HYDROLOGY OF THE AREA 7 



thickness, towards its northern boundary. Its surface temperature increases from 15-5 

 to 18' C. in the south to 23° C. in the north, when the tropical convergence is reached. 

 On crossing this the surface temperature rapidly increases to 26 or 27° C, marking the 

 appearance at the surface of the tropical layer next to be described. The sub-tropical 

 water continues on below this very warm tropical layer, the vertical discontinuity also 

 coinciding roughly with the 23° C. isotherm. The salinity of the sub-tropical surface 

 water ranges from about 34-90700 in the south to 36-0070: in the north. The amount 

 of nutrient ions present shows a striking decrease as compared with the sub-Antarctic 

 water, the phosphate content falling to about 10 mg. per m.^ north of the sub-tropical 

 convergence. Here also the nitrate content falls abruptly from 200 to 50 mg. nitrate 

 nitrogen per m.^* and to 10 mg. only a little farther north. The oxygen content of the 

 sub-tropical surface water is of the order of 93 per cent saturation, but north of the 

 tropical convergence, where this same water persists as an under-layer to the very warm, 

 shallow, tropical surface layer, its percentage saturation decreases from about 80 in 

 25° S, to 40 near the Equator, showing how very strongly marked is the discontinuity 

 between it and the tropical surface water, vertical mixing being practically impossible. 



This tropical surface water has a temperature range of from 23 to 28 or 29° C, the 

 latter being recorded just north of the Equator. The salinity maximum of over 37-00700 

 is found in about 15° S and the minimum of about 35-50 7 00 just north of the Equator. 

 It contains no detectable phosphate and only from o to 5 mg. of nitrate nitrogen per 

 m.^ As we have seen that vertical mixing with the deeper layers is impossible, the 

 probable source of this nitrate is the tropical thunderstorms. 



The tropical convergence (see Fig. i) is not so well defined as the other convergences, 

 but, as stated above, its position coincides roughly with the 23' C. summer isotherm. 

 It extends as far as 28" S in longitude 30° W, where the Brazil current is flowing south- 

 wards, but only to about 10-15° S in the east where the Benguela current is flowing 

 northwards, its boundary probably lying between the islands of St Helena and Ascension. 



One of the more deeply seated water masses may be of direct importance in con- 

 sidering the ph3i;oplankton. This is the warm water found immediately below the sur- 

 face layer within the Antarctic Zone. Previously this was thought to originate in the 

 North Atlantic according to the theory of Brennecke and Wust, but while this may 

 remain true of the more eastern portion of the South Atlantic, some recent hydrological 

 information^ suggests that the bulk of this water in the south-west may originate in the 

 Pacific Ocean, and enter the Scotia Sea through Drake Passage. It is therefore no longer 

 possible to regard this as a homogeneous water mass, though the similarity in tempera- 

 ture would seem to indicate that its ultimate mode of origin in the Pacific Ocean must 

 resemble that of the "North-Atlantic deep water". Pending further hydrological in- 

 formation the provisional term "warm deep water'' may be applied to it. Its nucleus is 

 shown by a maximum temperature at a depth of about 400 m. off South Georgia: farther 

 south it approaches the surface more closely. 



1 See Clowes, A. J., Nature, cxxxi, p. 189 (1933). 



