Of their effect on fishes and other life of the deep sea we have only the faintest conception. Swedish scientists say that the herring are carried or drawn into some of the fiords of Sweden when the deep internal waves roll over the submerged sills and into the fiords. In the open ocean, we know that the boundary between water masses of different temperatures or salinities is often a barrier that may not be passed by living creatures, delicately adjusted to certain conditions. Do these creatures themselves then move up and down with the roll of the deep waves? And what happens to the bottom fauna of the continental slope, adjusted, it may be, to water of unchanging warmth? What is their fate when the waves move in from a region of arctic cold, rolling like a storm surf against those deep, dark slopes? At present we do not know. We can only sense that in the deep and turbulent recesses of the sea are hidden mysteries far greater than any we have solved.
Wind, Sun, and the Spinning of the Earth
For thousands upon thousands of years the Sunlight and the Sea and the masterless Winds have held tryst together.
AS THE ALBATROSS III groped through fog over Georges Bank all of one week in the midsummer of 1949, those of us aboard had a personal demonstration of the power of a great ocean current. There was never less than a hundred miles of cold Atlantic water between us and the Gulf Stream, but the winds blew persistently from the south and the warm breath of the Stream rolled over the Bank. The combination of warm air and cold water spelled unending fog. Day after day the Albatross moved in a small circular room, whose walls were soft gray curtains and whose floor had a glassy smoothness. Sometimes a petrel flew, with swallow-like flutterings, across this room, entering and leaving it by passing through its walls as if by sorcery. Evenings, the sun, before it set, was a pale silver disc hung in the ship’s rigging, the drifting streamers of fog picking up a diffused radiance and creating a scene that set us to searching our memories for quotations from Coleridge. The sense of a powerful presence felt but not seen, its nearness made manifest but never revealed, was infinitely more dramatic than a direct encounter with the current.
The permanent currents of the ocean are, in a way, the most majestic of her phenomena. Reflecting upon them, our minds are at once taken out from the earth so that we can regard, as from another planet, the spinning of the globe, the winds that deeply trouble its surface or gently encompass it, and the influence of the sun and the moon. For all these cosmic forces are closely linked with the great currents of the ocean, earning for them the adjective I like best of all those applied to them—the planetary currents.
Since the world began, the ocean currents have undoubtedly changed their courses many times (we know, for example, that the Gulf Stream is no more than about 60 million years old); but it would be a bold writer who would try to describe their pattern in the Cambrian period, for example, or in the Devonian, or in the Jurassic. So far as the brief period of human history is concerned, however, it is most unlikely that there has been any important change in the major patterns of oceanic circulation, and the first thing that impresses us about the currents is their permanence. This is not surprising, for the forces that produce the currents show little disposition to change materially over the eons of earthly time. The primary driving power is supplied by the winds; the modifying influences are the sun, the revolving of the earth ever toward the east, and the obstructing masses of the continents.
The surface of the sea is unequally heated by the sun; as the water is warmed it expands and becomes lighter, while the cold water becomes heavier and more dense. Probably a slow exchange of polar and equatorial waters is brought about by these differences, the heated water of the tropics moving poleward in the upper layers, and polar water creeping toward the equator along the floor of the sea. But these movements are obscured and largely lost in the far greater sweep of the wind-driven currents. The steadiest winds are the trades, blowing diagonally toward the equator from the northeast and southeast. It is the trades that drive the equatorial currents around the globe. On wind and water alike, as on all that moves, be it a ship, a bullet, or a bird, the spinning earth exerts a deflecting force, turning all moving objects to the right in the Northern Hemisphere and to the left in the Southern. Through the combined action of these and other forces, the resulting current patterns are slowly circulating eddies, turning to the right, or clockwise, in the northern oceans, and to the counterclockwise, in the southern.
There are exceptions, and the Indian Ocean, which seems never to be quite like the others, is an important one. Ruled by capricious monsoons, its currents shift with the seasons. North of the equator, the direction of flow of immense masses of water may be either eastward or westward, depending on which of the monsoons is blowing. In the southern part of this ocean a fairly typical counterclockwise pattern exists: westward under the equator, south along the African coast, east to Australia on the westerly winds, northward by devious and seasonally shifting paths, here giving up water to the Pacific and there receiving contributions from it.
The Antarctic Ocean, being merely a continuous band of water encircling the globe, is another exception to the typical current pattern. Its waters are driven constantly into the east and the northeast by winds from the west and southwest, and the currents are given speed by the quantities of fresh water pouring in from melting ice. It is not a closed circulation; water is given off, in surface currents and by deep paths, to the adjacent oceans, and in return other water is received from them.
It is in the Atlantic and Pacific that we see most clearly the interplay of cosmic forces producing the planetary currents.
Perhaps because of the long centuries over which the Atlantic has been crossed and recrossed by trade routes, its currents have been longest known to seafaring men and best studied by oceanographers. The strongly running Equatorial Currrents were familiar to generations of seamen in the days of sail. So determined was their set to westward that vessels intending to pass down into the South Atlantic could make no headway unless they had gained the necessary easting in the region of the southeast trades. Ponce de Leon’s three ships, sailing south from Cape Canaveral to Tortugas in 1513, sometimes were unable to stem the Gulf Stream, and ‘although they had great wind, they could not proceed forward, but backward.’ A few years later Spanish shipmasters learned to take advantage of the currents, sailing westward in the Equatorial Current, but returning home via the Gulf Stream as far as Cape Hatteras, whence they launched out into the open Atlantic.
The first chart of the Gulf Stream was prepared about 1769 under the direction of Benjamin Franklin while he was Deputy Postmaster General of the Colonies. The Board of Customs in Boston had complained that the mail packets coming from England took two weeks longer to make the westward crossing than did the Rhode Island merchant ships. Franklin, perplexed, took the problem to a Nantucket sea captain, Timothy Folger, who told him this might very well be true because the Rhode Island captains were well acquainted with the Gulf Stream and avoided it on the westward crossing, whereas the English captains were not. Folger and other Nantucket whalers were personally familiar with the Stream because, he explained,