In the earliest ages of the human race the sea formed an absolute barrier. Men looked out upon its immense surface, now calm and bright, now lashed by storms, and always mysteriously attractive; but they could not grapple with it. Then they learned to make boats; at first small, simple craft, which could only be used when the sea was calm. But by degrees the boats were made larger and more perfect, so that they could venture farther out and weather a storm if it came. In antiquity the peoples of Europe accomplished the navigation of the Mediterranean, and the boldest maritime nation was able to sail round Africa and find the way to India by sea. Then came voyages to the northern waters of Europe, and far back in the Middle Ages enterprising seamen crossed from Norway to Iceland and Greenland and the north-eastern part of North America. They sailed straight across the North Atlantic, and were thus the true discoverers of that ocean.
Even in antiquity the Greek geographers had assumed that the greater part of the globe was covered by sea, but it was not till the beginning of the modern age that any at all accurate idea arose of the extent of the earth’s great masses of water. The knowledge of the ocean advanced with more rapid steps than ever before. At first this knowledge only extended to the surface, the comparative area of oceans, their principal currents, and the general distribution of temperature. In the middle of the last century Maury collected all that was known, and drew charts of the currents and winds for the assistance of navigation. This was the beginning of the scientific study of the oceanic waters; at that time the conditions below the surface were still little known. A few investigations, some of them valuable, had been made of the sea fauna, even at great depths, but very little had been done towards investigating the physical conditions. It was seen, however, that there was here a great field for research, and that there were great and important problems to be solved; and then, half a century ago, the great scientific expeditions began, which have brought an entire new world to our knowledge.
It is only forty years since the Challenger sailed on the first great exploration of the oceans. Although during these forty years a quantity of oceanographical observations has been collected with a constant improvement of methods, it is, nevertheless, clear that our knowledge of the ocean is still only in the preliminary stage. The ocean has an area twice as great as that of the dry land, and it occupies a space thirteen times as great as that occupied by the land above sea-level. Apart from the great number of soundings for depth alone, the number of oceanographical stations — with a series of physical and biological observations at various depths — is very small in proportion to the vast masses of water; and there are still extensive regions of the ocean of the conditions of which we have only a suspicion, but no certain knowledge. This applies also to the Atlantic Ocean, and especially to the South Atlantic.
Scientific exploration of the ocean has several objects. It seeks to explain the conditions governing a great and important part of our earth, and to discover the laws that control the immense masses of water in the ocean. It aims at acquiring a knowledge of its varied fauna and flora, and of the relations between this infinity of organisms and the medium in which they live. These were the principal problems for the solution of which the voyage of the Challenger and other scientific expeditions were undertaken. Maury’s leading object was to explain the conditions that are of practical importance to navigation; his investigations were, in the first instance, applied to utilitarian needs.
But the physical investigation of the ocean has yet another very important bearing. The difference between a sea climate and a continental climate has long been understood; it has long been known that the sea has an equalizing effect on the temperature of the air, so that in countries lying near the sea there is not so great a difference between the heat of summer and the cold of winter as on continents far from the sea-coast. It has also long been understood that the warm currents produce a comparatively mild climate in high latitudes, and that the cold currents coming from the Polar regions produce a low temperature. It has been known for centuries that the northern arm of the Gulf Stream makes Northern Europe as habitable as it is, and that the Polar currents on the shores of Greenland and Labrador prevent any richer development of civilization in these regions. But it is only recently that modern investigation of the ocean has begun to show the intimate interaction between sea and air; an interaction which makes it probable that we shall be able to forecast the main variations in climate from year to year, as soon as we have a sufficiently large material in the shape of soundings.
In order to provide new oceanographical material by modern methods, the plan of the Fram expedition included the making of a number of investigations in the Atlantic Ocean. In June, 1910, the Fram went on a trial cruise in the North Atlantic to the west of the British Isles. Altogether twenty-five stations were taken in this region during June and July before the Fram’s final departure from Norway.
The expedition then went direct to the Antarctic and landed the shore party on the Barrier. Neither on this trip nor on the Fram’s subsequent voyage to Buenos Aires were any investigations worth mentioning made, as time was too short; but in June, 1911, Captain Nilsen took the Fram on a cruise in the South Atlantic and made in all sixty valuable stations along two lines between South America and Africa.
An exhaustive working out of the very considerable material collected on these voyages has not yet been possible. We shall here only attempt to set forth the most conspicuous results shown by a preliminary examination.
Besides the meteorological observations and the collection of plankton — in fine silk tow-nets — the investigations consisted of taking temperatures and samples of water at different depths The temperatures below the surface were ascertained by the best modern reversing thermometers (Richter’s); these thermometers are capable of giving the temperature to within a few hundredths of a degree at any depth. Samples of water were taken for the most part with Ekman’s reversing water-sampler; it consists of a brass tube, with a valve at each end. When it is lowered the valves are open, so that the water passes freely through the tube. When the apparatus has reached the depth from which a sample is to be taken, a small slipping sinker is sent down along the line. When the sinker strikes the sampler, it displaces a small pin, which holds the brass tube in the position in which the valves remain open. The tube then swings over, and this closes the valves, so that the tube is filled with a hermetically enclosed sample of water. These water samples were put into small bottles, which were afterwards sent to Bergen, where the salinity of each sample was determined. On the first cruise, in June and July, 1910, the observations on board were carried out by Mr. Adolf Schröer, besides the permanent members of the expedition. The observations in the South Atlantic in the following year were for the most part carried out by Lieutenant Gjertsen and Kutschin.
