Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/827

 A T L A N T I 0 surfaces in summer causes the normal descending movement to be largely reinforced, hence the “ North Atlantic anticyclone ” is much larger, and its circulation more vigorous, in summer than in winter. Again, during the winter months pressure is relatively high over North America, Western Eurasia, and the Arctic regions; hence vast quantities of air are brought down to the surface, and Circulation must be kept up by ascending currents over the ocean. The Atlantic anticyclone is therefore at its weakest in winter, and on its polar side the polar eddy becomes a trough of low pressure, extending roughly from Labrador to Iceland and Jan Mayen, and traversed by a constant succession of cyclones. The net effect of the surrounding land is, in fact, to reverse the seasonal variations of the planetary circulation, but without destroying its type. In the intermediate belt between the two high-pressure areas the meteorological equator remains permanently north of the geographical equator, moving between it and about 11 ° N. lat. The wind circulation may shortly be described thus from north to south :■— 1. “Iceland” depression—low-pressure belt, traversed by numerous cyclones, some of great intensity, in winter; abundant rainfall. 2. A belt of “ westerly variables,” the polar side of the Atlantic anticyclone, merging into (1). 3. Calm and dry belt, the axis of the Atlantic anticyclone—the “ horse latitudes,” or “calms of Cancer.” 4. Belt of N.E. and E. winds. These coming from a high-pressure area are dry, hence there is little rainfall, and great evaporation from the sea surface, which attains its greatest salinity in this region ; the “ trade winds.” 5. Equatorial belt of calms and rains; the “ doldrums.” 6. Belt of S.E. and E. winds—the “ trade winds.” 7. Tropical calm and dry belt, or “ calms of Capricorn.” 8. Belt of W. and N.W. winds, merging into the “ roaring forties.” The part of this circulation which is steadiest in its action is the trade winds, and this is therefore the most effective in producing drift movement of the surface waters. The trade winds give rise, in the region most exposed to their influence, to two westwardmoving drifts—the equatorial currents, which are separated in parts of their course by currents moving in the opposite direction along the equatorial belt. These last may be of the nature of “ reaction ” currents ; they are collectively known as the equatorial counter-current. On reaching the South American coast, the southern equatorial current splits into Brazil two parts at Cape St Roque
 * one branch,

urre/i s. ^ current, is deflected southwards and follows the coast as a true stream current at least as far as the River Plate. The second branch proceeds northwestwards towards the West Indies, where it mingles with the waters of the northern equatorial; and the two drifts, blocked by the <3 -shape of the land, raise the level of the surface in the Gulf of Mexico, the Caribbean Sea, and in the whole area outside the West Indies. This congestion is relieved by what is probably the most rapid and most voluminous stream current in the world, the Gulf Stream, which runs along the coast of North America, separated from it by a narrow strip of cold water, the “ cold wall,” to a point off the south-east of Newfoundland. At this point the Gulf Stream water mixes with that from the Labrador current (see below), and a drift current eastwards is set up under the influence of the prevailing westerly winds: this is generally called the Gulf Stream drift. When the Gulf Stream drift approaches the eastern side of the Atlantic it splits into two parts, one going southwards .along the north-west coast of Africa, the Canaries current,

0 C E A N

771

and another turning northwards and passing to the west of the British Isles. Most of the Canaries current re-enters the' northern equatorial, but a certain proportion keeps to the African coast, unites with the equatorial return currents, and penetrates into the Gulf of Guinea. This last feature of the circulation is still somewhat obscure; it is probably to be accounted for by the fact that on this part of the coast the prevailing winds, although to a considerable extent monsoonal, are off-shore winds, blowing the surface waters out to sea, and the place of the water thus removed is filled up by water derived either from lower levels or from “ reaction ” currents. The movements of the northern branch of the Gulf Stream drift have been the object of more careful and more extended study than all the other currents of the ocean put together, except, perhaps, the Gulf Stream itself. The cruises of the Porcupine and Lightning, which led directly to the despatch of the Challenger expedition, were altogether within its “ sphere of influence ”; so also was the great Norwegian North Atlantic expedition. More recently, the area has been further explored by the German expedition in the ss. National, the Danish Ingolf expedition, and the minor expeditions of the Michael Sars, Jackal, Research, &c. Much has also been done by the discussion of observations made on board vessels belonging to the mercantile marine of various countries. It may now be taken as generally admitted that the current referred to breaks into three main branches. The first passes northwards, most of it between the Faroe and Shetland Islands, to the coast of Norway, and so on to the Arctic basin, which, as Nansen has shown, it fills to a great depth. The second, the Irminger stream, passes up the west side of Iceland ; and the third goes up the Greenland side of Davis Strait to Baffin Bay. These branches are separated from one another at the surface by currents moving southwards: one, the importance of which has only recently been recognized, to the east of Iceland; the second, the Greenland current, skirts the east coast of Greenland; and the third, the Labrador current already mentioned, follows the western side of Davis Strait. Pettersson has recently shown that these cold surface currents are chiefly due to the melting of sea ice, caused by the influx of immense quantities of warm water from the Gulf Stream drift, and this result has been confirmed by Dickson, who shows, from an investigation of the seasonal changes of the circulation, that a drift of the kind described is inadequate to account for all the facts. He maintains that, just as in the case of the equatorial currents, a heaping-up of water takes plaice off the south west coast of Europe and the north-west of Africa, and that this heaping-up takes place especially in summer, when the Atlantic anticyclone produces its maximum effect. The heaped-up water escapes by stream currents of the same nature as the Gulf Stream; and it is these currents, not the drifts, which are able to make their way into high latitudes, guided by the outline of the land. During winter, when the Atlantic anticyclone is at its weakest and the surface circulation is especially under the control of the cyclones of the Iceland depression, a general drift circulation is set up, large quantities of water move south-eastwards from Davis Strait, cross the Atlantic, and pass north-eastwards into the Norwegian Sea. The development of the equatorial and the Brazil currents in the South Atlantic has already been described. On the polar side of the high-pressure area a west wind drift is under the control of the “ roaring forties,” and on reaching South Africa part of this is deflected and sent northwards along the west coast as the cold Benguela current, which rejoins the equatorial. In the central parts of the two high-pressure areas there is practically no surface