SURFACE-CURRENTS 153 



it died away completely. These calm conditions persisted until the inner end of the Luderitz line, 

 but towards the Orange river line the wind built up again. It did not, however, become very strong — 

 force 3-4 from the east of south. Off the Orange river mouth the wind dropped. 



On Survey II the wind was light after we left Simon's Town, but increased after Saldanha Bay to 

 force 4-5 and remained so until the seaward end of the Orange river line, except for a short calm spell 

 off the Orange river mouth. The passage north to Hollam's Bird Island was very calm, but as we 

 steamed seawards the wind increased from west of south, remaining between S.S.W. and S.S.E., force 

 4-5, up to Walvis Bay. The course seawards from Walvis Bay was attended by an increasing southerly 

 wind which reached gale force at the outer end of the line, and the ship was hove to against this for 

 nearly two days. The wind then fell quickly and the ship returned to Walvis Bay in light northerly airs. 

 At Walvis Bay the Berg wind already mentioned was encountered, and conditions remained variable 

 with a fair proportion of north-westerly wind up to Cape Frio, north of which the passage became 

 increasingly calm. 



SURFACE- CURRENTS 



Previous data 

 Since the ship was working to a strict time-schedule it was impracticable to make any direct observa- 

 tions on the amount of set and drift to which she was subjected. A few occasions, however, presented 

 themselves when trustworthy fixes (stellar or shore) could be correlated with an uninterrupted 

 passage of the ship, thereby giving an estimate of the amount of drift due to wind and currents. With 

 accurate wind observations it was then possible to get a fairly good idea of how much of the drift was 

 caused by the currents alone. These data, although useful in relation to the other oceanographical 

 observations, are insufficient to give any comprehensive picture of the circulation, and for this it has 

 been necessary to draw on other sources of information. 



In the South Atlantic Ocean the surface-currents take the form of a large anti-cyclonic gyral of water 

 movement, a circulation impelled principally by the south-east trade winds. These drive the surface 

 water in a westerly direction away from the African coast, and reaching South America it is returned 

 southwards in the Brazil current, and eventually back to Africa in a current flowing more or less in the 

 same direction as the Southern Ocean current but separated from the latter by the subtropical 

 convergence. 



Off the South-west African coast there is, therefore, a wind-driven transport of water, principally to 

 the west but rather more northerly in the south of the region (Table 4). It has already been noted 

 that the south-east trade, although very constant in force and direction offshore, becomes much more 

 variable nearer the coast in the proximity of the continental shelf, and consequently, considering 

 purely a wind-driven circulation, one would expect the currents also to behave with much greater 

 irregularity in this region. Unfortunately very few current observations have been made in the coastal 

 region since it lies off the track of the main shipping lanes, but sufficient exist to confirm the irregular 



Table 4. Mean set and drift of the south-east trade wind drift 



Latitude 12°-i8° S. Latitude i8°-24° S. Latitude 24°-30° S. 



