Among unusual creations of the tide, perhaps the best known are the bores. The world possesses half a dozen or more famous ones. A bore is created when a great part of the flood tide enters a river as a single wave, or at most two or three waves, with a steep and high front. The conditions that produce bores are several: there must be a considerable range of tide, combined with sand bars and other obstructions in the mouth of the river, so that the tide is hindered and held back, until it finally gathers itself together and rushes through. The Amazon is remarkable for the distance its bore travels upstream—some 200 miles—with the result that the bores of as many as 5 flood tides may actually be moving up the river at one time.
On the Tsientang River, which empties into the China Sea, all shipping is controlled by the bore—the largest, most dangerous, and best known in the world. The ancient Chinese used to throw offerings into the river to appease the angry spirit of this bore, whose size and fury appear to have varied from century to century, or perhaps even from decade to decade, as the silting of the estuary has shifted and changed. During most of the month the bore now advances up the river in a wave 8 to 11 feet high, moving at a speed of 12 to 13 knots, its front ‘a sloping cascade of bubbling foam, falling forward and pounding on itself and on the river.’ Its full ferocity is reserved for the spring tides of the full moon and the new moon, at which times the crest of the advancing wave is said to rise 25 feet above the surface of the river.
There are bores, though none so spectacular, in North America. There is one at Moncton, on New Brunswick’s Petitcodiac River, but it is impressive only on the spring tides of the full or new moon. At Turnagain Arm in Cook Inlet, Alaska, where the tides are high and the currents strong, the flood tide under certain conditions comes in as a bore. Its advancing front may be four to six feet high and is recognized as being so dangerous to small craft that boats are beached well above the level of the flats when the bore is approaching. It can be heard about half an hour before its arrival at any point, traveling slowly with a sound as of breakers on a beach.
The influence of the tide over the affairs of sea creatures as well as men may be seen all over the world. The billions upon billions of sessile animals, like oysters, mussels, and barnacles, owe their very existence to the sweep of the tides, which brings them the food which they are unable to go in search of. By marvelous adaptations of form and structure, the inhabitants of the world between the tide lines are enabled to live in a zone where the danger of being dried up is matched against the danger of being washed away, where for every enemy that comes by sea there is another that comes by land, and where the most delicate of living tissues must somehow withstand the assault of storm waves that have the power to shift tons of rock or to crack the hardest granite.
The most curious and incredibly delicate adaptations, however, are the ones by which the breeding rhythm of certain marine animals is timed to coincide with the phases of the moon and the stages of the tide. In Europe it has been well established that the spawning activities of oysters reach their peak on the spring tides, which are about two days after the full or the new moon. In the waters of northern Africa there is a sea urchin that, on the nights when the moon is full and apparently only then, releases its reproductive cells into the sea. And in tropical waters in many parts of the world there are small marine worms whose spawning behavior is so precisely adjusted to the tidal calendar that, merely from observing them, one could tell the month, the day, and often the time of day as well.
Near Samoa in the Pacific, the palolo worm lives out its life on the bottom of the shallow sea, in holes in the rocks and among the masses of corals. Twice each year, during the neap tides of the moon’s last quarter in October and November, the worms forsake their burrows and rise to the surface in swarms that cover the water. For this purpose, each worm has literally broken its body in two, half to remain in its rocky tunnel, half to carry the reproductive products to the surface and there to liberate the cells. This happens at dawn on the day before the moon reaches its last quarter, and again on the following day; on the second day of the spawning the quantity of eggs liberated is so great that the sea is discolored.
The Fijians, whose waters have a similar worm, call them ‘Mbalolo’ and have designated the periods of their spawning ‘Mbalolo lailai’ (little) for October and ‘Mbalolo levu’ (large) for November. Similar forms near the Gilbert Islands respond to certain phases of the moon in June and July; in the Malay Archipelago a related worm swarms at the surface on the second and third nights after the full moon of March and April, when the tides are running highest. A Japanese palolo swarms after the new moon and again after the full moon in October and November.
Concerning each of these, the question recurs but remains unanswered: is it the state of the tides that in some unknown way supplies the impulse from which springs this behavior, or is it, even more mysteriously, some other influence of the moon? It is easier to imagine that it is the press and the rhythmic movement of the water that in some way brings about this response. But why is it only certain tides of the year, and why for some species is it the fullest tides of the month and for others the least movements of the waters that are related to the perpetuation of the race? At present, no one can answer.
No other creature displays so exquisite an adaptation to the tidal rhythm as the grunion—a small, shimmering fish about as long as a man’s hand. Through no one can say what processes of adaptation, extending over no one knows how many millennia, the grunion has come to know not only the daily rhythm of the tides, but the monthly cycle by which certain tides sweep higher on the beaches than others. It has so adapted its spawning habits to the tidal cycle that the very existence of the race depends on the precision of this adjustment.