The Atlantic Ocean is traversed by a series of main currents, which are of great importance on account of their powerful influence on the physical conditions of the surrounding regions of sea and atmosphere. By its oceanographical investigations in 1910 and 1911 the Fram expedition has made important contributions to our knowledge of many of these currents. We shall first speak of the investigations in the North Atlantic in 1910, and afterwards of those in the South Atlantic in 1911.
The waters of the Northern Atlantic Ocean, to the north of lats. 80° and 40° N., are to a great extent in drifting motion north-eastward and eastward from the American to the European side. This drift is what is popularly called the Gulf Stream. To the west of the Bay of Biscay the eastward flow of water divides into two branches, one going south-eastward and southward, which is continued in the Canary Current, and the other going north-eastward and northward outside the British Isles, which sends comparatively warm streams of water both in the direction of Iceland and past the Shetlands and Faroes into the Norwegian Sea and north-eastward along the west coast of Norway. This last arm of the Gulf Stream in the Norwegian Sea has been well explored during the last ten or fifteen years; its course and extent have been charted, and it has been shown to be subject to great variations from year to year, which again appear to be closely connected with variations in the development and habitat of several important species of fish, such as cod, coal-fish, haddock, etc., as well as with variations in the winter climate of Norway, the crops, and other important conditions. By closely following the changes in the Gulf Stream from year to year, it looks as if we should be able to predict a long time in advance any great changes in the cod and haddock fisheries in the North Sea, as well as variations in the winter climate of North–Western Europe.
But the cause or causes of these variations in the Gulf Stream are at present unknown. In order to solve this
difficult question we must be acquainted with the conditions in those regions of the Atlantic itself through which this
mighty ocean current flows, before it sends its waters into the Norwegian Sea. But here we are met by the difficulty
that the investigations that have been made hitherto are extremely inadequate and deficient; indeed, we have no
Fig. 1. — Hypothetical Representation of the Surface Currents in the Northern Atlantic in April. After Nansen, in the Internationale Revue der gesamten Hydrobiologie and Hydrographie, 1912. knowledge even of the course and extent of the current in this ocean. A thorough investigation of it with the improved methods of our time is therefore an inevitable necessity.
As the Gulf Stream is of so great importance to Northern Europe in general, but especially to us Norwegians, it was not a mere accident that three separate expeditions left Norway in the same year, 1910 — Murray and Hjort’s expedition in the Michael Sars, Amundsen’s trial trip in the Fram, and Nansen’s voyage in the gunboat Frithjof — all with the object of investigating the conditions in the North Atlantic. The fact that on these three voyages observations were made approximately at the same time in different parts of the ocean increases their value in a great degree, since they can thus be directly compared; we are thus able to obtain, for instance, a reliable survey of the distribution of temperature and salinity, and to draw important conclusions as to the extent of the currents and the motion of the masses of water.
Amundsen’s trial trip in the Fram and Nansen’s voyage in the Frithjof were made with the special object of studying the Gulf Stream in the ocean to the west of the British Isles, and by the help of these investigations it is now possible to chart the current and the extent of the various volumes of water at different depths in this region at that time.
A series of stations taken within the same region during Murray and Hjort’s expedition completes the survey, and provides valuable material for comparison.
After sailing from Norway over the North Sea, the Fram passed through the English Channel in June, 1910, and the first station was taken on June 20, to the south of Ireland, in lat. 50° 50’ N. and long. 10° 15’ W., after which thirteen stations were taken to the westward, to lat. 58° 16’ N. and long. 17° 50’ W., where the ship was on June 27. Her course then went in a northerly direction to lat. 57° 59’ N. and long. 15° 8’ W., from which point a section of eleven stations (Nos. 15 — 25) was made straight across the Gulf Stream to the bank on the north of Scotland, in lat. 59° 88’ N. and long. 4° 44’ W. The voyage and the stations are represented in Fig. 2. Temperatures and samples of water were taken at all the twenty-four stations at the following depths: surface, 5, 10, 20, 30, 40, 50, 75, 100, 150, 200, 300, 400, and 500 metres (2.7, 5.4, 10.9, 16.3, 21.8, 27.2, 40.8, 54.5, 81.7, 109, 163.5, 218, and 272.5 fathoms) — or less, where the depth was not so great.
The Fram’s southerly section, from Station 1 to 13 (see Fig. 3) is divided into two parts at Station 10, on the Porcupine Bank, south-west of Ireland. The eastern part, between Stations 1 and 10, extends over to the bank south of Ireland, while the three stations of the western part lie in the deep sea west of the Porcupine Bank.
In both parts of this section there are, as shown in Fig. 3, two great volumes of water, from the surface down to depths greater than 500 metres, which have salinities between 35.4 and 35.5 per mille. They have also comparatively high temperatures; the isotherm for 10° C. goes down to a depth of about 500 metres in both these parts.
