184 DISCOVERY REPORTS 



It may be said, therefore, that on the basis of the temperature-salinity relationship, it is not 

 practicable to distinguish any significantly distinct water masses in the layer 200-600 m., and also 

 that it is all characteristic of the South Atlantic central water. As will be shown later, however, when 

 dealing with the non-conservative properties oxygen and phosphate, a very marked distinction is 

 present in this layer. 



The antarctic intermediate water 

 The depth distribution of the observed salinity minimum in this layer has already been given in 

 Table 7. It is noteworthy that at its core off South-west Africa only 50% of the original water type 

 is present. Later it will be shown that the upwelling takes place from considerably lesser depths than 

 this, and it is in fact South Atlantic central water which is upwelled and not, as has often been supposed, 

 the Antarctic intermediate water. It is true that the former may contain a little of the latter but the 

 proportion of antarctic intermediate water must be extremely low. 



UPWELLING 



Previous work on the mechanism of upwelling 

 Several explanations have been put forward to account for the presence of the cold water along the 

 south-west African coast. The earliest idea held was that the Benguela current was a continuation of 

 the west wind drift, bringing down cold water from the higher latitudes of the antarctic. Ross (1847) 

 (see p. 132), however, demonstrated that this could not be true because warmer water was present to 

 the south of the Benguela current, cutting off any continuous flow from the antarctic. Yet this view 

 was maintained by some authorities even as late as 1910 (Engeler). Ross did not offer any satisfactory 

 alternative explanation to the South Polar current theory, but later workers gradually developed the 

 idea that the cold water must come up from subsurface layers. Several suggestions were made as 

 to how this uplift, or upwelling, came about. Witte (1880) deduced on theoretical considerations that 

 the upwelling must be brought about either by the effect of the earth's rotation on such a meridional 

 current as this, or possibly by offshore winds driving surface-water away from the coast. Murray 

 (1888, 1 891) showed the latter to occur in the lochs on the west coast of Scotland, where, in summer- 

 time, offshore winds produced a depression of the surface temperature along the shore. Buchanan 

 (1880) and Buchan (1895) held that the same process was of general application to the major upwelling 

 regions off the west coasts of the continents. Schott (1902), however, while considering that the 

 upwelling along the north-west African coast could be explained on these grounds (as the winds blow 

 either parallel to the coast or offshore), pointed out that this could not hold good for the south-west 

 African coast. Here the winds were principally onshore in the coastal region. 



Schott offered an alternative explanation. Since the main impulsive force of the Benguela current 

 was the south-east trade wind, the current would be deflected, under the influence of the earth's 

 rotation, away from the coast. The water thus removed must be replaced, and owing to the direction 

 of the coastline this would have to be a vertical compensation. This theory Schott supported with the 

 fact that where the African coast recedes to the north-east (in about 17 S.) the upwelling fades out 

 and becomes an irregular phenomenon, the compensation flow then being able to take place hori- 

 zontally on the sea surface from the north-east. 



Thorade (1909) states, with regard to the California current, that 'Auch dieser Grund kann fur uns 

 nicht in Betracht kommen, denn einmal ist unser Auftriebgebiet bedeutend weiter von Aquatorial- 

 strom entfernt, und dann entwickelt sich dieser [the trades] am Kraftigsten gerade wahrend der 

 Wintermonate, in denen die Auftrieberscheinung im Riickgange begriffen ist'. Thorade proceeded 