Shortly after the full moon of the months from March to August, the grunion appear in the surf on the beaches of California. The tide reaches flood stage, slackens, hesitates, and begins to ebb. Now on these waves of the ebbing tide the fish begin to come in. Their bodies shimmer in the light of the moon as they are borne up the beach on the crest of a wave, they lie glittering on the wet sand for a perceptible moment of time, then fling themselves into the wash of the next wave and are carried back to sea. For about an hour after the turn of the tide this continues, thousands upon thousands of grunion coming up onto the beach, leaving the water, returning to it. This is the spawning act of the species.
During the brief interval between successive waves, the male and female have come together in the wet sand, the one to shed her eggs, the other to fertilize them. When the parent fish return to the water, they have left behind a mass of eggs buried in the sand. Succeeding waves on that night do not wash out the eggs because the tide is already ebbing. The waves of the next high tide will not reach them, because for a time after the full of the moon each tide will halt its advance a little lower on the beach than the preceding one. The eggs, then, will be undisturbed for at least a fortnight. In the warm, damp, incubating sand they undergo their development. Within two weeks the magic change from fertilized egg to larval fishlet is completed, the perfectly formed little grunion still confined within the membranes of the egg, still buried in the sand, waiting for release. With the tides of the new moon it comes. Their waves wash over the places where the little masses of the grunion eggs were buried, the swirl and rush of the surf stirring the sand deeply. As the sand is washed away, and the eggs feel the touch of the cool sea water, the membranes rupture, the fishlets hatch, and the waves that released them bear them away to the sea.
But the link between tide and living creature I like best to remember is that of a very small worm, flat of body, with no distinction of appearance, but with one unforgettable quality. The name of this worm is Convoluta roscoffensis, and it lives on the sandy beaches of northern Brittany and the Channel Islands. Convoluta has entered into a remarkable partnership with green alga, whose cells inhabit the body of the worm and lend to its tissues their own green color. The worm lives entirely on the starchy pro
ducts manufactured by its plant guest, having become so completely dependent upon this means of nutrition that its digestive organs have degenerated. In order that the algal cells may carry on their function of photosynthesis (which is dependent upon sunlight) Convoluta rises from the damp sands of the intertidal zone as soon as the tide has ebbed, the sand becoming spotted with large green patches composed of thousands of the worms. For the several hours while the tide is out, the worms lie thus in the sun, and the plants manufacture their starches and sugars; but when the tide returns, the worms must again sink into the sand to avoid being washed away, out into deep water. So the whole lifetime of the worm is a succession of movements conditioned by the stages of the tide—upward into sunshine on the ebb, downward on the flood.
What I find most unforgettable about Convoluta is this: sometimes it happens that a marine biologist, wishing to study some related problem, will transfer a whole colony of the worms into the laboratory, there to establish them in an aquarium, where there are no tides. But twice each day Convoluta rises out of the sand on the bottom of the aquarium, into the light of the sun. And twice each day it sinks again into the sand. Without a brain, or what we would call a memory, or even any very clear perception, Convoluta continues to live out its life in this alien place, remembering, in every fiber of its small green body, the tidal rhythm of the distant sea.
III
Man and the Sea About Him
The Global Thermostat
Out of the chamber of the South cometh the storm,
and cold out of the North.
THE BOOK OF JOB
WHEN THE BUILDING of the Panama Canal was first suggested, the project was severely criticized in Europe. The French, especially, complained that such a canal would allow the waters of the Equatorial Current to escape into the Pacific, that there would then be no Gulf Stream, and that the winter climate of Europe would become unbearably frigid. The alarmed Frenchmen were completely wrong in their forecast of oceanographic events, but they were right in their recognition of a general principle—the close relation between climate and the pattern of ocean circulation.
There are recurrent schemes for deliberately changing—or attempting to change—the pattern of the currents and so modifying climate at will. We hear of projects for diverting the cold Oyashio from the Asiatic coast, and of others for controlling the Gulf Stream. About 1912 the Congress of the United States was asked to appropriate money to build a jetty from Cape Race eastward across the Grand Banks to obstruct the cold water flowing south from the Arctic. Advocates of the plan believed that the Gulf Stream would then swing in nearer the mainland of the northern United States and would presumably bring us warmer winters. The appropriation was not granted. Even if the money had been provided, there is little reason to suppose that engineers then—or later—could have succeeded in controlling the sweep of the ocean’s currents. And fortunately so, for most of these plans would have effects different from those popularly expected. Bringing the Gulf Stream closer to the American east coast, for example, would make our winters worse instead of better. Along the Atlantic coast of North America, the prevailing winds blow eastward, across the land toward the sea. The air masses that have lain over the Gulf Stream seldom reach us. But the Stream, with its mass of warm water, does have something to do with bringing our weather to us. The cold winds of winter are pushed by gravity toward the low-pressure areas over the warm water. The winter of 1916, when Stream temperatures were above normal, was long remembered for its cold and snowy weather along the east coast. If we could move the Stream inshore, the result in winter would be colder, stronger winds from the interior of the continent—not milder weather.
