250 251 (12)

CHAPTER l8

THE DISTRIBUTION OF SEA ICE AND ICEBERGS Introduction

Though obscrvations of icc in thc polar regions have been collcctcd ovcr scvcral ccnturies, tlie data havc bccn fragmcntary and conscqucntly insufTicient to dcfine with any accuracy the ice limits in either hemisphcre. Rcports from vessels might dctcrmine the position of the ice edgc in a particular place and timc but such rcports wcrc widely scattercd and interpolation to fili the gaps bctween them was, of necessity, largcly a matter of gucss-work. Even with thc introduction of routinc acrial reconnaissancc in the post-war years in the Arctic, and the dramatic incrcase in the rcports from Antarctica during and sincc the International Gcophysical Year (1957-58), thcrc still rcmained largc arcas whercin the data were inadeąuatc to specify thc icc conditions.

The introduction of meteorological satcllitcs in near-polar orbits has com-plctcly altered thc availability of data, at least on the hemisphcrical scalc, so that a much clcarcr understanding of ice conditions around botli polar regions is rapidly unfolding. The sca-icc discussion which follows is based to a very large extcnt on satellitc data. For morę detailed information on sca-ice conditions in any limited region the reader is referred to thc appropriatc Admirally Pilot.

The distribution and scasonai variation of sca ice differ grcatly from the one polar region to the other. In thc Arctic, sea ice covcrs thc greater part of thc Arctic Ocean throughout the year and spreads southwards, sometimes a considcrablc distance, into the adjaccnt sea arcas in the winter scason. The averagc rcduction from winter to summer in thc arca affected by sea icc is about 25%. In thc Antarctic, sea icc occurs in a belt around thc continent. Hcre, in spite of thc fact that thc arca covcrcd by sca ice (in the Southern winter) is half as much again as the arca covered by the Arctic sca icc in thc northern winter, thc averagc reduction from winter to summer in the arca affected by sca icc is about 85%. This is partly explained by the fact that thc Antarctic sea icc occurs in lowcr latitudcs than its Arctic countcrpart.

The long-tcrm variations in mean sca-icc conditions on a hemisphcrical, or global, scalę are extremcly difficult to assess at this stage. Severc ice conditions in one region of either hemisphcre arc usually compcnsated by light conditions in another. For cxample, a ncw extrcmc maximum pack-icc limit was estab-lishcd in the Barents Sca in latc winter 1966. In the same season very light sea-ice conditions prcvailed over thc Gulf of St Lawrence and off the Labrador coast. Only when many years of satellite data havc bccn accumulatcd will it be possible to determine thc long-tcrm variations in mean icc conditions.

SEA ICE IN THE NORTHERN HEMISPHERE

Figurę 18.1 shows thc position of thc cxtrcmc maximum and minimum ice edges during the period 1962-74. The conccntrations used to dcfine these edges are, rcspectively, 1/10 conccntration (i.c. the ratio of thc arca covcrcd by ice to the total arca of ice plus water, reckoned over a smali arca, is 1/10)

DISTRIBUTION OF ICE AND ICEBERGS

and 7/10. The outer of these limits givcs an idea of thc extent of thc region within which thcrc is sonie risk of encountcring sca icc at thc worst limę of thc year, and the inner limit indicatcs thc region within which sca icc can bc cxpcctcd to be a serious hindrancc to convcntional shipping, even at thc most favourable time of thc year. Also shown in thc figurc arc thc average positions of thc icc edge (,j/10 conccntration) at thc times of its greatest (usually about Kebruary March and least (usually about August,September cxtcnt. Except for thc arca around Japan and North China, these avcragc positions arc based upon thc monthly sca-icc charts published by thc Mcteorological OHicc and cover thc period 1966 74. It will be appreciatc 1 that these limits. particularly the cxtrcmes, may need to bc modified when a longcr period of obscrvations becomes availablc.

Distribution of Pack Ice in the Season when it is Least Extensive

Sca icc within thc Arctic region rcccdcs to its least extcnt in about August or September in an avcragc year. Though pack icc covers the greater part of thc Arctic Basin at this tiinc, thc pcripheral coastlines arc mostly icc-frcc cxcept for thc north and east coasts of Grcenland, tlić northern parts of thc Canadian Arctic Archipelago and parts of Zernlya Frantsa Iosifa and Sevcrnaya Zemlya.

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