OCEANIC CIRCULATION The remarkable uniformity in the composition of sea water from the equator to the poles, and from the surface to the lowest depths, is undoubtedly a proof of a general and constant interchange of the waters of the ocean; for otherwise, owing to excessive evaporation in some localities, and excessive precipitation in others, the proportion of saline ingredients would vary considerably ; and the causes of the difference being in many places almost constant, the variation would also be constant and increasing ; so that in time some parts of the sea would be nearly, if not quite, fresh, while other parts would be excessively salt. But the saltness of the water is very nearly the same throughout the ocean. There must be, therefore, an actual interchange, or "circulation," of the waters of the ocean.
The "drifting" of the bottles, drift-wool, and other marine ddbris (Art. 123), practically proves the existence of currents at the surface of the ocean, constantly flowing in certain definite directions. Mariners have also not imfre quently observed immense icebergs moving rapidly against both the surface current and the wind. The submerged por tions of the icebergs must therefore have been acted on by under-currents flowing in an opposite direction to the surface drifts. Recent researches show that the warm water even under the equator forms a comparatively shallow layer, over lying a mass of cold water, near and occasionally below the freezing point of fresh water, and therefore actually below the general temperature of the earth's crust. There must therefore be not only surface and under-currents to a moderate depth, but also a vast influx of cold water along the bottom from the poles towards the equator, being, in fact, a bottom inflow from the poles, compensating a surface overflow from the equator.
There is, therefore, such a thing as a general circulation of the waters of the ocean ; but although its existence is beyond doubt, its cause or causes is as yet a moot point. One high authority) asserts that the general system of oceanic currents is produced by the winds,—not by the trade-winds alone, nor by the prevailing winds proper alone, but by the combined action of all the prevailing winds of the globe, regarded as one system of circulation. Dr. Carpenter, Keith
Johnston, and others, assert that currents are caused by the difference of temperature of the polar and equatorial regions of the globe. But Sir Wyville Thomson says that "he has never seen, whether in the Atlantic, the Southern Sea, or the Pacific, the slightest grounds for supposing that such a thing exists as a general vertical circulation of the water of the ocean depending upon differences of specific gravity." Con stant aerial currents, such as the north-east and the south-east trade-winds, are most probably the cause of the westerly drift in the Atlantic and Pacific ; but the general oceanic circulation is mainly owing to the unequal densities of sea-water, arising from differences of temperature or saltness.
Dr. Carpenter's theory is, that "a vertical circulation is maintained between the polar and equatorial waters by the difference of their temperature, the level of polar waters being reduced, and its density increased, by the surface cold to which it is subjected, whilst a downward motion is also imparted to each stratum successively exposed to it; and the level of equa torial water being raised, and its density diminished, by the surface heat to which it is exposed. The first of these agencies is by far the more effective, since it extends to the whole depth of the water, whilst the second only affects in any considerable degree the superficial stratum. Thus a movement will be imparted to the upper stratum of oceanic water, from the equator to the poles ; whilst a movement will be imparted to the deeper stratum, from the poles to the equator." This theory is supported by two well known facts, viz. :—the general movement of the upper stratum of water in the tropics towards the poles, and the existence of a lower stratum of icy-cold water over the bottom of the great oceans, communicating with either or both of the polar seas.