Gulf Stream and Oceanic Currents. The Gulf Stream is the best known, the best defined, and the most remarkable of all the ocean currents (see map at ATLANTIC). It derives its name from the Gulf of Mexico, out of which, as a great current of warm water, it flows through the Strait of Florida, along the eastern coast of the United States of America, and is then deflected near the banks of Newfoundland diagonally across the Atlantic. This great body of warm water indirectly modifies the climate of western Europe, and it is possible to trace its effects as far as the coasts of Spitzbergen and Nova Zembla. It is essential in describing the Gulf Stream to take into consideration the general question of oceanic circulation, and the thermal conditions of the ocean made known from the explorations of the Challenger, Blake, and other recent expeditions.
The prevailing winds of the globe are determined by the distribution of atmospheric pressure, and the position of barometric maxima and minima are in turn determined by the distribution of land-masses and water-surfaces. The wind blows out of and around high-pressure or anticyclonic areas, and into and around low pressure or cyclonic areas (see ATLANTIC). By comparing the maps of the prevailing winds with those of the oceanic currents, it will be seen that the latter roughly coincide with the winds blowing out of and around the high-pressure areas in the Atlantic and Pacific oceans. There have been many theories to account for oceanic circulation, but recent researches show that all the principal surface currents have their origin in, and are maintained by, the action of the prevailing winds of the globe, modified locally by variations in temperature, density, evaporation, gravity, and rotation of the earth.
The phenomenon of oceanic circulation is to be seen in its simplest form in the westerly wind-driven currents of the trade-wind regions of the Atlantic and Pacific. The heated surface waters of the tropics are there driven to the west, and banked up towards the eastern shores of America, Africa, Asia, and Australia. On the other hand, the cold deep water is drawn up along the western shores of America and Africa to take the place of the surface water driven before the trade-winds. The temperatures of the water towards the western parts of the oceans are thus higher and more uniform to considerable depths below the surface than in the eastern, where they are lower and have a wider range at different seasons or different states of the wind. The writer has even measured this effect of the wind in Loch Ness in Scotland; he found the cold water from the bottom drawn to the surface in the course of a few hours, and the warm surface water banked up to the north end of the loch, during a southerly gale. Recent observations show that similar effects are produced in the great oceans during a continuance of winds off shore.
In the Atlantic a large proportion of the waters of the equatorial current are forced into the Caribbean Sea through the passages in the Windward Islands, and then into the Gulf of Mexico, principally through the Yucatan Channel. The amount of accumulation or heaping up of water in the Gulf of Mexico, through the action of the trade-winds, has been measured by the officers of the United States Coast Survey; it has been found that the Atlantic Ocean at Sandy Hook is 3 feet 4 inches lower than the waters of the Gulf of Mexico at the mouth of the Mississippi. This is partly the origin of the force constantly at work to keep up the flow of the Gulf Stream through the Strait of Florida. The stream as it flows through the narrowest part of the strait is 50 miles wide, and has an average depth of 350 fathoms. In the axis of the stream the velocity is four or five knots an hour, two miles an hour or even less along the edges, and probably the same near the bottom. It is estimated to be 150 miles wide off Charleston, and 300 miles wide off Sandy Hook; it then spreads fan-like over the surface of the North Atlantic. Off Cape Hatteras the velocity is about three miles an hour, off the banks of Newfoundland one and a half mile an hour, then the rate slowly merges into that of the north-easterly drift of the Atlantic—four or five miles a day. The mean surface temperature in the straits is F.; off Sandy Hook, F. The average bottom temperature in the strait at 400 fathoms is but F.; off Charleston, at 300 fathoms, F.; and off Sandy Hook, in 200 fathoms, F. The bottom in the strait, and for some distance north on the 'Blake plateau,' appears to be swept by the current so that no fine ooze is allowed to form; but the bottom, where hard, is made up of the remains of surface and bottom-living organisms, often cemented together into nodules and phosphatic concretions. The diagram in the article ATLANTIC shows the distribution of temperature across the Gulf Stream between New York and Bermuda. The Gulf Stream water of the North Atlantic is carried towards the coasts of Europe by the south-west winds; one branch passes on to the coasts of Norway, and another south to the coasts of Spain and Africa. As this water is carried into colder latitudes it sinks on becoming cooled because of its greater density, so that off the coasts of Britain warmer water is found at a depth of three-fourths of a mile than at a like depth off the tropical coast of Africa, where the winds are off shore. While a warm current passes to the Arctic Ocean along the coasts of Norway and Lapland, a cold current from the Arctic comes down the coasts of Greenland, and along the coasts of Labrador and the United States, inside the Gulf Stream, and ultimately sinks beneath it into the deeper parts of the North Atlantic basin. The passage from the green, cold, turbid waters along the American coast into the deep blue, warm waters of the Gulf Stream is sometimes sudden and well marked, and is usually observed by all who sail from the shore seaward.
