ATLANTIC OCEAN, the name given to the vast stretch of sea dividing the continents of Europe and Africa from the New World. The term is supposedly derived from Atlantis, presumed to be a submerged continent be low the present ocean.
For the Arctic basin we have F. Nansen's maximum sounding of 3,850m., and we know that at the North Pole Peary failed to reach bottom with a sounding of 2,743m. In the Norwegian sea there are many depths of more than 3,000m., but the Barents sea is fairly shallow (300-400m.) . The submarine ridge from East Greenland through Iceland and the Faeroes to North Scot land is such that a lowering of the water-level of 400m. would link Europe with Greenland. This ridge keeps the cold Polar bottom-water of the northern deeps away from the Atlantic. From this transverse ridge two branches stretch south-westwards, both probably of volcanic origin. The first projects from Ice land, the second, stretching from the Faeroes, includes the island of Rockall Sookm. west of the Hebrides. The grand banks of New foundland are less than aoom. below sea level and between them and Iceland is the so-called "telegraph plateau," the level of which varies between depths of soo and 4,000m. South of this plateau begins the dominant feature of the relief of the whole Atlantic, namely the central Atlantic rise. This keeps to the middle of the ocean as far south as Lat. 5o° S., parallel to main curvings of the continental coasts on both sides; it stretches through over boo° of latitude. On this rise, the depths of water are usually less than 3,000m. and often less than 2,000m. On it lie the Azores, St. Paul's island, Ascension, Tristan da Cunha and Bouvet Island, and at the last of these the rise bends east wards and goes to the Indian Ocean. On both sides of the rise are greater depths, of more than 5,000 or 6,000m. The greatest, those of over 6,000m. or approximately 3,00o fathoms, are called "Deeps" by Sir John Murray and he named them after famous marine explorers, e.g., Nares Deep, north of Haiti and Porto Rico, with a depth of 8,526m., only 15okm. from the coast, the deepest spot known in the Atlantic. More recently purely geographical names have been used, and we speak of a North American basin, a Brazilian basin and an Argentine basin. In the last, depths of 8,00em. have been found just North of the South Sandwich islands. East of the Central rise lie the deep Cape Verde de pression, the West African depression and the Cape depression, always deeper than 5,000m., often than 6,000m. Some ridges branching out from the central rise in the South Atlantic influ ence greatly the water movements and temperatures of the depths.
Such are, to the West the Rio Grande ridge (30° S.), to the East the Walfish ridge, stretching S.W.—N.E. from Tristan da Cunha S.) to the African coast near Walfish bay. On these two ridges depths of 900 or even only loom. on some local pinnacles have been found, and the Walfish ridge is three times as long as the European Alps, with heights 4,000m. above the depths. The Atlantic thus possesses submarine mountains, of course with slopes less sharp than those of terrestrial ones.
The origins and age of the Atlantic are unsettled. Neumayr (1885) thought that in Jurassic times there was a bridge-continent between North America and North Europe and one between South America and A.f rica, so that then the ocean lay only be tween the West Indies and south Europe. Wegener (192 2) on the other hand thinks the Atlantic was formed in the Cretaceous period through a fracture between the Old and the New World, fol lowed by the drift of America westward. J. Murray thought the Atlantic in the main extremely old, thus upholding a view of the permanence of this ocean basin.
Temperature decreases—save in the polar regions—from the surface to the depths ; but decreases in different places at different rates. In the inter-tropical regions at a depth of 400m. tempera tures as low as 10° C. or even 8° C. and less have been noted. In middle latitudes, e.g., in the Sargasso sea (q.v.) near the Bermuda islands the temperature may be as high as 17° C. at the same depth, and in latitude 30° S. we get C. and over at this depth. This striking fact can be explained only by supposing that in the equatorial region cold water rises nearly vertically from depths of 400-600m., whereas in middle latitudes warm surface water descends vertically to the depths.
In those parts of the Atlantic which are more than i,000m. deep the temperature of the water depends mainly on the latitude at least south of the Tropic of Cancer. The Atlantic off Europe, helped by warm and very salt Mediterranean water flowing out through the Strait of Gibraltar, even at these great depths is warmer than the deep water of the inter-tropical region. The bottom temperatures are C. in the North Atlantic, o.5° C. in the Brazilian depression, 2.2° in the West African depres sion, zero and —0.4° C. in high southern latitudes. Differences of bottom temperatures are explained mainly by the form of the floor; transverse ridges stop the flow of the coldest water from higher latitudes, and the bottom temperature on their equatorial side is thus higher. In a similar manner the Greenland-Iceland Shetland ridge with depth less than 500m. prevents the cold bot tom water (temperature may be as low as —1.2° C.) of the Arctic and the Norwegian sea from entering the North Atlantic. This fact further helps the temperature of the European Atlantic waters, which are therefore of exceptionally high temperature for their latitude at every depth.
