THE WATER MASSES 179 



minimum finally disappears at T = 6-6° C, S = 34-95 % in about 15 N., so we can consider that the 

 antarctic intermediate layer off South-west Africa contains only about 50 % of the original water type. 



Above the antarctic intermediate water core, and to within 100 m. of the surface, all the 'William 

 Scoresby' observations fall fairly close to a nearly straight line joining the points T = 14-0° C, 

 S = 35-20 % and T = 4-7° C, S = 34-40 % . It will be seen that this line lies somewhat to the left 

 of Sverdrup's curve for the South Atlantic central water (Fig. 32) (Sverdrup et al. 1946), but it 

 should be remembered that Sverdrup has taken a mean curve for the whole South Atlantic. 



Above 100 m. external influences come into play and the water becomes subjected to heating, 

 cooling, evaporation, etc., and these account for the more widely spaced distribution of the T-S points. 

 But the effect of these external influences is fairly well defined and with caution it is possible to derive 

 at least some information concerning the surface water-layers from their T-S relationships. 



Water masses of the upper layers (0-200 m.) 

 Oceanic and coastal surface-waters 

 The different types of surface-water which were recognized from the general distribution of tempera- 

 ture and salinity (p. 174), form, of course, a natural grouping for the T-S curves of the upper water 

 masses. As the upwelled water has been brought to the surface from some subsurface depth offshore, 

 its T-S characteristics are similar to those of the water at the depth from which it was upwelled, 

 except that it has undergone a certain amount of modification in the process of uplift. 



If we compare an inshore and offshore station on the first survey — stations WS 1000 and 996 

 (Fig. 33 a) — it is clear that at the inshore station the upper 50 m. is composed of a mass of water 

 corresponding to that at 200 m. at the offshore station, but warmed up, reaching a temperature of 

 18 C. at the surface. The low salinity (34-9 % ) of this water on the surface contrasts strongly with 

 the high salinity (35-18 % ) of the oceanic surface-water. Similarly on survey II (in Fig. 336) there 

 is again a contrast, although rather less pronounced. 



The T-S diagrams also demonstrate clearly the mixed nature of the water inshore at Walvis Bay 

 on survey I. Comparing Fig. 33 c with Fig. 33a, we can see that the surface-water at stations WS 979 

 and 980 lie somewhere between the true oceanic and true coastal characteristics. 



Finally, for the 25 S. line on the first survey the T-S diagrams (Fig. 33d) show the 'oceanic' 

 characteristics of the inshore 'station'. 



We see, therefore, that the upwelled water originates from the South Atlantic central water, at 

 depths of 200-300 m. Apparently the upwelled water is not subjected to any significant mixing with 

 the warmer and more saline 'offshore' surface-waters in its process of uplift. Indeed, everything 

 suggests that it remains quite discrete from the latter. On the sea surface, as we have already seen, 

 there is in most cases a sharp boundary between the two types of water except in areas where mixing 

 has obviously taken place. 



It is evident that these two water masses form, on both surveys, a series of eddies along the coast, 

 tongues of upwelled water diverging offshore and inter-locking with wedges of offshore water con- 

 verging towards the coast. It is difficult to generalize about the depth to which the eddies extend, as 

 some of them appear to retain their identity to the depth of upwelling while others are very much 

 shallower. 



Dynamic height anomalies 

 Owing to the lack of direct observations of surface-currents it has been necessary to resort to some 

 indirect method of estimating the actual water movements which were taking place during the surveys. 

 The most widely used of such methods is that based upon the theorem developed by Bjerknes 



