that shown in previous publications.

The data on which the earlier position was based is known to have been in-adcquate and the newer limit can be regarded with morę confidence as representing the avcrage for the years 1969-73. What must rcmain in some doubt for the time being is the extent to which this mean can be taken as representing thc long-term average. As morę satellite data become available a morę representative picture of the mean minimum conditions will be revealed; mcanwhile thc minimum limit shown in Figurę 18.2 should be interpreted with caution.

The available data are insufficient to define extreme maximum and minimum limits.

Distribution of Sea Ice at the Time of Least Extent

The mean limit of pack ice (about 5/10 concentration) at the timc of least extent (usually Fcbruary or March) is shown in Figurę 18.2. It can be seen that there are two main regions where considerable quantities of sea icc survive thc summer melt. One is located in thc Wcddelt Sea where a vast area of pack ice extends eastwards from the Antarctic Peninsula as far east as about longitude 35°w at latitude 74°s, though normally a wide lead occurs on the Southern side of this pack-ice zonę reaching westwards to about longitude 45°w. The persis-tence of pack ice in this region is thought to be chiefly due to south-easterly winds causing the Weddcll Sea pack icc to pile up against the Antarctic Peninsula. The other main ice region lies to the west of the Antarctic Peninsula bctwcen thc meridians of 80°w and I75°w. The action of currents and thc absence of off-shore winds combine to prevcnt the clearance of pack ice from this region. Most parts of the coast within thc Indian Ocean scctor rcmain afTected by sea ice throughout thc summer (see Figurę 18.2). Since the outer limits of the pack ice in this scctor approximate to the position of the 300-metre depth contour it may be that thc prcsence of numerous grounded bergs obstructs the clearance of sca ice in this region. In all Coastal regions sca ice may pile up on the castern sides of land-icc tongues owing to thc west-going currents which predominate along most parts of the coast.

It is to be expected that ice conditions at this timc of least extent will show considerable variability but the limits of this variability are not yet known. In a very light ice year thc lead on the Southern side of the Weddcll Sea may exceed 100 n. mile in width and reach westwards to longitude 60°w, and the Bellingshauscn Sea may be almost icc-free westwards to about longitude 90°w. In a bad ice year, at this time of least extent, the Weddcll Sea may remain largely ice-covcred and parts of its castward-reaching ice tongue will survive through the summer, perhaps as far east as the Grcenwich meridian, while the Bellingshausen Sca rcmains ice-covercd.

The Advance of the Ice Edge in Winter

By mid March new ice has usually formed around the coast, indicating the onset of the new freezing scason. The advance of the ice edge seems to occur at a somewhat slower ratę than its retreat in summer owing to the Iongcr winter season (March to September). It may well be that the Wcddell Sea pack-ice tongue advances across the Grcenwich meridian, between latitudes 60 °s and 65°s before the region immediately on its Southern flank freezes over latcr in the winter. The advancc of the ice edge continucs until late September or early October when it rcaches its maximum extent.

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