It is obvious that both these comparatively salt and warm volumes of water belong to the Gulf Stream. The more
westerly of them, at Stations 11 and 12, and in part 13, in the deep sea to the west of the Porcupine Bank, is probably
in motion towards the north-east along the outside of this bank and then into Rockall Channel — between Rockall Bank
and the bank to the west of the
Fig. 3. — Temperature and Salinity in the “Fram’s” Southern Section, June, 1910. British Isles — where a corresponding volume of water, with a somewhat lower salinity, is found again in the section which was taken a few weeks later by the Frithjof from Ireland to the west-north-west across the Rockall Bank. This volume of water has a special interest for us, since, as will be mentioned later, it forms the main part of that arm of the Gulf Stream which enters the Norwegian Sea, but which is gradually cooled on its way and mixed with fresher water, so that its salinity is constantly decreasing. This fresher water is evidently derived in great measure directly from precipitation, which is here in excess of the evaporation from the surface of the sea.
The volume of Gulf Stream water that is seen in the eastern part (east of Station 10) of the southern Fram section, can only flow north-eastward to a much less extent, as the Porcupine Bank is connected with the bank to the west of Ireland by a submarine ridge (with depths up to about 300 metres), which forms a great obstacle to such a movement.
The two volumes of Gulf Stream water in the Fram’s southern section of 1910 are divided by a volume of water, which lies over the Porcupine Bank, and has a lower salinity and also a somewhat lower average temperature. On the bank to the south of Ireland (Stations 1 and 2) the salinity and average temperature are also comparatively low. The fact that the water on the banks off the coast has lower salinities, and in part lower temperatures, than the water outside in the deep sea, has usually been explained by its being mixed with the coast water, which is diluted with river water from the land. This explanation may be correct in a great measure; but, of course, it will not apply to the water over banks that lie out in the sea, far from any land. It appears, nevertheless, on the Porcupine Bank, for instance, and, as we shall see later, on the Rockall Bank, that the water on these ocean banks is — in any case in early summer — colder and less salt than the surrounding water of the sea. It appears from the Frithjof section across the Rockall Bank, as well as from the two Fram sections, that this must be due to precipitation combined with the vertical currents near the surface, which are produced by the cooling of the surface of the sea in the course of the winter. For, as the surface water cools, it becomes heavier than the water immediately below, and must then sink, while it is replaced by water from below. These vertical currents extend deeper and deeper as the cooling proceeds in the course of the winter, and bring about an almost equal temperature and salinity in the upper waters of the sea during the winter, as far down as this vertical circulation reaches. But as the precipitation in these regions is constantly decreasing the salinity of the surface water, this vertical circulation must bring about a diminution of salinity in the underlying waters, with which the sinking surface water is mixed into a homogeneous volume of water. The Frithjof section in particular seems to show that the vertical circulation in these regions reaches to a depth of 500 or 600 metres at the close of the winter. If we consider, then, what must happen over a bank in the ocean, where the depth is less than this, it is obvious that the vertical circulation will here be prevented by the bottom from reaching the depth it otherwise would, and there will be a smaller volume of water to take part in this circulation and to be mixed with the cooled and diluted surface water. But as the cooling of the surface and the precipitation are the same there as in the surrounding regions, the consequence must be that the whole of this volume of water over the bank will be colder and less salt than the surrounding waters. And as this bank water, on account of its lower temperature, is heavier than the water of the surrounding sea, it will have a tendency to spread itself outwards along the bottom, and to sink down along the slopes from the sides of the bank. This obviously contributes to increase the opposition that such banks offer to the advance of ocean currents, even when they lie fairly deep.
These conditions, which in many respects are of great importance, are clearly shown in the two Fram sections and the Frithjof section.
The Northern Fram section went from a point to the north-west of the Rockall Bank (Station 15), across the northern
end of this bank (Station 16), and across the northern part of the wide channel (Rockall Channel) between it and
Scotland. As might be expected, both temperature and salinity are lower in this section than in the southern one, since
in the course of their slow northward movement the waters are cooled, especially by the vertical circulation in winter
already mentioned, and are mixed with water containing less salt, especially precipitated water. While in the southern
section the isotherm for 10° C. went down to 500 metres, it here lies at a depth of between 50 and 25 metres. In the
comparatively short distance between the two sections, the whole volume of water has been cooled between 1° and 2° C.
This represents a great quantity of warmth, and it is chiefly given off to the air, which is thus warmed over a great
area. Water contains more than 3,000 times as much warmth as the same volume of air at the same temperature. For
example, if 1 cubic metre of water is cooled 1°, and the whole quantity of warmth thus taken from the water is given
Fig. 4. — Temperature and Salinity in the “Fram’s” Northern Section, July 1910 to the air, it is sufficient to warm more than 3,000 cubic metres of air 1°, when subjected to the pressure of one atmosphere. In other words, if the surface water of a region of the sea is cooled 1° to a depth of 1 metre, the quantity of warmth thus taken from the sea is sufficient to warm the air of the same region 1° up to a height of much more than 3,000 metres, since at high altitudes the air is subjected to less pressure, and consequently a cubic metre there contains less air than at the sea-level. But it is not a depth of 1 metre of the Gulf Stream that has been cooled 1° between these two sections; it is a depth of about 500 metres or more, and it has been cooled between 1° and 2° C. It will thus be easily understood that this loss of warmth from the Gulf Stream must have a profound influence on the temperature of the air over a wide area; we see how it comes about that warm currents like this are capable of rendering the climate of countries so much milder, as is the case in Europe; and we see further how comparatively slight variations in the temperature of the current from year to year must bring about considerable variations in the climate; and how we must be in a position to predict these latter changes when the temperature of the currents becomes the object of extensive and continuous investigation. It may be hoped that this is enough to show that far-reaching problems are here in question.
