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 1.5º 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 2º C. warmer in 1911 than in 1906; at 400 metres (218 fathoms) the difference was over 1º, and even at 800 metres (486 fathoms) it was about 0.75º C. warmer in 1911 than in 1906. At 1,000 metres (545 fathoms) the difference was only 0.3º.
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.