Salinity.—The Atlantic on the surface is by far the saltest of the great oceans. Its saltest waters are found in the two trade wind belts, one extending east and west in the North Atlantic between 20° and 30° N. lat., and another of almost equal salinity extending eastwards from the coast of South America in r o° to S. lat. The average salinity in these two belts is In the equatorial region between these belts the salinity is markedly less, especially in the eastern part, where only is observed. North of the North Atlantic maximum the waters become steadily fresher as latitude increases until the channels opening into the Arctic basin are reached. In all of these, water of relatively high salinity usually appears for a long distance towards the north on the eastern side of the channel, while on the western side the water is comparatively fresh, especially in the vicinity of the Newfoundland Banks, but great variations occur at different seasons and in different years. In the higher latitudes of the South Atlantic the salinity diminishes steadily to 35%0 and tends to be uniform from east to west, except near the southern extremity of South America where the surface waters are very fresh (less than 34%o). In the true Polar sections of the Atlantic, i.e., the Weddell sea in the south and the Arctic basin in the north, the surface water is relatively poor in salt, 34-30%,,,, especially where icebergs melt or the great Siberian rivers bring fresh water to the sea. Though this fresh water is very cold, its lack of salt makes it so light that it remains on the surface. Under this, one finds, however, warmer but salter water, a condition very different from that obtaining in the open ocean. In the Atlantic depths away from the ice regions the salt content of the water and its temperature both diminish with increasing depths, though this is strictly true only in the northern hemisphere. In the southern Atlantic depths, salt content and temperature decrease only down to 800-1,000m. and then increase again slightly until a depth of 3,000m. is reached.
Circulation in the Atlantic Depths.—Modern chemical re search on salt content at various depths has become very im portant and the observations made by the "Challenger" are now better understood. Fig. 2 shows a section through the Atlantic about longitude 30° W. with indications of the slow movements of great masses of water determined by very small, but distinct differences of salinity. The surface currents, important for cli mate and shipping, of course only affect the uppermost layers at the very most down to 3oom., and are due to the winds ; they have little relation to questions of salinity. In higher latitudes, at the ice-limit between the Falklands and Bouvet island, cold water poor in salt sinks to Soo-1,0°0m. depth and moves thence northward as an Antarctic under-current (No. I. in fig. 2) beyond the equator to at least 20° N. In the Sargasso sea, on the other hand, warmer and very salt water sinks and moves, as an under current at a depth of 1,500 to 3,000m., to the Antarctic zone; this is the North Atlantic under-current (No. II. in fig. 2). Affecting almost all the greater depths, we have a South Atlantic bottom current (No. III. in fig. 2) uniting with a weaker North Atlantic bottom current (No. IV. in fig. 2). There are thus in the Atlantic depths great horizontal movements, just as in the atmosphere there are layers with different characters and move ments (trades, westerlies, etc.). But, whereas the equatorial belt divides the atmospheric circulatory system on its two sides, there are immense exchanges of water in the depths right across the equator.
Meteorology.—The difference between water-temperature and air-temperature just over the water is usually I° C. or less in equatorial and temperate latitudes. Only in the region of the Gulf Stream near the American coast and towards the Polar regions is the air as much as 4°-6° C., colder than the water. North-westerly winter storms drive cold air (0°-5° C.) from North America eastwards into the Atlantic into regions where the water temperature may be C. On the other hand, on the Newfoundland banks with icebergs and cold water one finds in spring and summer that, if the wind is from the south, the air may be C. warmer than the water, and this is a main factor of the dangerous fogs there. On the whole, however, air tempera tures over the Atlantic are distributed very much as are water temperatures at the surface. The wind circulation is much simpler than in the Indian ocean and is symmetrical about the meteorological equator which lies between lat. 2° N. and i o° N. Thus the two hemispheres have very similar and independent cir culatory systems of air, though there are differences. The circu lation in the South Atlantic is nearer than that of the North to a theoretical standard for a simple globe, as it is less influenced by land masses. Between latitudes and 30° S. is a region of maximum pressure (anticyclone) and atmospheric pressure diminishes slowly northward and sharply southward. On the north side, the S.E. trade blows weakly in the southern summer but in the southern winter strongly, especially on the coast of North Brazil. On the African side, the S.E. trade always reaches beyond the equator and contributes to the rainy S.W. monsoon of the Gulf of Guinea (Cape Palmas to Gaboon) . From the high pressure region towards the south there blow the N.W.-W. winds over a belt reaching from lat. 35° to 6o° or 65° S. and they are felt around Cape Horn and Cape of Good Hope. They have been called "the roaring forties" because formerly ships sailing to India and Australia in lat. 40° to lat. 50° S. utilized these strong winds; storms are frequent here.
