S o DISCOVERY REPORTS 



upper stratum of this deep water has a high salinity which suggests that it is formed 

 chiefly from a current which climbs from a much greater depth at St. 668. It seems safer 

 to conclude that most of the poorly saline water is carried away towards the north, and 

 to assume that the southward movement at 800 m. at St. 668 is only part of an eddy 

 (see pp. 94, 95). 



THE SURFACE CURRENT 



With the possible exception of a small stretch in the eastern part of the Pacific Ocean 

 the sub-Antarctic Zone lies wholly within the region of the prevailing westerly wind. 

 The currents set up by this wind are, according to Ekman's theory, a pure wind drift 

 towards the north-east at the surface, and a convection current, with or without a slope 

 current, towards the east in the deeper water ; the resultant movement at the surface 

 and at depths less than the limit of the wind's frictional influence will therefore be 

 directed to some point north of east. The general existence of such a current is on the 

 whole borne out by all the evidence available. In 30 W, for example, the uniform 

 nature of the water in the first 60-80 m. suggests that it moves in more or less the same 

 direction as the surface current, which the current charts show to flow north of east. 

 Below 80 m. there are indications of a different current — a subsurface current towards 

 the south, and the existence of such a movement at that depth is a reliable indication 

 that the northward movement due to the wind is confined to the first 80 m. 



Whilst the wind drives the surface water towards the north, another factor — the 

 difference of climate between the southern and northern parts of the zone, sets up a 

 density gradient which tends to cause a current in the opposite direction. The amount 

 of radiation falling on the southern part of the zone is considerably less than that which 

 falls on the northern part, and without the influence of the wind the colder and heavier 

 water found in the south would sink and be replaced by a southward current at the 

 surface. Sverdrup (1934, pp. 315-17) has calculated that the southward movement 

 formed in this way is likely to be of the same order of magnitude as the northward 

 current set up by the wind. The calculations are of course only approximate, but they 

 indicate that the combined effect of wind and thermohaline differences will leave the 

 surface water with no positive movement either to the north or south. 



A further factor which is likely to influence the speed and direction of the surface 

 current is the addition of Antarctic water to the sub-Antarctic Zone from the south. 

 Krummel (1911, II, pp. 680-1), who compared the current observations received at the 

 Deutsche Seewarte from the strip of ocean between 40-45 S and 20-1 20 E, with the 

 theoretical wind currents based on the charts drawn by Koppen, found that the surface 

 current was stronger in summer than in winter, although the wind current should be 

 weaker. Michaelis (1923, p. 29) also shows that the West Wind Drift has a more 

 northerly direction in summer than in winter, and attributes the difference to a much 

 greater northward movement of thaw-water from melting ice in summer. This con- 

 clusion does not conflict with the theory that the bulk of the Antarctic water sinks below 

 the surface at the Antarctic convergence because the sub-Antarctic current has its 

 origin in the region of mixed water which absorbs the Antarctic current. 



