In the southern portion of the north Atlantic the surface drift is westward at a rate of four or five miles a day. The waters, forced along principally by the trade-winds, flow through the numerous passes between the Lesser Antilles and enter the Caribbean Sea, and from thence are carried through the Yucatan channel into the Gulf of Mexico. The waters are piled up, as it were, in that great landlocked basin, at the same time becoming warmer, and receive additions of fresh water from rain and inflowing streams. Each of these causes tends to decrease the density of the water, while evaporation has a counterbalancing influence. The escape for the waters, both salt and fresh, which enter the Gulf, is by evaporation and by flowing through the only notch in the rim of the Gulf basin which is not in the path of the equatorial current, namely, the strait separating Florida from Cuba and the Bahama Islands. These outflowing waters form the justly celebrated Gulf Stream.
Between Florida and the shoal waters on the Bahama Banks the Gulf Stream is about 50 miles wide, approximately 350 fathoms deep, and flows northward at the rate of from four to five miles an hour. Its temperature is about 80° F. It is estimated that this great river in the ocean carries 90,000,000,000 tons of water per hour past a given cross-section. Its course is northward along the immediate border of the continental shelf until it arrives opposite the Carolina coast, and thence northeastward, thus giving it a constantly increasing distance from the land. To the north of the Bahamas it receives as a tributary the portion of the equatorial current, perhaps even greater in volume than the true Gulf Stream, which is deflected northward by the West India Islands and their associated banks. Continuing its course, it is deflected still more towards the northeast owing to the influence of the earth's rotation, at the same time expanding and losing velocity so as to become a surface drift rather than a well-defined current. Under the influence of the prevailing westerly winds of the north Atlantic, the waters delivered by the Gulf
Stream pass the vicinity of the British Islands and in part enter the Greenland Sea.
Plate I.—Orographical features.
The transfer of the vast amount of warm water carried by the Gulf Stream far to the north is counterbalanced in part by a southward-flowing cold current which emerges from Davis Strait, and being joined by another cold current from the eastward of Greenland, continues southward under the name of the Labrador current, past Newfoundland and Nova Scotia to the Massachusetts coast, and is thought to exert an influence on the temperature of the sea even as far south as Cape Hatteras. While the Gulf Stream in the northern portion of its course curves eastward and departs from the American coast, the southward-flowing Labrador current is turned westward and follows close along the border of the land, and mainly over the continental shelf.
The chief effect of the cold current from the north in proximity to the coast of the continent is to bring to the adjacent land a lower mean annual temperature and especially colder and more stormy winters than it would otherwise experience. This tendency is augmented by the icebergs carried southward with the Labrador current. In a similar way, the northward-flowing warm current gives Florida and the Carolinas a subtropical climate, admits of the growth of reef-building corals about the Bermuda Islands, and carries so much warmth to northwestern Europe that its climate is milder and more humid than one would expect from its geographical position.
The currents of the north Pacific are analogous to those of the north Atlantic, but simpler, as there is nothing similar to the true Gulf Stream, and as communication with the Arctic Ocean is practically closed, there is no cold current flowing southward from that ocean; but the conditions, so far as they influence the climate of North America, are reversed. A warm current flowing northward off the coast of Japan, and hence known as the Japan current, crosses the Pacific, and on approaching the coast of Alaska and British Columbia is deflected southward. The climate of the northwest coast is thus ameliorated, the prevailing westerly winds are warm and humid, and the mean annual
precipitation from western Alaska to Oregon is in the neighbourhood of 100 inches. Under the influence of a mild equitable temperature and abundant moisture, the land bordering the Pacific from southern Alaska to northern California is clothed with the most magnificent forests that the continent affords. The marked contrasts in climate, vegetation, and the conditions that influence civilization between the two sides of the North American continent, produced by the cold Labrador current on the east and the warm Japan current on the west, is shown in a marked way by the sweep of the lines of equal mean annual temperature (isotherms) represented on the map forming Plate II, and again by the distribution of forests, as will be described later. It is instructive to note that the climate of Sitka, in north latitude 57°, is far more temperate and equable than that of New York city, latitude 40° 45', although the cool summers on the northwest coast make the mean annual temperature somewhat lower than on the coast of New York, or even of New England.