The salinity of the Gulf Stream water decreases considerably between the Fram’s southern and northern sections. While in the former it was in great part between 35.4 and 35.5 per mille, in the latter it is throughout not much more than 35.3 per mille. In this section, also, the waters of the Gulf Stream are divided by an accumulation of less salt and somewhat colder bank water, which here lies over the Rockall Bank (Station 16). On the west side of this bank there is again (Station 15) salter and warmer Gulf Stream water, though not quite so warm as on the east. From the Frithjof section, a little farther south, it appears that this western volume of Gulf Stream water is comparatively small. The investigations of the Fram and the Frithjof show that the part of the Gulf Stream which penetrates into the Norwegian Sea comes in the main through the Rockall Channel, between the Rockall Bank and the bank to the west of the British Isles; its width in this region is thus considerably less than was usually supposed. Evidently this is largely due to the influence of the earth’s rotation, whereby currents in the northern hemisphere are deflected to the right, to a greater degree the farther north they run. In this way the ocean currents, especially in northern latitudes, are forced against banks and coasts lying to the right of them, and frequently follow the edges, where the coast banks slope down to the deep. The conclusion given above, that the Gulf Stream comes through the Rockall Channel, is of importance to future investigations; it shows that an annual investigation of the water of this channel would certainly contribute in a valuable way to the understanding of the variations of the climate of Western Europe.
We shall not dwell at greater length here on the results of the Fram’s oceanographical investigations in 1910. Only when the observations then collected, as well as those of the Frithjof’s and Michael Sars’s voyages, have been fully worked out shall we be able to make a complete survey of what has been accomplished.
In the South Atlantic we have the southward Brazil Current on the American side, and the northward Benguela Current on the African side. In the southern part of the ocean there is a wide current flowing from west to east in the west wind belt. And in its northern part, immediately south of the Equator, the South Equatorial Current flows from east to west. We have thus in the South Atlantic a vast circle of currents, with a motion contrary to that of the hands of a clock. The Fram expedition has now made two full sections across the central part of the South Atlantic; these sections take in both the Brazil Current and the Benguela Current, and they lie between the eastward current on the south and the westward current on the north. This is the first time that such complete sections have been obtained between South America and Africa in this part of the ocean. And no doubt a larger number of stations were taken on the Fram’s voyage than have been taken — with the same amount of detail — in the whole South Atlantic by all previous expeditions put together.
When the Fram left Buenos Aires in June, 1911, the expedition went eastward through the Brazil Current. The first
station was taken in lat. 36° 18’ S. and long. 43° 15’ W.; this was on June 17. Her course was then north-east or east
until Station 32 in lat. 20° 30’ S. and long. 8° 10’ E.; this station lay in the Benguela Current, about 800 miles from
the coast of Africa, and it was taken on July 22. From there she went in a gentle curve
Fig. 5. — The “Fram’s” Stations in the South Atlantic (June — August, 1911). past St. Helena and Trinidad back to America. The last station (No. 60) was taken on August 19 in the Brazil Current in lat. 24° 39’ S. and about long. 40° W.; this station lay about 200 miles south-east of Rio de Janeiro.
There was an average distance of 100 nautical miles between one station and the next. At nearly all the stations investigations were made at the following depths: surface, 5, 10, 25, 50, 100, 150, 200, 250, 300, 400, 500, 750, and 1,000 metres (2.7, 5.4, 13.6, 27.2, 54.5, 81.7, 109, 136.2, 163.5, 218, 272.5, and 545 fathoms). At one or two of the stations observations were also taken at 1,500 and 2,000 metres (817.5 and 1,090 fathoms).
The investigations were thus carried out from about the middle of July to the middle of August, in that part of the
southern winter which corresponds to the period between the middle of
Fig. 6. — Currents in the South Atlantic (June — August, 1911). December and the middle of February in the northern hemisphere We must first see what the conditions were on the surface in those regions in the middle of the winter of 1911.
It must be remembered that the currents on the two sides of the ocean flow in opposite directions. Along the coast of Africa, we have the Benguela Current, flowing from south to north; on the American side the Brazil Current flows from the tropics southward. The former current is therefore comparatively cold and the latter comparatively warm. This is clearly seen on the chart, which shows the distribution of temperatures and salinities on the surface. In lat. 20° S. it was only about 17° C. off the African coast, while it was about 23° C. off the coast of Brazil.
The salinity depends on the relation between evaporation and the addition of fresh water. The Benguela Current comes
Fig. 7. — Salinities and Temperatures at the Surface in the South Atlantic (June — August, 1911) regions where the salinity is comparatively low; this is due to the acquisition of fresh water in the Antarctic Ocean, where the evaporation from the surface is small and the precipitation comparatively large. A part of this fresh water is also acquired by the sea in the form of icebergs from the Antarctic Continent. These icebergs melt as they drift about the sea.
