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).