When upwelling takes place along coastlines, it is the result of the interplay of several forces—the winds, the surface currents, the rotation of the earth, and the shape of the hidden slopes of the continent’s foundations. When the winds, combined with the deflecting effect of rotation, blow the surface waters offshore, deep water must rise to replace it.
Upwelling may occur in the open sea as well, but from entirely different causes. Wherever two strongly moving currents diverge, water must rise from below to fill the place where the streams separate. One such place lies at the westernmost bounds of the Equatorial Current in the Pacific, where the powerfully moving stream turns and pours part of its waters back into the counter-current, and part northward toward Japan. These are confused and turbulent waters. There is the strong pull to the north by which the main stream, sensitive to the force of the rotating earth, turns to the right. There are the swirls and eddies by which the lesser stream turns again upon itself and flows back into the eastern Pacific. There is the rushing up from below to fill the otherwise deepening groove between the streams. In the resulting disquietude of the ocean waters, chilled and enriched from below, the smaller organisms of the plankton thrive. As they multiply, they provide food for the larger plankton creatures, which, in turn, provide food for squid and fish. These waters are prodigiously rich in life, and there is evidence that they may have been so for many thousands of years. Swedish oceanographers recently found that under these areas of divergence the sediment layer is exceptionally thick—the layer composed of all that remains of the billions upon billions of minute creatures that have lived and died in this place.
The downward movement of surface water into the depths is an occurrence as dramatic as upwelling, and perhaps it fills the human mind with an even greater sense of awe and mystery, because it cannot be seen but can only be imagined. At several known places the downward flow of enormous quantities of water takes place regularly. This water feeds the deep currents of whose courses we have only the dimmest knowledge. We do know that it is all part of the ocean’s system of balances, by which she pays back to one part of her waters what she had latterly borrowed for distribution to another.
The North Atlantic, for example, receives quantities of surface water (some 6 million cubic meters a second) from the South Atlantic via the Equatorial Current. The return payment is made at deep levels, partly in very cold arctic water, and partly in some of the saltiest, warmest water in the world, that of the Mediterranean. There are two places for the down-flow of arctic water. One is in the Labrador Sea. Another is southeast of Greenland. At each the quantity of sinking water is prodigious—some 2 million cubic meters a second. The deep Mediterranean water flows out over the sill that separates the basin of the Mediterranean from the open Atlantic. This sill lies about 150 fathoms beneath the surface of the sea. The water that spills over its rocky edge does so because of the unusual conditions that prevail in the Mediterranean. The hot sun beating down on its nearly enclosed water creates an extraordinarily high rate of evaporation, drawing off into the atmosphere more water than is added by the inflow of rivers. The water becomes ever saltier and more dense; as evaporation continues the surface of the Mediterranean falls below that of the Atlantic. To correct the inequality, lighter water from the Atlantic pours past Gibraltar in surface streams of great strength.
Now we give the matter little thought, but in the days of sail, passage out into the Atlantic was a difficult problem because of this surface current. An old ship’s log of the year 1855 has this to say of the current and its practical effect:
Weather fine; made 1¼ pt. leeway. At noon, stood in to Almira Bay, and anchored off the village of Roguetas. Found a great number of vessels waiting for a chance to get to the westward, and learned from them that at least a thousand sail are weather-bound between this and Gibraltar. Some of them have been so for six weeks, and have even got so far as Malaga, only to be swept back by the current. Indeed, no vessel has been able to get out into the Atlantic for three months past.
Later measurements show that these surface currents flow into the Mediterranean with an average velocity of about three knots. The bottom current, moving out into the Atlantic, is even stronger. Its outward flow is so vigorous that it has been known to wreck oceanographic instruments sent down to measure it, apparently pounding them against stones on the bottom; and once the wire of the Falmouth cable near Gibraltar ‘was ground like the edge of a razor, so that it had to be abandoned and a new one laid well inshore.’
The water that sinks in the arctic regions of the Atlantic, as well as that spilling over the Gibraltar sill, spreads out widely into the deeper parts of the ocean basins. Traversing the North Atlantic, it crosses the equator and continues to the south, there passing between two layers of water that are moving northward from the Antarctic Sea. Some of this antarctic water mingles with the Atlantic water—that from Greenland and Labrador and the Mediterranean—and with it returns south. But other antarctic water moves northward across the equator and has been traced as far as the latitude of Cape Hatteras.
The flow of these deep waters is hardly a ‘flow’ at all; its pace is ponderously slow, the measured creep of icy, heavy water. But the volumes involved are prodigious, and the areas covered world-wide. It may even be that the deep ocean water, on such global wanderings, acts to distribute some of the marine fauna—not the surface forms but the dwellers in deep, dark layers. From our knowledge of the source of the currents, it seems significant that some of the same species of deep-water invertebrates and fishes have been collected off the coast of South Africa and off Greenland. And about Bermuda, where a greater variety of deep-water forms has been found than anywhere else, there is a mingling of deep water from the Antarctic, the Arctic, and the Mediterranean. Perhaps in these sunless streams the weird inhabitants of deep waters drift, generation after generation, surviving and multiplying because of the almost changeless character of these slowly moving currents.
There is, then, no water that is wholly of the Pacific, or wholly of the Atlantic, or of the Indian or the Antarctic. The surf that we find exhilarating at Virginia Beach or at La Jolla today may have lapped at the base of antarctic icebergs or sparkled in the Mediterranean sun, years ago, before it moved through dark and unseen waterways to the place we find it now. It is by the deep, hidden currents that the oceans are made one.