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CHAPTER XV.

OCEAN CURRENTS AND SEA TEMPERATURE.

NEXT to the winds in importance to climatology come the Ocean Currents, which are, therefore, most properly considered in connection with the winds. Of late years the origin of ocean currents has been discussed with great fulness by Carpenter and Croll in this country and by Colding in Denmark; but for the purposes of the present work it will suffice to consider them as in the first instance produced mainly by the winds, and, following Major Rennell, the first investigator of their phenomena, to divide them into two great classes -Drift Currents, produced directly by the wind, and moving more or less parallel to the wind in each district; and Stream Currents, which are produced when drift currents are deflected by meeting with an obstacle, such as a line of coast, and flow on in the new direction until the impetus they have received is expended.

Plate VIII., reduced from the General Current Chart published in the Wind and Current Charts' issued by the Hydrographic Office in 1872, will give the reader an idea of the general oceanic circulation, and its broad feature will be seen to be that in all the three oceans, the Atlantic, Pacific, and Indian, there is in the torrid zone a general set of the waters from

the east to the west; in fact, there is an equatorial westerly current.

Here, at the outset, the reader must be warned that currents are named according to the direction towards which they flow, while winds are named according to the direction from which they come.

An Easterly wind produces a westerly drift current, and so the Trades, being Easterly winds, produce a general movement of the surface-water from east to west along the equator.

Commencing with the Pacific, this equatorial current flows till it meets the East Indian Archipelago to the east of New Guinea; it there divides and is deflected, the northern portion, known as the Kuro Siwo, or the Great Black Stream of the Japanese seas, sweeping up into high latitudes outside the chain of islands formed by the Philippines, the Loo Choo group, and Japan, while the southern branch flows on till it strikes the coast of Australia, and there turns eastward again.

In the Indian Ocean the water is embayed on the northern side, so that there is no exit for the stream in that direction. The currents, therefore, in the Arabian Gulf and the Bay of Bengal are purely drift currents, depending on the Monsoons and changing with them.

The main equatorial current is principally developed to the south of the Line, and, when it meets the African coast, turns southward. Part flows outside Madagascar, and there is gradually converted into an easterly current-a change similar to that occurring on the east coast of Australia; but the main portion of the current sweeps down the Mozambique Channel and along the coast, as a strong warm stream, flowing out beyond the south point of Africa.

The Atlantic equatorial current may be said to divide on Cape St. Roque. One portion turns southward along the coast of Brazil and flows on as far as the latitude of the River Plate, where it turns eastward across the South Atlantic. The other, and larger, portion passes northwards, and, combining with the westerly current of the North-east Trade, enters the Caribbean Sea, where it, in its turn, is embayed and turned back on itself. It finally flows forth from the Gulf of Mexico as a rapid stream-the Gulf Streamwhich skirts the eastern coast of the United States throughout nearly its whole extent, and then sweeps across the North Atlantic to the shores of Northern Europe.

This rough summary shows us that we meet with westerly currents in low latitudes all round the world, and in every case, as we shall subsequently see, there is a counter-current flowing eastward close along the equator, between the drift currents of the two Trades.

In the middle latitudes the currents are reversed, and are generally easterly. In fact, in each of the five great oceans-the North and South Atlantic, the North and South Pacific, and the South Indian Ocean-great bodies of water are continually circling round in eddies, the equatorial current taking the water westwards, and the return currents, like the Gulf Stream, bringing it eastwards again.

The necessity for the existence of such systems of compensating currents is at once intelligible on the principle put prominently forward by Dr. Haughton in his lectures on Physical Geography,' p. 141. It is that used by mathematicians in hydrodynamics under the name of the Equation of Continuity. This may be

expressed very simply by the following self-evident theorem:-Given a permanent system of constant continuous circulation in any ocean; if any vertical plane be supposed drawn across the ocean, equal quantities of water must cross that plane from right to left, and from left to right, in a given time; for, if possible, let it not be so, then there will arise a difference of sea level at the two sides of the supposed plane, and the hypothesis of constant continuous circulation will become impossible.'

The equatorial currents are essentially warm water currents, but in high latitudes cold water currents appear. The best known of these is the American Arctic current, which flows out of Baffin's Bay, hugging the coast, as the action of the earth's rotation, which we have already described at p. 245, causes the stream to press against its right bank, which is here the continent of America. The influence of this cold stream is felt as far south as Cape Cod, in lat. 42°.

The North Pacific has no polar current of the same importance as that of the Atlantic, for Behring's Strait, the only outlet of the Arctic Ocean in that region, is too shallow to allow much water to pass through it.

It is, however, in the southern hemisphere, with its immense expanse of water, that the polar currents form a prominent feature of the physical geography. It seems, as Sir F. Evans remarks (Brit. Assoc. Report,' 1876, p. 175), as if all the surface water between the Antarctic Circle and the parallel of 45° S. is drifting northwards and eastwards. In this latitude it joins with the easterly streams produced by the rebound of the southern branches of the respective equatorial currents, chills them, and appears on the

western coasts of America, Africa, and Australia as cold northerly currents, causing the isotherms on these shores to dip down towards the equator, as shown on Plates I.-III.

The best known of these currents, and that standing first on the list, is the Peruvian, or Humboldt's, current, on the coast of South America, but that in the Atlantic is well marked, and on the west coast of Africa the influence of the cold current is such that the temperature of the sea near Cape Town is sometimes 20° lower than in the corresponding latitude on the eastern side of the continent.

I shall now proceed to treat of these currents more in detail, and shall commence with the Atlantic, as the district which has been most studied.

In the first place, there are the two westerly equatorial currents, with the counter-current between them. Of these two the southern is the more important, as it comes from a greater body of water. It strikes upon Cape St. Roque and divides into two branches, of which the northern passes along the north-east coast of the continent and finally joins the northern equatorial. The combined current enters the Caribbean Sea, circulates round the Gulf of Mexico, and finally pours out between Cuba and the coast of Florida as a mighty stream of hot water, which passes the Straits of Bemini with a width of about thirty miles and a discharge of some forty cubic miles per hour, at a rate of over 6 feet per second, according to Colding. It flows or, expanding in width, until it is found off Newfoundland, at a distance of 1,800 miles from Cape Florida, as a stream 320 miles wide, but with its velocity reduced to one-third of its initial