Immediately off the African coast there is a belt where the salinity is under 35 per mille on the surface; farther out in the Benguela Current the salinity is for the most part between 35 and 36 per mille. As the water is carried northward by the current, evaporation becomes greater and greater; the air becomes comparatively warm and dry. Thereby the salinity is raised. The Benguela Current is then continued westward in the South Equatorial Current; a part of this afterwards turns to the north-west, and crosses the Equator into the North Atlantic, where it joins the North Equatorial Current. This part must thus pass through the belt of calms in the tropics. In this region falls of rain occur, heavy enough to decrease the surface salinity again. But the other part of the South Equatorial Current turns southward along the coast of Brazil, and is then given the name of the Brazil Current. The volume of water that passes this way receives at first only small additions of precipitation; the air is so dry and warm in this region that the salinity on the surface rises to over 37 per mille. This will be clearly seen on the chart; the saltest water in the whole South Atlantic is found in the northern part of the Brazil Current. Farther to the south in this current the salinity decreases again, as the water is there mixed with fresher water from the South. The River La Plata sends out enormous quantities of fresh water into the ocean. Most of this goes northward, on account of the earth’s rotation; the effect of this is, of course, to deflect the currents of the southern hemisphere to the left, and those of the northern hemisphere to the right. Besides the water from the River La Plata, there is a current flowing northward along the coast of Patagonia — namely, the Falkland Current. Like the Benguela Current, it brings water with lower salinities than those of the waters farther north; therefore, in proportion as the salt water of the Brazil Current is mixed with the water from the River La Plata and the Falkland Current, its salinity decreases. These various conditions give the explanation of the distribution of salinity and temperature that is seen in the chart.
Between the two long lines of section there is a distance of between ten and fifteen degrees of latitude. There is, therefore, a considerable difference in temperature. In the southern section the average surface temperature at Stations 1 to 26 (June 17 to July 17) was 17.9° C.; in the northern section at Stations 36 to 60 (July 26 to August 19) it was 21.6° C. There was thus a difference of 3.7° C. If all the stations had been taken simultaneously, the difference would have been somewhat greater; the northern section was, of course, taken later in the winter, and the temperatures were therefore proportionally lower than in the southern section. The difference corresponds fairly accurately with that which Kr:ummel has calculated from previous observations.
We must now look at the conditions below the surface in that part of the South Atlantic which was investigated by the Fram Expedition.
The observations show in the first place that both temperatures and salinities at every one of the stations give the same values from the surface downward to somewhere between 75 and 150 metres (40.8 and 81.7 fathoms). This equalization of temperature and salinity is due to the vertical currents produced by cooling in winter; we shall return to it later. But below these depths the temperatures and salinities decrease rather rapidly for some distance.
The conditions of temperature at 400 metres (218 fathoms) below the surface are shown in the next little chart. This chart is based on the Fram Expedition, and, as regards the other parts of the ocean, on Schott’s comparison of the results of previous expeditions. It will be seen that the Fram’s observations agree very well with previous soundings, but are much more detailed.
The chart shows clearly that it is much warmer at 400 metres (218 fathoms) in the central part of the South Atlantic than either farther north — nearer the Equator — or farther south. On the Equator there is a fairly large area where the temperature is only 7° or 8° C. at 400 metres, whereas in lats. 2O° to 30° S. there are large regions where it is above 12° C.; sometimes above 13° C., or even 14°C. South of lat. 30° S. the temperature decreases again rapidly; in the chart no lines are drawn for temperatures below 8° C., as we have not sufficient observations to show the course of these lines properly. But we know that the temperature at 400 metres sinks to about 0° C. in the Antarctic Ocean.
At these depths, then, we find the warmest water within the region investigated by the Fram. If we now compare the distribution of temperature at 400 metres with the chart of currents in the South Atlantic, we see that the warm region lies in the centre of the great circulation of which mention was made above. We see that there are high temperatures on the left-hand side of the currents, and low on the right-hand side. This, again, is an effect of the earth’s rotation, for the high temperatures mean as a rule that the water is comparatively light, and the low that it is comparatively heavy. Now, the effect of the earth’s rotation in the southern hemisphere is that the light (warm) water from above is forced somewhat down on the left-hand side of the current, and that the heavy (cold) water from below is raised somewhat. In the northern hemisphere the contrary is the case. This explains the cold water at a depth of 400 metres on the Equator; it also explains the fact that the water immediately off the coasts of Africa and South America is considerably colder than farther out in the ocean. We now have data for studying the relation between the currents and the distribution of warmth in the volumes of water in a way which affords valuable information as to the movements themselves. The material collected by the Fram will doubtless be of considerable importance in this way when it has been finally worked out.
Below 400 metres (218 fathoms) the temperature further decreases everywhere in the South Atlantic, at first rapidly to a depth between 500 and 1,000 metres (272.5 and 545 fathoms), afterwards very slowly. It is possible, however, that at the greatest depths it rises a little again, but this will only be a question of hundredths, or, in any case, very few tenths of a degree.
