The Open Sea: The World of Plankton. Alister Hardy
of water in the deep layers of the ocean as well as near the surface, they have to consider a second kind of cause: the action of what are often termed Archimedian forces. These are the forces due to internal changes in the water-mass causing alterations in its density. Such changes may be due to the expansion or contraction of the water on being warmed or cooled; they may also be due to an increase in the salt-content caused by excessive evaporation of water at the surface, as in the tropics, or to a decrease in saltness caused by large additions of fresh water from melting ice or excessive rainfall. Whether these causes actually produce the deep water movements, or do no more than make the water take the path of least resistance, they have far-reaching effects, particularly in giving rise to vertical as well as horizontal differences in the great ocean basins. One of the most surprising discoveries of worldwide oceanography was made between the two world wars—indeed through the work of our own Discovery Expeditions and the German Meteor Expedition, in the Antartic and Atlantic Oceans; it is the fact that the heavy snow or rainfall and the melting ice in the Antarctic seas has an immense influence that extends across the equator into the northern hemisphere. It is so striking an example of these Archimedian forces that I cannot resist using it as an illustration.
FIG. 7
A section through the Atlantic Ocean, from latitude 55°S to 15°N along the meridian 30°W, showing the water of varying saltness (34–00/00 to 37–0 0/00) and the directions of the main water movements at different depths. It shows how the great Antarctic icecap extends its influence into the northern hemisphere. Redrawn in diagrammatic form from Deacon (1933).
The great ice-cap at the south pole dominates the oceans of the world. The Antarctic continent rising in a plateau to elevations of some 8,000 feet is covered with a sheet of ice many hundreds of feet thick; this is continually being added to by the frequent heavy falls of snow, and is constantly and slowly moving as a vast glacier to the coast and beyond into the ocean where it juts out as the floating ice barrier. This is shown on the left of Fig. 7. At its edge this barrier from time to time breaks up into the massive tabular icebergs so characteristic of the south polar seas. This is freshwater ice, which on melting, helps to form a cold but light surface layer; in spite of being colder it is lighter than the normal sea-water because its salt content is reduced by the addition of the fresh-water. All round the pole this cold surface layer flows away to the north. Below this is water that is heavier because it is just cooled and not diluted with fresh-water; this sinks and forms a cold current, also flowing north but over the ocean floor. To take the place of these two streams of water flowing away from the pole, a mass of warmer water flows southwards and wells up against the ice, to be itself diluted, cooled and turned north again. The surface current thus formed continues till it meets warmer water which, although more saline, is lighter because it is so much warmer; the cold current now dips below this warmer water but still travels northwards and can be traced to a point some 30° of latitude north of the equator; it then sinks and joins in the return flow going south again to complete the circulation.
Here we have a striking case of waters at different levels travelling in opposite directions: a layer going south flows in between two layers coming north. We shall see in a later chapter how the behaviour of some plankton animals is adapted in a most remarkable manner to take advantage of this fact.
Dr. G. E. R. Deacon, who has done so much to increase our knowledge of this remarkable system (1933 and 1937), while on one of the Discovery expeditions took water-samples all the way along the path of this northward-flowing current after it had dipped below the surface; when he had analysed them he found something very extraordinary. We have said that this water was of low salt-content because of the melted ice; it also has a high oxygen-content because of a great production of planktonic plants in the polar surface waters (due to the rich nutrient salts and to the long hours of daylight in high latitudes). Now as this water travels north the salt-content increases by diffusion from the surrounding layers and the oxygen-content is lowered by the respiratory requirements of animals. As he went along the path of the current Dr. Deacon obtained a clear indication that the salt and oxygen values did not increase and decrease respectively in a perfectly steady manner as one might have expected, but in a series of waves. As far could be judged from the graph of the increase in saltness there were seven undulations in the curve from south to north; likewise in a graph of oxygen-decrease there were also seven undulations. Now the crests of the undulations of one curve corresponded in position with the troughs of the undulations of the other curve; in other words as we passed along the stream of water, regions of higher oxygen-content and lower salinity alternated with regions of lower oxygen-content and higher salinity. This water had come originally from the Antarctic surface layer in which during the summer more ice melts and also more plants are produced than in the winter. More ice melting means a lowering of salinity and more plants mean a greater production of oxygen. Clearly these regions of lower salinity and higher oxygen-content alternating with regions of higher salinity and lower oxygen-content represent the water which left the antarctic surface layers in past summers and winters respectively. Dr. Deacon tells me that he now fears that there are not sufficient observations along the path of the current to make their number quite certain; but since the indicated rates of water movement agree very well with most other estimates, he feels that they give us a fairly reliable time scale for this great circulating system. This water takes at least seven years on its journey from the antarctic to the northern hemisphere!
There is a somewhat similar system of a cold and less saline surface current flowing away from the north polar basin due to melting ice and the fall of snow and rain, but it is not so far-reaching as that from the south because this precipitation is less and in addition the Arctic Ocean is almost entirely enclosed by submarine ridges; this cold stream dips below the warmer water at the northern boundary of the Gulf Stream. It is partly to replace this cold water stream that the extension of the Gulf Stream—the North Atlantic Drift—is carried so far to the northward of our islands and up the northern coasts of Scandinavia. Here we see how these Archimedian forces may contribute to the North Atlantic system.
The third factor affecting ocean-currents is a much more subtle one, due directly to the actual spin of our planet. This deflecting force of the earth’s rotation is sometimes called Corioli’s force, after the French physicist, though in fact it was carefully worked out by his countryman Laplace 60 years before; it applies to the atmosphere as well as the sea. It is not a cause of the initial motion of the water but a cause of its deflection. A body of water moving in any direction is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. The effect is greater towards the poles and reduced towards the equator; on the actual equator itself there is no such effect at all. It applies not only to water but to any moving object; we can perhaps understand it best by considering the effect upon a swinging pendulum. Let us suppose we could hang a fairly heavy weight, say of some 20 lbs., on a long string from a 100 foot tall gallows-like structure at the north pole; now if we set the weight swinging to-and-fro in the same direction as a straight line drawn in the snow beneath it, we should soon observe that its line of swing would deviate from the line in the snow. Its swing would be deflected in a regular fashion in a clockwise direction; even in ten minutes its path would be deflected 2½. Unperceived by us the earth and the gallows would be rotating in an anti-clockwise direction, but the heavy pendulum weight is swinging free and its path is not affected although the string at the top will twist. If we watched it for a full twenty-four hours we should see the path of the pendulum complete a deflection of 360° and once more for a moment swing directly above the line in the snow. If we repeated this experiment at the equator—drawing our line in the sand—we should see no such effect; if the pendulum was set swinging say north and south it would continue to swing thus as it would also continue to swing in any other direction in which we might choose to start it; here the earth makes no turning motion in relation to the swing; for the line in the sand and the line of swing are carried on together by the earth’s motion round its axis. At the south pole we should of course get a similar effect to that at the north pole except that the pendulum would appear to be deflected in an anti-clockwise direction. Swinging such a pendulum at places in different degrees of latitude will give a different