TEMPERATURE OF THE OCEAN HORIZONTAL AND VERTICAL Horizontally, the temperature of the ocean, like that of the land, depends on the amount of solar heat, and varies similarly with the latitude—being highest in the tropics, and falling away towards the poles. But the distribu tion of the temperature of the sea is far more even than on land. The surplus heat of one part of the land can only be conveyed to another colder region by variable aerial currents, while the heated waters of the tropical seas are actually transferred in vast constant currents from the equator towards the poles. So that, as regards surface temperature, the latitudinal distinctions which may be ap propriately applied to the land are scarcely applicable to the sea,—the thorough co-mingling of the surface waters of the ocean, from the tropics to the polar regions, by a vast system of currents, resulting in a far more even distribution of heat. Still it is generally true that, as on land, so at sea, the temperature sinks from the equator towards the poles. Thus, in the equatorial part of the Atlantic, the temperature of the surface water ranges from to F., while that of the North Atlantic is from 44° to F. The mean temperature of the surface water in the North Pacific is about 70° F., and in the South Pacific about F. The waters of land locked tropical seas are abnormally warm, their excess of heat not being readily diffused through the colder open ocean. Thus the surface temperature in the Gulf of Mexico is fre quently 88° F., and in the Red Sea 94° F. has occasionally been recorded.
Of the temperature of the depths of the sea nothing certain was known until recent years. Sir James Ross's researches in the Southern Ocean led him to conclude that a uniform mean temperature of 39i° F. would be found at depths varying from 7,200 feet at the equator-3,600 feet along the parallel of 45° S.—at the surface within the belt between the parallels of 56° and 59° S.—and at a depth of 4,500 feet along the parallel of 70° S. ; although the surface at the equator was 80° F., and at 70° S. lat. only 30° F. The recent observations of Dr. Carpenter and Sir Wyville Thomson have entirely dis pelled the idea of a uniform mean temperature of 39° at varying depths from the equator towards the pole& It is now proved beyond all doubt that the temperature decreases gradually from the surface to the bottom, the lowest stratum of water, even under the equator, being icy cold, generally near—and not unfrequently below—the freezing point; and that, through out the ocean, the stratum of water having a temperature above 40° F. is comparatively shallow, the mass of the
ocean being of cold water from 40° F., to icy cold at 30° F. In the North Atlantic, in a line from Sandy Hook to Bermuda, (and consequently across the Gulf Stream), it was shown that the Gulf Stream, with a temperature of 75', overlies a mass of water which quickly falls to F., and gradually to 35.3° F. at the bottom, at a depth of 2,425 fathoms, or 14,550 feet ; the mass of cold water, from 35° to 40° F., being about two-thirds of the whole. The general bottom-temperature of the North Atlantic is 35.3°. In the South Atlantic the temperature falls to 40° F. at a depth of 300 fathoms, or 1,800 feet ; at the bottom there is an average temperature of 32.9°, or nearly 3° colder than in the North Atlantic. In the North Pacific the bottom temperature is generally below 35° F.; and off Cape Otway, in the South Pacific, it falls to 32.5° F. ; still further south the mass of the water from a depth of 1,000 fathoms, or 6,000 feet to the bottom, was icy cold, being from 32° to 31° F.
The lowest layer of water throughout the ocean is thus nearly at, and occasionally below, the freezing point of fresh water, being, in fact, actually below the general temperature of the outer crust of the earth. If the mass of water at great depths be immovable, as formerly supposed, surely its tempera ture would, in course of time, approximate that of the crust it overlies. This ,cold bottom-water cannot, then, be anything but an influx from the Polar Seas of icy cold water, which, sinking below the warmer—and therefore specifically lighter surface-water " creeps " along the floor of the ocean towards the equator. Whether this cold influx comes from the north or south, or if from both, in what proportion, is easily determined by a moment's consideration of the communication between the Arctic and Antarctic basins, and those of the other great oceans. The Pacific, for instance, is nearly landlocked on the north, and communicates with the Arctic basin only by a strait of no great width, and only 40 fathoms in depth. This channel being mainly occupied by a warm current from the Pacific to the Arctic, the under current of cold water from the latter is unimportant. The influx of cold bottom-water in the Pacific must therefore be from the Antarctic. On the south the basins of these two oceans unite broadly for thou sands of miles, and, for the most part, uninterrupted by continuous submarine elevations.