It is known from previous investigations in the South Atlantic, that the waters at the greatest depths, several thousand metres below the surface, have a temperature of between 0° and 3° C. Along the whole Atlantic, from the extreme north (near Iceland) to the extreme south, there runs a ridge about half-way between Europe and Africa on the one side, and the two American continents on the other. A little to the north of the Equator there is a slight elevation across the ocean floor between South America and Africa. Farther south (between lats. 25° and 35° S.) another irregular ridge runs across between these continents. We therefore have four deep regions in the South Atlantic, two on the west (the Brazilian Deep and the Argentine Deep) and two on the east (the West African Deep and the South African Deep). Now it has been found that the “bottom water” in these great deeps — the bottom lies more than 5,000 metres (2,725 fathoms) below the surface — is not always the same. In the two western deeps, off South America, the temperature is only a little above 0° C. We find about the same temperatures in the South African Deep, and farther eastward in a belt that is continued round the whole earth. To the south, between this belt and Antarctica, the temperature of the great deeps is much lower, below 0° C. But in the West African Deep the temperature is about 2° C. higher; we find there the same temperatures of between 2° and 2.5° C. as are found everywhere in the deepest parts of the North Atlantic. The explanation of this must be that the bottom water in the western part of the South Atlantic comes from the south, while in the north-eastern part it comes from the north. This is connected with the earth’s rotation, which has a tendency to deflect currents to the left in the southern hemisphere. The bottom water coming from the south goes to the left — that is, to the South American side; that which comes from the north also goes to the left — that is, to the African side.
The salinity also decreases from the surface downward to 600 to 800 metres (about 300 to 400 fathoms), where it is only a little over 34 per mille, but under 34.5 per mille; lower down it rises to about 34.7 per mille in the bottom water that comes from the south, and to about 34.9 per mille in that which comes from the North Atlantic.
We mentioned that the Benguela Current is colder and less salt at the surface than the Brazil Current. The same
thing is found in those parts of the currents that lie below the surface. This is clearly shown in Fig. 9, which gives
the distribution of temperature at Station 32 in the Benguela Current, and at Station 60 in the Brazil Current; at the
Fig. 9. — Temperatures at Station 32 (in the Benguela Current, July 22, 1911), and at Station 60 (in the Brazil Current, August 19, 1911). various depths down to 500 metres (272.5 fathoms) it was between 5° and 7° C. colder in the former than in the latter. Deeper down the difference becomes less, and at 1,000 metres (545 fathoms) there was only a difference of one or two tenths of a degree.
Fig. 10 shows a corresponding difference in salinities; in the first 200 metres below the surface the water was about 1 per mille more saline in the Brazil Current than in the Benguela Current. Both these currents are confined to the upper waters; the former probably goes down to a depth of about 1,000 metres (545 fathoms), while the latter does not reach a depth of much more than 500 metres. Below the two currents the conditions are fairly homogeneous, and there is no difference worth mentioning in the salinities.
The conditions between the surface and a depth of 1,000 metres along the two main lines of course are clearly shown in the two sections (Figs. 11 and l2). In these the isotherms for every second degree are drawn in broken lines. Lines connecting points with the same salinity (isohalins) are drawn unbroken, and, in addition, salinities above 35 per mille are shown by shading. Above is a series of figures, giving the numbers of the stations. To understand the sections rightly it must be borne in mind that the vertical scale is 2,000 times greater than the horizontal.
Fig. 11. Salinities and temperatures in the Southern section (June-July, 1911).
Salinities between 35 per mille and 26 per mille is shown by horizontal shading; above 36 per mille by cross-hatching.
Fig. 12. Salinities and temperatures in the Northern section (July-August, 1911).
Many of the conditions we have already mentioned are clearly apparent in the sections: the small variations between the surface and a depth of about 100 metres at each station; the decrease of temperature and salinity as the depth increases; the high values both of temperature and salinity in the western part as compared with the eastern. We see from the sections how nearly the isotherms and isohalins follow each other. Thus, where the temperature is 12° C., the water almost invariably has a salinity very near 35 per mille. This water at 12° C., with a salinity of 35 per mille, is found in the western part of the area (in the Brazil Current) at a depth of 500 to 600 metres, but in the eastern part (in the Benguela Current) no deeper than 200 to 250 metres (109 to 136 fathoms).
We see further in both sections, and especially in the southern one, that the isotherms and isohalins often have an undulating course, since the conditions at one station may be different from those at the neighbouring stations. To point to one or two examples: at Station 19 the water a few hundred metres down was comparatively warm; it was, for instance, 12° C. at about 470 metres (256 fathoms) at this station; while the same temperature was found at about 340 metres (185 fathoms) at both the neighbouring stations, 18 and 20. At Station 2 it was relatively cold, as cold as it was a few hundred metres deeper down at Stations 1 and 3.
These undulating curves of the isotherms and isohalins are familiar to us in the Norwegian Sea, where they have been shown in most sections taken in recent years. They may be explained in more than one way. They may be due to actual waves, which are transmitted through the central waters of the sea. Many things go to show that such waves may actually occur far below the surface, in which case they must attain great dimensions; they must, indeed, be more than 100 metres high at times, and yet — fortunately — they are not felt on the surface. In the Norwegian Sea we have frequently found these wave-like rises and falls. Or the curves may be due to differences in the rapidity and direction of the currents. Here the earth’s rotation comes into play, since, as mentioned above, it causes zones of water to be depressed on one side and raised on the other; and the degree of force with which this takes place is dependent on the rapidity of the current and on the geographical latitude. The effect is slight in the tropics, but great in high latitudes. This, so far as it goes, agrees with the fact that the curves of the isotherms and isohalins are more marked in the more southerly of our two sections than in the more northerly one, which lies 10 or 15 degrees nearer the Equator.
But the probability is that the curves are due to the formation of eddies in the currents. In an eddy the light and warm water will be depressed to greater depths if the eddy goes contrary to the hands of a clock and is situated in the southern hemisphere. We appear to have such an eddy around Station 19, for example. Around Station 2 an eddy appears to be going the other way; that is, the same way as the hands of a clock. On the chart of currents we have indicated some of these eddies from the observations of the distribution of salinity and temperature made by the Fram Expedition.
