HYDROLOGY OF THE SOUTH ATLANTIC 185 



North of 66° S the wind moves water northwards and assists the thermohahne influence. 

 It is considered by some to be the sole cause of the south to north movement and also 

 of the circulation in the ocean as a whole. Defant (1929, p. 133) says: "The thermo- 

 hahne circulation is therefore quite a surface phenomenon; the lower layers in the 

 ocean have no part in it ", and on p. 134, " It is not to be doubted that in contrast with 

 the thermohaline influences just mentioned, ocean currents produced by winds are 

 more important, and give rise to the final structure of current systems in the sea". 

 Defant also says on p. 136: "While acknowledging the winds to be the principal factor 

 giving rise to a horizontal circulation, we must not overlook the importance of density 

 differences as the origin of vertical circulation ". There seems to be some contradiction 

 between the three quotations, unless a circulation caused by density differences is dis- 

 tinct from a thermohaline circulation. 



Our observations show that all the movement in the Antarctic surface layer cannot 

 be explained entirely as the result of wind currents although the zonal movements are 

 probably due almost entirely to wind forces. The south to north movements, particu- 

 larly below the surface stratum, are part of a continuous circulation, which is the result 

 of density differences. In the opinion of the author the density differences are largely 

 thermohaline differences which are the result of the different climates in different parts 

 of the ocean. The distribution of density may however be modified by the effects of 

 wind and of neighbouring land masses, which give rise to movements of water from 

 one climatic region to another. In the deep layers the density distribution is influenced 

 by the topography of the sea bottom : a movement of heavy deep water may be forced 

 to follow a particular channel, or it may be stopped by a ridge. 



Before discussing the effect of wind on the movement of Antarctic surface water, it is 

 necessary to describe the general conclusions which Ekman (1928) has reached on the 

 effect of winds on the sea. Ekman distinguishes four types of current : 



(i) A PURE DRIFT CURRENT caused by the effect of the resultant tangential force of the 

 wind on the surface of the sea. The velocity of the water at the surface is deduced on the 

 assumption that the sea is homogeneous, and that the coefficient of friction is constant, 

 to be directed at 45° cum sole^ from the direction of the wind. The direction of the surface 

 current turns uniformly cum sole with depth, and at the same time its velocity decreases 

 according to a logarithmic spiral until it is only one twenty-third of its surface value 

 when the direction of movement has turned through two right angles. The depth at 

 which this takes place has been called by Ekman "the depth of frictional influence", 

 since, if small velocities less than 5 per cent of the original one are neglected, it shows 

 the greatest depth to which horizontal motion can be transferred by friction. The depth 



of frictional influence is written D, and according to Ekman D = / . , , where iv is 



Vsm(/) 



the speed of the wind in m./sec, and </> is the latitude. The formula, as pointed out by 



Ekman, can only be regarded as approximate; it gives a depth of 80 m. for a wind of 



^ Cum sole is the direction of the sun's apparent azimuthal motion — anticlockwise in the southern hemi- 

 sphere. Contra solein is the opposite direction — clockwise in the southern hemisphere. 