In the North Atlantic also there is a region of high pressure (anticyclone) across the ocean between lat. 20° and 30° N. The highest pressures are noted south of the Azores and west of Madeira. The northeast trade blowing thence southward is stronger in summer than in winter, especially near the Canaries and the Antilles; at this season it is felt as far north as the latitude of Gibraltar. On the north side of the high pressure region the conditions over the American continent cause considerable sea sonal variations in the westerly winds. These blow relatively weakly towards North Europe in summer, but in winter, when North America has high pressure conditions, the barometric gradient towards the Atlantic is steep and the space between Labrador, Iceland, Jan Mayen and the North Cape is the great highway of a procession of cyclonic storms from the west ; cyclonic storms also pass more or less parallel with them over the British isles towards the east.
Surface Currents.—In the "west wind zone," though this is the main direction, air movements are very varied as they are mostly movements in spirals of small diameter. The actual transport of air through space in these zones is theref ore much less than in the zone of the almost constant if often more moderate trade wind. It thence follows that the trade winds have a far greater influence on the movement of surface water than have the wester lies of higher latitudes. The two Atlantic trade winds give rise to two great equatorial westward currents. On account of the earth's rotation the northern equatorial current turns into a N.W. direction, the southern into a S.W. one. As, further, the ocean ographical and the meteorological equator lie north of the geo graphical equator, through the year a good deal of the water of the southern equatorial current passes north of Cape San Roque (N. Brazil) into the northern hemisphere and so reaches the north equatorial current on the coasts of Guiana and in the West Indies, and this current is thus much stronger than the south equatorial current on the coast of S. Brazil. Almost all water that streams westwards between latitudes 5° S. and 20° N. is eventually banked up in the Caribbean sea and just north of the Great Antilles with the result that there is a northern out-flow through the strait between Florida and the Bahamas. This is the origin of the Gulf Stream, though only a small fraction of the water of it comes from the Gulf of Mexico. This is one of the most important currents of the world, but it is markedly warm and rapid only in the uppermost layers. Even in Florida strait at a depth of Zoom. the temperature is only io° to 18° C. and the speed o.4–o.8m. per second as contrasted with over 25° C. and 1.2 to 1.7m. per second at the surface. Generally speaking the surface currents of the Atlantic and of all seas affect the sea to a depth less than 400-500m.
The Gulf Stream is separated by cold water (the cold wall) from the east coast of U.S.A. as it flows northward to a region south of the Newfoundland banks. Here it mixes with the cold water of the Labrador current that comes from the west side of Baffin Bay; the interaction of these two currents causes powerful whirls near the southeast tail of the Newfoundland banks. Hence forth the Gulf Stream is so indefinite as to vary with the winds, and it is customary to speak of a Gulf Stream drift or an Atlantic drift, which moves towards and along the coast of Europe as far north as Norway and Spitzbergen. Here the sea temperatures on the S.W. coast remain above o° C., and the sea water may have more than of salts. In the Arctic this water sinks into the depths. A branch of the Gulf Stream drift, called the Irminger current, keeps the south and west coasts of Iceland free from ice. A third branch goes further north-westward and northward to the west coast of Greenland which is thus ice free for a large part of the year. Another branch reaches around Scotland into the North sea giving a mild and rainy climate. Not all the Gulf Stream drift goes north of lat. 50° N. An important part is de flected increasingly to the right near the Azores and forms a weak and variable Canaries current, which makes a small con tribution to the northern equatorial current in the vicinity of the Cape Verde Islands. In the west centre of the mid-north At lantic towards the Bermudas occurs the Sargasso sea with enormous masses of floating golden-yellow Gulf-weed (Sargassum bacci f erurn) ; this weed does not hinder ship movements. It is now thought that this Gulfweed is a really pelagic plant and that only small amounts are torn from the coasts of the Antilles. Finally, in the north hemisphere, we must mention the Guinea current which can be discerned off the Liberia coast going south eastwards but becomes far more marked from Cape Palmas east ward to the Gulf of the Cameroons. This is partly a re-action cur rent compensating for the water driven west by the south equatorial current, but it is partly due to the south-west mon soon of this coast.