While this, then, is the probable explanation of the irregularities shown by the lines of the sections, it is not impossible that they may be due to other conditions, such as, for instance, the submarine waves alluded to above. Another possibility is that they may be a consequence of variations in the rapidity of the current, produced, for instance, by wind. The periodical variations caused by the tides will hardly be an adequate explanation of what happens here, although during Murray and Hjort’s Atlantic Expedition in the Michael Sars (in 1910), and recently during Nansen’s voyage to the Arctic Ocean in the Veslemöy (in 1912), the existence of tidal currents in the open ocean was proved. It may be hoped that the further examination of the Fram material will make these matters clearer. But however this may be, it is interesting to establish the fact that in so great and deep an ocean as the South Atlantic very considerable variations of this kind may occur between points which lie near together and in the same current.
As we have already mentioned in passing, the observations show that the same temperatures and salinities as are found at the surface are continued downward almost unchanged to a depth of between 75 and 150 metres; on an average it is about 100 metres. This is a typical winter condition, and is due to the vertical circulation already mentioned, which is caused by the surface water being cooled in winter, thus becoming heavier than the water below, so that it must sink and give place to lighter water which rises. In this way the upper zones of water become mixed, and acquire almost equal temperatures and salinities. It thus appears that the vertical currents reached a depth of about 100 metres in July, 1911, in the central part of the South Atlantic. This cooling of the water is a gain to the air, and what happens is that not only the surface gives off warmth to the air, but also the sub-surface waters, to as great a depth as is reached by the vertical circulation. This makes it a question of enormous values.
This state of things is clearly apparent in the sections, where the isotherms and isohalins run vertically for some way below the surface. It is also clearly seen when we draw the curves of distribution of salinity and temperature at the different stations, as we have done in the two diagrams for Stations 32 and 60 (Fig. 9). The temperatures had fallen several degrees at the surface at the time the Fram’s investigations were made. And if we are to judge from the general appearance of the station curves, and from the form they usually assume in summer in these regions, we shall arrive at the conclusion that the whole volume of water from the surface down to a depth of 100 metres must be cooled on an average about 2° C.
As already pointed out, a simple calculation gives the following: if a cubic metre of water is cooled 1° C., and the whole quantity of warmth thus taken from the water is given to the air, it will be sufficient to warm more than 3,000 cubic metres of air 1° C. A few figures will give an impression of what this means. The region lying between lats. 15° and 35° S. and between South America and Africa — roughly speaking, the region investigated by the Fram Expedition — has an area of 13,000,000 square kilometres. We may now assume that this part of the ocean gave off so much warmth to the air that a zone of water 100 metres in depth was thereby cooled on an average 2° C. This zone of water weighs about 1.5 trillion kilogrammes, and the quantity of warmth given off thus corresponds to about 2.5 trillion great calories.
It has been calculated that the whole atmosphere of the earth weighs 5.27 trillion kilogrammes, and it will require something over 1 trillion great calories to warm the whole of this mass of air 1°C. From this it follows that the quantity of warmth which, according to our calculation, is given off to the air from that part of the South Atlantic lying between lats. 15° and 35° S., will be sufficient to warm the whole atmosphere of the earth about 2° C., and this is only a comparatively small part of the ocean. These figures give one a powerful impression of the important part played by the sea in relation to the air. The sea stores up warmth when it absorbs the rays of the sun; it gives off warmth again when the cold season comes. We may compare it with earthenware stoves, which continue to warm our rooms long after the fire in them has gone out. In a similar way the sea keeps the earth warm long after summer has gone and the sun’s rays have lost their power.
Now it is a familiar fact that the average temperature of the air for the whole year is a little lower than that of the sea; in winter it is, as a rule, considerably lower. The sea endeavours to raise the temperature of the air; therefore, the warmer the sea is, the higher the temperature of the air will rise. It is not surprising, then, that after several years’ investigations in the Norwegian Sea we have found that the winter in Northern Europe is milder than usual when the water of the Norwegian Sea contains more than the average amount of warmth. This is perfectly natural. But we ought now to be able to go a step farther and say beforehand whether the winter air will be warmer or colder than the normal after determining the amount of warmth in the sea.
It has thus been shown that the amount of warmth in that part of the ocean which we call the Norwegian Sea varies from year to year. It was shown by the Atlantic Expedition of the Michael Sars in 1910 that the central part of the North Atlantic was considerably colder in 1910 than in 1873, when the Challenger Expedition made investigations there; but the temperatures in 1910 were about the same as those of 1876, when the Challenger was on her way back to England.