The winds blow out of and around an area situated in the North Atlantic, between the north of Africa and America, and the surface currents of water also circulate around this area, which is known as the Sargasso Sea. Here are found immense banks of floating Gulfweed (q.v.), covered with peculiar species of animals: Crustaceans, Polyzoa, Annelids, Molluscs, Hydroids, and Fishes, all the same colour as the weed, presenting remarkable examples of protective resemblance.
A very similar, but not such a well-defined or constant stream as the Atlantic one, is found in the North Pacific, and is known as the Kuro Siwo ('Black Stream') or Japan Stream. On approaching Japan in April from the south the Challenger found a belt of water running to the eastward at the rate of three miles per hour. In this stream the temperature changed from to F. suddenly several times without any alteration in the rate of the current. In June no current was found 30 to 40 miles from the coast, but close to the south coast of the main island there was a northward current of two miles per hour and a mean temperature of F. Alternating bands of cold and warm water were also found by the Challenger in the Gulf Stream near its shore edge. The origin of these alternate belts of water in the Japan Stream is probably due to the monsoons. The northern equatorial current striking against the eastern side of the Philippine Islands is, as is well known, diverted to the northward, along the eastern side of Formosa, after passing which it appears gradually to lose its distinctive character. During the north-east monsoon a cold surface current is running to the southward from the Japan and Yellow seas. It appears therefore highly probable that the equatorial current, instead of losing itself as is supposed, when it meets with the cold water from the Japan and Yellow seas, is diverted to the eastward along with a cold northerly current, the two running together side by side without intermingling their waters. When the north-east monsoon ceases the current from the Japan and Yellow seas also ceases, which causes the slackness of the Kuro Siwo, south of the main island, in June, as it is then only due to the equatorial current. Later on, in July and August, when it is further augmented by the surface drift from the China Sea in the south-west monsoon, it runs again with great rapidity, and is wholly a warm current. These peculiar effects are probably not experienced to the eastward of the meridian of E.; there apparently the stream is always a warm one. The current that runs from the Arctic Ocean through Behring Strait is insignificant compared with the Arctic currents of the Atlantic. There is an ill-defined Sargasso Sea in the North Pacific, in some respects resembling that of the North Atlantic. The surface currents in the Indian Ocean are, as is well known, changed with the shifting of the monsoons.
From the Challenger observations it appears to be proved that the dense warm equatorial waters which pass along the eastern shores of South America, Africa, and Australia into the Great Southern Ocean that surrounds the world in latitudes beyond S., become cooled in these latitudes, and sink to the bottom through the other waters on account of their greater density. This water is then drawn slowly north as a great indraught to supply the loss by surface currents and evaporation in the equatorial regions of the Atlantic, Pacific, and Indian oceans. It appears then that by far the larger part of the cold water that fills these great ocean-basins is cooled and sinks to the bottom in about S. lat. A portion of this water seems also to be drawn southward to supply the place of the relatively light though cold surface currents that flow north from the Antarctic in the regions of floating ice. The great bulk of the ocean has a low temperature—below F.; it is ice cold in the Atlantic at the bottom even under the equator. The warm surface water is a relatively thin film, but this film is much deeper towards the western parts of the oceans in the tropics than in the eastern. On the other hand, in the regions of the westerly winds of temperate latitudes the layer of warm water is deeper in the eastern parts of the oceans, as has been already noticed in referring to the deep-water temperatures off Britain and tropical Africa. These facts are clearly shown on the Challenger maps, showing the distribution of temperature at 10, 20, 50, 60, 100, and 300 fathoms.
The surface currents may, as we have seen, have a considerable velocity, but there is no evidence that any such currents exist in the deeper waters at the bottom of the ocean; the movements there must be slow and massive. It is true that between oceanic islands and in positions like the Wyville-Thomson Ridge, between Scotland and the Faroe Islands, where the tidal wave is confined, the ridges are swept by currents at a great depth; but these are exceptional cases. In the open ocean the temperature decreases with increase of depth, except in the Arctic or Antarctic, where there is melting ice on the surface. In enclosed seas, like the Mediterranean, Caribbean Sea, Gulf of Mexico, Sulu Sea, and many others, there is a large body of water at the bottom of a nearly uniform temperature; the depth at which this uniform temperature is reached depends on the height of the ridges cutting enclosed seas off from general ocean circulation. The deeper water in these can only be renewed by vertical currents set in motion by the winds or by convection currents. The direct influence of ocean currents on climate is undoubtedly great, but this influence is most marked by the indirect effects of the prevailing winds blowing from off these currents towards the land, carrying with them heat and moisture.