In the South Atlantic the Brazil current and South Equatorial current have been mentioned. The Benguella current which con tributes to the south equatorial current can be traced over a broad zone west of South Africa. In its path lie St. Helena and Ascen sion and it is composed of rather cool water flowing rapidly; this is specially marked near the coast of South West Africa. In the enormous stretches between lats. 3o° and 6o° S., under the in fluence of variable west winds, the surface current almost everywhere tends E.N.E. or N.E. (not E.S.E. or S.E. as we might perhaps expect from analogy with the Gulf Stream) ; this fact is of enormous climatic importance, for cold water is thus brought to the neighbourhood of the Falkland islands, South Georgia, etc., and the eastern South Atlantic thus shows a large negative thermal anomaly being relatively 2 °-5 ° C. too cold, whereas the eastern North Atlantic is abnormally warm.
Ice.—Pack ice from frozen sea water, and icebergs as frag ments of the Antarctic ice-sheet and glaciers come especially from the Gulf between Graham Land and Coats Land, i.e., from the Weddell sea which stretches between lat. 6o° S. and 78° S. The famous drift of Shackleton on the "Endurance" (1915) shows that there the ice moves clockwise and to the N.E. and attains open water near the South Orkneys and South Georgia. The average limit of large ice masses is thus near the South Shetlands in the west and near Bouvet island in the east. In some years one finds ice near Cape Horn and even north of the Falklands towards lat. 4o° S. in such quantity as to be dangerous to ships. Such years were 1891-1894, and 1906. This ice moves in the so called Falkland current which sends cold water northwards parallel to the Patagonian shelf, and may be compared with the Labrador current ; it is probable that this Falkland ice comes from the southernmost Pacific. In the North Atlantic the occurrence of ice-fields and icebergs near the Newfoundland banks (about lat. 45° N.) is very important as this is near the most frequented of all ocean routes. It is the Labrador current that brings the ice southward, and the icebergs, especially, move along over the deep water outside, i.e., east of the banks; thereafter they may swirl round for weeks between the Gulf Stream and the Labrador current. This ice never reaches the coasts of Nova Scotia or U.S.A. and any ice sighted there comes from the Gulf of St. Lawrence. The ice of the Labrador current is seen between Feb ruary and August and reaches farthest south (lat. 4 2 ° N. 43 ° N.) in May and June. From September to January the great seaway is practically free from ice. In some years the ice may reach as far south as lat. 4o° N. or even 38° and the most dangerous zone is between long. 45° and so° W. Collisions with icebergs have led to the southward deflection of the ocean route during the ice-season and the presence of Newfoundland fogs in the same months has contributed to the same end. A vessel of the U.S. coast-guard patrols the dangerous zone with signal apparatus during the critical periods. Polar ice is very important on the whole of the east side of Greenland, moving along the cold East Greenland current from the Arctic basin. The east coast of Greenland therefore can rarely be reached without difficulty and it is almost uninhabited. The north and east coasts of Iceland are also blockaded for a large part of the year by ice that comes from the north and fills the bays. In the north of the Norwegian sea the boundary of ice-floes stretches past Jan Mayen to the region west of Spitzbergen ; in the Barents Sea it still remains far to the north of Norway and of the Russian coast.
vertical tidal differences and the direction of the tidal currents near the Atlantic coasts and harbours are well known, but conditions over the open ocean are still very uncer tain. The reasons for this are that one needs observations from a ship anchored over a period of 121 to 25 hours and that we still lack instruments for measuring vertical changes in the conditions of the water in the open ocean. Probably the vertical tidal differ ence in the open Atlantic has a maximum of o.5-i.om., as these values have been fixed for oceanic islands with sharp slopes down to the depths—Bermudas i.om., Ascension o.6m., St. Helena o.gm. The tidal difference seems to be less between the tropics than in temperate and high latitudes. If the tidal waves from the great southern ocean between Cape horn and the Cape of Good Hope are continued northward in the Atlantic deflection due to the earth's rotation, to the left in southern latitudes, to the right in northern ones, must strengthen the tides on the coasts of South Brazil and the Argentine and on the coasts of Europe.
Large tidal differences have been observed in the harbours of Patagonia, moderate ones in the harbours of South and West Africa, large ones in the Spanish, French and West British har bours; they are quite small in the harbours of U.S.A. The funnel shaped Bay of Fundy, Bristol Channel, Gulf of St. Malo all con centrate tides and give a local vertical tidal difference of more than iom.
Apparently the tidal currents are not restricted to coastal waters though as yet we have only a small number of observa tions, e.g., south of the Azores, north of the Cape Verde Islands, the tidal current changes its direction from hour to hour and affects water to a great depth. Apparently this must be the same everywhere, as ebb and flow of the tide are cosmic phenomena.
(G. Sc.)