Fig. 13. — Temperatures at one of the “Fram’s” and one of the “Challenger’s” Stations, to the South of the South Equatorial Current
We can now make similar comparisons as regards the South Atlantic. In 1876 the Challenger took a number of stations
in about the same region as was investigated by the Fram. The Challenger’s Station 339 at the end of March, 1876, lies
near the point where the Fram’s Station 44 was taken at the beginning of August, 1911. Both these stations lay in about
lat. 17.5° S., approximately half-way between Africa and South America — that is, in the region where a relatively
slack current runs westward, to the south of the South Equatorial Current. We can note the difference in Fig. 13, which
shows the distribution of temperature at the two stations. The Challenger’s station was taken during the autumn and the
Fram’s during the winter. It was therefore over 3° C. warmer at the surface in March, 1876, than in August, 1911. The
curve for the Challenger station shows the usual distribution of temperature immediately below the surface in summer;
the temperature falls constantly from the surface downward. At the Fram’s station we see the typical winter conditions;
we there find the same temperature from the surface to a depth of 100 metres, on account of cooling and vertical
circulation. In summer, at the beginning of the year 1911, the temperature curve for the Fram’s station would have
taken about the same form as the other curve; but it would have shown higher temperatures, as it does in the deeper
zones, from 100 metres down to about 500 metres. For we see that in these zones it was throughout 1° C. or so warmer in
1911 than in 1876; that is to say, there was a much greater store of warmth in this part of the ocean in 1911 than in
1876. May not the result of this have been that the air in this region, and also in the east of South America and the
west of Africa, was warmer during the winter of 1911 than during that of 1876? We have not sufficient data to be able
to say with certainty whether this difference in the amount of warmth in the two years applied generally to the whole
ocean, or only to that part which surrounds the position of the station; but if it was general, we ought probably to be
able to find a corresponding difference in the climate of the neighbouring regions. Between 500 and 800 metres (272 and
486 fathoms) the temperatures were exactly the same in both years, and at 900 and 1,000 metres (490 and 545 fathoms)
there was only a difference of two or three tenths of a degree. In these deeper parts of the ocean the conditions are
probably very similar; we have there no variations worth mentioning, because the warming of the surface and sub-surface
waters by the sun has no effect there, unless, indeed, the currents at these depths may vary so
Fig. 14. — Temperatures at one of the “Fram’s” and one of the “Valdivia’s” Stations, in the Benguela Current. much that there may be a warm current one year and a cold one another year. But this is improbable out in the middle of the ocean.
In the neighbourhood of the African coast, on the other hand, it looks as if there may be considerable variations even in the deeper zones below 500 metres (272 fathoms). During the Valdivia Expedition in 1898 a station (No. 82) was taken in the Benguela Current in the middle of October, not far from the point at which the Fram’s Station 31 lay. The temperature curves from here show that it was much warmer (over 1.5° C.) in 1898 than in 1911 in the zones between 500 and 800 metres (272 and 486 fathoms). Probably the currents may vary considerably here. But in the upper waters of the Benguela Current itself, from the surface down to 150 metres, it was considerably warmer in 1911 than in 1898; this difference corresponds to that which we found in the previous comparison of the Challenger’s and Fram’s stations of 1876 and 1911. Between 200 and 400 metres (109 and 218 fathoms) there was no difference between 1898 and 1911; nor was there at 1,000 metres (545 fathoms).
In 1906 some investigations of the eastern part of the South Atlantic were conducted by the Planet. In the middle of March a station was taken (No. 25) not far from St. Helena and in the neighbourhood of the Fram’s Station 39, at the end of July, 1911. Here, also, we find great variations; it was much warmer in 1911 than in 1906, apart from the winter cooling by vertical circulation of the sub-surface waters. At a depth of only 100 metres (54.5 fathoms) it was 2° C. warmer in 1911 than in 1906; at 400 metres (218 fathoms) the difference was over 1°, and even at 800 metres (486 fathoms) it was about 0.75° C. warmer in 1911 than in 1906. At 1,000 metres (545 fathoms) the difference was only 0.3°.
From the Planet’s station we also have problems of salinity, determined by modern methods. It appears that the
salinities at the Planet station, in any case to a depth of 400 metres, were lower, and in part much lower, than those
of the Fram Expedition. At 100 metres the difference was even greater than 0.5 per mille; this is a great deal in the
same region of open sea. Now, it must be remembered that the current in the neighbourhood of St. Helena may be regarded
as a continuation of the Benguela Current, which comes from the south and has relatively low salinities. It looks,
therefore, as if there were yearly variations of salinity in these
Fig. 15. — Temperatures at the “Planet’s” Station 25, and the “Fram’s” Station 39 — Both in the Neighbourhood of St. Helena
Fig. 16. — Salinities at the “Planet’s” Station 25 (March 19, 1906) And the “Fram’s” Station 39 (July 29, 1911). regions. This may either be due to corresponding variations in the Benguela Current — partly because the relation between precipitation and evaporation may vary in different years, and partly because there may be variations in the acquisition of less saline water from the Antarctic Ocean. Or it may be due to the Benguela Current in the neighbourhood of St. Helena having a larger admixture of the warm and salt water to the west of it in one year than in another. In either case we may expect a relatively low salinity (as in 1906 as compared with 1911) to be accompanied by a relatively low temperature, such as we have found by a comparison of the Planet’s observations with those of the Fram.
We require a larger and more complete material for comparison; but even that which is here referred to shows that there may be considerable yearly variations both in the important, relatively cold Benguela Current, and in the currents in other parts of the South Atlantic. It is a substantial result of the observations made on the Fram’s voyage that they give us an idea of great annual variations in so important a region as the South Atlantic Ocean. When the whole material has been further examined it will be seen whether it may also contribute to an understanding of the climatic conditions of the nearest countries, where there is a large population, and where, in consequence, a more accurate knowledge of the variations of climate will have more than a mere scientific interest.
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