The South Pole Part 36

Fig. 9. -- Temperatures at Station 32 (In the Benguela Current, July 22, 1911), and at Station 6O (In the Brazil Current, August 19, 1911).

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 h.o.m.ogeneous, 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

[Fig. 10 and caption]

the sections rightly it must be borne in mind that the vertical scale is 2,000 times greater than the horizontal.

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 lat.i.tude. The effect is slight in the tropics, but great in high lat.i.tudes. This, so far as it goes, agrees with the

[Fig. 11 and captions]

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 Veslemoy (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 pa.s.sing, 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 a.s.sume 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 quant.i.ty 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 a.s.sume 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 quant.i.ty 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 ma.s.s of air 1C. From this it follows that the quant.i.ty 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

[Fig. 13]

Fig. 13. -- Temperatures at one of the "Fram's" and one of the "Challenger's" Stations, to the South of the South Equatorial Current were about the same as those of 1876, when the Challenger was on her way back to England.

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]

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]

Fig. 15. -- Temperatures at the "Planet's" Station 25, and the "Fram's"

Station 39 -- Both in the Neighbourhood of St. Helena

[Fig. 16]

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.

NOTES

[1] -- Fram means "forward," "out of," "through." -- Tr.

[2] -- This retrospective chapter has here been greatly condensed, as the ground is already covered, for English readers, by Dr. H. R. Mill's "The Siege of the South Pole," Sir Ernest Shackleton's "The Heart of the Antarctic," and other works. -- Tr.

[3] -- Anniversary of the dissolution of the Union with Sweden. -- Tr.

[4] -- Daengealso means "thrash." -- Tr.

[5] -- Unless otherwise stated, "miles" means English statute miles. -- Tr.

[6] -- A language based on that of the country districts, as opposed to the literary language, which is practically the same as Danish. The maal is more closely related to Old Norse. -- Tr.

[7] -- Named after Dr. Nansen's daughter. -- Tr.

[8] -- A vessel sailing continuously to the eastward puts the clock on every day, one hour for every fifteen degrees of longitude; one sailing westward puts it back in the same way. In long. 180deg. one of them has gone twelve hours forward, the other twelve hours back; the difference is thus twenty-four hours. In changing the longitude, therefore, one has to change the date, so that, in pa.s.sing from east to west longitude, one will have the same day twice over, and in pa.s.sing from west to east longitude a day must be missed.

[9] -- For the benefit of those who know what a buntline on a sail is, I may remark that besides the usual topsail buntlines we had six extra buntlines round the whole sail, so that when it was clewed up it was, so to speak, made fast. We got the sail clewed up without its going to pieces, but it took us over an hour. We had to take this precaution, of having so many buntlines, as we were short-handed.