But if the eastern North American climate is not dominated by the Gulf Stream, it is far otherwise for the lands lying ‘down-stream.’ From the Newfoundland Banks, as we have seen, the warm water of the Stream drifts eastward, pushed along by the prevailing westerly winds. Almost immediately, however, it divides into several branches. One flows north to the western shore of Greenland; there the warm water attacks the ice brought around Cape Farewell by the East Greenland Current. Another passes to the southwest coast of Iceland, and, before losing itself in arctic waters, brings a gentling influence to the southern shores of that island. But the main branch of the Gulf Stream or North Atlantic Drift flows eastward. Soon it divides again. The southernmost of these branches turns toward Spain and Africa and re-enters the Equatorial Current. The northernmost branch, hurried eastward by the winds blowing around the Icelandic ‘low,’ piles up against the coast of Europe the warmest water found at comparable latitudes anywhere in the world. From the Bay of Biscay north its influence is felt. And as the current rolls northeastward along the Scandinavian coast, it sends off many lateral branches that curve back westward to bring the breath of warm water to the arctic islands and to mingle with other currents in intricate whirls and eddies. The west coast of Spitsbergen, warmed by one of these lateral streams, is bright with flowers in the arctic summer; the east coast, with its polar current, remains barren and forbidding. Passing around the North Cape, the warm currents keep open such harbors as Hammerfest and Murmansk, although Riga, 800 miles farther south on the shores of the Baltic, is choked with ice. Somewhere in the Arctic Sea, near the island of Novaya Zemlya, the last traces of Atlantic water disappear, losing themselves at last in the overwhelming sweep of the icy northern sea.
It is always a warm-water current, but the temperature of the Gulf Stream nevertheless varies from year to year, and a seemingly slight change profoundly affects the air temperatures of Europe. The British meteorologist, C. E. P. Brooks, compares the North Atlantic to ‘a great bath, with a hot tap and two cold taps.’ The hot tap is the Gulf Stream; the cold taps are the East Greenland Current and the Labrador Current. Both the volume and the temperature of the hot-water tap vary. The cold taps are nearly constant in temperature but vary immensely in volume. The adjustment of the three taps determines surface temperatures in the eastern Atlantic and has a great deal to do with the weather of Europe and with happenings in arctic seas. A very slight winter warming of the eastern Atlantic temperatures means, for example, that the snow cover of northwestern Europe will melt earlier, that there will be an earlier thawing of the ground, that spring plowing may begin earlier, and that the harvest will be better. It means, too, that there will be relatively little ice near Iceland in the spring and that the amount of drift ice in the Barents Sea will diminish a year or two later. These relations have been clearly established by European scientists. Someday long-range weather forecasts for the continent of Europe will probably be based in part on ocean temperatures. But at present there are no means for collecting the temperatures over a large enough area, at frequent enough intervals.*
For the globe as a whole, the ocean is the great regulator, the great stabilizer of temperatures. It has been described as ‘a savings bank for solar energy, receiving deposits in seasons of excessive insolation and paying them back in seasons of want.’ Without the ocean, our world would be visited by unthinkably harsh extremes of temperature. For the water that covers three-fourths of the earth’s surface with an enveloping mantle is a substance of remarkable qualities. It is an excellent absorber and radiator of heat. Because of its enormous heat capacity, the ocean can absorb a great deal of heat from the sun without becoming what we would consider ‘hot,’ or it can lose much of its heat without becoming ‘cold.’
Through the agency of ocean currents, heat and cold may be distributed over thousands of miles. It is possible to follow the course of a mass of warm water that originates in the trade-wind belt of the Southern Hemisphere and remains recognizable for a year and a half, through a course of more than 7000 miles. This redistributing function of the ocean tends to make up for the uneven heating of the globe by the sun. As it is, ocean currents carry hot equatorial water toward the poles and return cold water equator-ward by such surface drifts as the Labrador Current and Oyashio, and even more importantly by deep currents. The r
edistribution of heat for the whole earth is accomplished about half by the ocean currents, and half by the winds.
At that thin interface between the ocean of water and the ocean of overlying air, lying as they do in direct contact over by far the greater part of the earth, there are continuous interactions of tremendous importance.
The atmosphere warms or cools the ocean. It receives vapors through evaporation, leaving most of the salts in the sea and so increasing the salinity of the water. With the changing weight of that whole mass of air that envelops the earth, the atmosphere brings variable pressure to bear on the surface of the sea, which is depressed under areas of high pressure and springs up in compensation under the atmospheric lows. With the moving force of the winds, the air grips the surface of the ocean and raises it into waves, drives the currents onward, lowers sea levels on windward shores, and raises it on lee shores.
But even more does the ocean dominate the air. Its effect on the temperature and humidity of the atmosphere is far greater than the small transfer of heat from air to sea. It takes 3000 times as much heat to warm a given volume of water 1° as to warm an equal volume of air by the same amount. The heat lost by a cubic meter of water on cooling 1° C. would raise the temperature of 3000 cubic meters of air by the same amount. Or to use another example, a layer of water a meter deep, on cooling .1° could warm a layer of air 33 meters thick by 10°. The temperature of the air is intimately related to atmospheric pressure. Where the air is cold, pressure tends to be high; warm air favors low pressures. The transfer of heat between ocean and air therefore alters the belts of high and low pressure; this profoundly affects the direction and strength of the winds and directs the storms on their paths.