Ocean surface-currents depend in general on the prevailing winds, the configuration of the coast and its submarine approaches. The trade-wind regions correspond pretty closely with westward flowing currents, while in the equatorial calm belts there are eastward-running counter-currents. In a similar manner the Bra zil, the Agulhas and the East Australian currents originate from the drift of the south-east trades, and the Japan current arises from the north-east trade drift. In the region of the westerly winds on the poleward side of 40° N. and S. the currents again flow generally eastward.
The general accordance of wind and surface-currents is so obvious that it was fully recognized by seafaring men in the time of the first circumnavigators. Modern investigations have shown, however, that the relationship is by no means simple. We must remember that the ocean is a continuous sheet of water of a certain depth ; if a single water particle is set in motion, the whole ocean must respond, as Varenius pointed out in 165o. Only a complete system of circulation can exist in the ocean, and hori zontal and vertical movements of water are definitely related to one another. Modern oceanographic investigations of temperature and salt content in the depths, as well as dynamic analyses show that surface water where two surface-currents come together, sinks into the depths (fig. 3), but that where two surface-currents diverge water from the depths rises (fig. 4). The Sargasso sea is an important convergence region. A divergence region lies south ward of the Cape Verde islands. Many hitherto enigmatic phe nomena are explained by such convergence and divergence lines ; e.g., long ribbons of foam, drifting bodies in long lines, ripples, and small patches of cold water in the midst of warm water.
We also know that the sur face-currents have only a very small depth in consequence of the combined action of earth rotation and eddying friction. Following a suggestion by F. Nan sen, W. Ekman in 1906 pointed out that current at the absolute surface is deflected to the right (Northern Hemisphere), or to the left (Southern Hemi sphere) of the wind direction; the north-east trades therefore pro duce not a south-westerly but a westerly current, a fact estab lished by observation. Analogous deflections are characteristic for all depths, and already at a depth of ioo-150--200 metres the direc tion is generally opposite to that at the surface as fig. 5 shows.
The depth at which the current practically ends is called the fric tion-depth; it is dependent on the velocity at the surface and on the latitude. The eddying friction which reduces the velocity of water below the surface, on the other hand causes surface move ments to become rapidly movements beneath the surface.
W. Ekman showed further that a pure wind-drift current when towards the coast, causes a heaping up of the water, and if away from the land, a welling up of deep water. The latter phenomenon is most clearly shown by the stripes of cold water along coasts of north-west and south-west Africa and the west coasts of North and South America. Upwelling coastal water is coldest close to the shore. The heaping up of surface water upon the coast, gives rise in the depths to a counter-current from the coast. This bottom counter-current maintains its level, is also subject to de flection on account of the rotation of the earth, and has a large share in securing vertical circulation and ventilation of the ocean.
There are also currents produced by variations in the density of water (p. 688). Ekman calls these convection streams; they depend ultimately on the varying rates of cooling, warming or evaporation of water at various places. The consequent variations in density produce variations in pressure at one and the same depth; the heavier wa ter endeavours to flow under the lighter, and the lighter strives to spread itself out over the heavier.
The great horizontal compensa tory ocean streams between the Northern and Southern Hemis pheres are to be explained most probably by differences of pres sure; but we have also density or convection streams on the surface of the ocean, associated with or separate from the wind or drift currents. The so-called "At lantic Current" on the west coast of Europe—a so-called con tinuation of the Gulf Stream—is only in small part a wind drift. It is principally the result of a density current, for in the Norway sea the density is very high while between lat. 4o° and 5o° it is low. In particular cases, in straits linking seas of differently constituted water, the density or convection currents develop in such a way that a surface-current goes in one direction, then appears a boundary zone, where the velocity is zero, and under this there is a deep current flowing in a direction contrary to that of the surface-current.