236 237 (13)

METEOROLOGY FOR MARINERS

236

In thc casc of some of thc cold currents the Iow temperaturę of the surface water is not simply duc to advcction from lower latitudcs. In the Benguela currcnt for examplc the Iow temperatures are largely duc to the upwclling of sub-surfacc water as prcviously dcscribcd.

Seasonal and Monsoon Currents

In many regions thc avcragc current expericnccd varies according to the seasons. Where monthly data are availablc many varicties of seasonal variation may be dctected. One of the most familiar patterns is that in which there is relatively little change for several months in the winter followcd, after a brief transition in spring, by a period of scveral summer months wherein the current flow differs markedly from that of the winter period. A brief transition period in autumn leads back to the winter conditions.

In some regions three main seasons may be dctected, while in others there would seem to be a fairly steady cycle of dcvelopment throughout thc months of the year.

The most striking seasonal variation is that induccd by thc Asian monsoons. The great annual pressure oscillation which replaces the summer Iow pressure over Southern Asia by the winter high pressure ovcr central Asia produces an almost complete seasonal reversal of the prcvailing winds over the northern parts of the Indian Ocean and ofT the Pacific coasts of Asia. In accord with these wind changcs the principal currents in these arcas undergo a seasonal reversal. Thus, in the Indian Ocean, in the northern winter, the currents betwecn the equator and latitude I5°N set in a wcsterly direction in accord with thc north-east monsoon. In the northern summer, however, during the Southwest monsoon the currents of this region are revcrsed and set in castcrly dircc-tions. This revcrsal is the morę notable because the mean speed is comparatively high (about 1-1} kn) in both directions. Off much of thc Pacific coast of Asia, also, there is a complete rcvcrsal of current from winter to summer. Thus, off the south-east of Victnam, the averagc current is south-wcstcrly (1-1$ kn) in winter and north-castcrly (about 1 kn) in summer.

Annual and Secular Variation

It is highly probable that thc average current in a given month and position varics from one year to another. In generał, however, the number of observa-tions availablc cach year in a given location, even considering a onc-degree square, is insufiicient to determine the variation satisfactorily. There may well be variation also on a longcr-term scalę, for example when comparing one thirty-year period with another. Herc again any real variation is likely to be obscured by thc paucity of observations, cspecially in earlier periods. Bccause of thc high variability of currents and the inherent difficulty of mcasuring them accuratcly, there are barcly sufficicnt obscrvations in many arcas to determine the current regime even using all the observations cxtending ovcr a period of morę than 100 ycars. In these circumstances it is doubtful if comparisons ovcr shorter periods can bc regarded as mcaningful.

PART VI. ICE AND EXCHANGE OF ENERGY

BETWEEN SEA AND ATMOSPHERE

CHAPTER 17

THE FORMATION AND MOVEMENT OF SEA ICE AND ICEBERGS

Introduction

Ice is found at high latitudes in both hcmisphercs but bccausc of their physical dissimilaritics the climatic and ice regimes of the Arctic and Antarctic regions diffcr grcatly. The Arctic Basin is an arca of ocean about 3000 metres dccp which is covercd by a thin shell of ice about 3-5 metres thick. Antarctica, similar in area, is a continent covercd by an ice cap which is up to 3000 metres thick. The annual mcan temperaturę at the South Pole is ~49°c (the lowest temperaturę yet recorded in Antarctica is -88-3°c), whereas at thc North Pole thc annual mean temperaturę is estimated to be - 20^cC (the lowest temperaturę yet recorded in the Arctic Basin is only a little bclow - 50°c).

The ice cap covcring the Antarctic continent accounts for morę than 90% of thc earth’s permanent ice. The ice constituting thc ice cap is constantly moving outwards towards thc coasts where many thousands of icebergs are calvcd cach year from the glaciers and ice shelvcs which rcach out over thc sca. As a conscqucnce, large numbers of icebergs are to be found in a wide belt which complctcly surrounds thc continent. In contrast, the icebergs of the Arctic region are almost entircly confined to the sea areas off thc cast and west coasts of Greenland and off the eastern seaboard of Canada. The Arctic Basin remains almost completely covered by pack ice throughout the year whereas thc greater part of thc pack ice surrounding Antarctica melts cach summer. This is because thc Antarctic pack ice is locatcd in lower latitudes than its Arctic countcrpart.

Sea ice is a complex substance varying in shape, sizc, age, thickness and many other characteristics so that any dcscription must use terms which are readily understood and acccptcd by thc rcader. The terms uscd in thc following descrip-tion conform to the Sea Ice Aomenclature published by thc World Meteorological Organization in 1970 (incorporating amendments up to 1975). This nomen-claturc is also published in the Marinę Observer's Handbook and in The Marineis Handbook, N.P. 100.

Classification of Ice

The many forms of ice which may be encountcrcd at sca are convenicntly classificd under threc main headings. Of these the most important is called sea ice, which is formed by thc freezing of sea water. The other major categories are icebergs and river ice, which are derived from fresh water. Icebergs are a scrious hazard to navigation around Antarctica and also in some parts of the northern hemisphere, notably thc eastern seaboard of Canada. Thcy will be described later. River ice is sometimes encountcrcd in harbours and off shorc elose to estuaries during the spring breakup. As it is then in a State of dccay it gcncrally presents only a temporary hindrancc to shipping. The description which now follows is confined to sea ice.

237