THE ANTARCTIC CONVERGENCE 23 



southward current of warmer water. Actually, the Antarctic water is unable to sink 

 vertically because it is borne up by the highly saline warm deep water which has an even 

 greater density, but it gives rise to a northward movement just above the warm deep 

 water. The surface water also flows northwards owing to the influence of the prevailing 

 west wind, and thus, in the northern part of the zone, there is generally a northward 

 movement in the whole of the Antarctic layer. 



All the Antarctic water is heavier than the warm surface waters farther north, and 

 although such a density distribution is stabilized to some extent by the wind, the water 

 would sink if it were not prevented by the highly saline deep water. As soon, however, 

 as the northward current has passed the point where the deep water climbs steeply 

 towards the surface it is no longer prevented from sinking, and the sections suggest that 

 the Antarctic water flows over the steep ascent of the warm deep water like a stream 

 over a waterfall. 



Where the Antarctic water has left the surface there is a sufficient depth of water above 

 its new level to permit a movement of warmer sub-Antarctic water towards the south. 

 Such a movement is known to exist, although not actually at the surface, and it brings 

 the warm sub-Antarctic water into contact with the cold Antarctic water. Where the 

 two waters meet there is generally a sharp boundary which at sea-level is known as the 

 Antarctic convergence. 



The surface water in the sub-Antarctic Zone generally has a northward movement 

 which appears to be caused in a greater measure than the similar movement in the 

 Antarctic Zone by the influence of the prevailing west wind. The existence of a sharp 

 boundary at the surface, such as the Antarctic convergence, which is marked by a sudden 

 change of surface temperature, implies, however, that there is a break between the 

 northward currents in the two zones : the Antarctic current does not flow without in- 

 terruption across the convergence and become a sub-Antarctic current, but sinks below 

 the sub-Antarctic water. 



Formerly I thought that a sharp convergence was formed because the Antarctic 

 water flowed northwards more quickly than the sub-Antarctic water (1933, p. 187, and 

 1934, 1, p. 129). A later suggestion by Sverdrup (1934, pp. 316-17) was that the effect of 

 the wind on the sub-Antarctic water might be exceeded by the effect of the thermohaline 

 differences, which tend to set up a southward movement. If, however, the convergence 

 is formed because the northward current of Antarctic water sinks from the surface at a 

 point determined by the deep water movements, a density difference between the two 

 surface waters is all that is necessary to set up a sharp boundary. Where there is, for 

 some reason, only a small difference between the densities of the Antarctic and sub- 

 Antarctic waters a sharp convergence will not be expected. 



The close agreement between the geographical position of the surface boundary and 

 the zone of steepest ascent of the deep water is shown by Fig. 5, which gives, in addition 

 to the temperature observations at a depth of 2500 m., the position of the Antarctic 

 convergence determined from surface observations. 



Since the latitude in which the deep water makes its sudden upward movement is 



