THE SOUTHERN OCEANS 201 



The surface water of the southernmost, the Antarctic, zone is formed near the Ant- 

 arctic continent and is a mixture of fresh water from melting ice and snow with up- 

 welling deep water from the north (a part of the warm deep current to be described 

 later). It is cold and poorly saline. It moves with the prevailing winds — towards the 

 west near the continent, towards the east farther north — but it has as well a northerly 

 flow which is probably small compared with the east or west movement. At a latitude 

 which is determined by the deep-water movements (see Deacon, 1937, pp. 21-24) the 

 Antarctic water sinks below the surface to form a deep current. Here, in this same 

 latitude, it meets at the surface the lighter water of the next zone, the sub-Antarctic, and 

 it is below this that it sinks sharply. The line along which this happens is known as the 

 Antarctic convergence. ^ The depth of the Antarctic surface layer varies from about 

 80 m. in the far south between the east and west currents to 250 m. near the Antarctic 

 convergence. Where the sub-Antarctic surface water meets the warmer water of the 

 subtropical Zone it sinks below the surface along a line known as the subtropical 

 convergence. There are reasons for believing that there is a less definite boundary 

 separating subtropical and tropical water farther north. The Antarctic convergence 

 runs uninterrupted by land around the southern hemisphere between the latitudes of 

 50 and 60° S. The course of the subtropical convergence between 37 and 47° S is 

 broken by South America and New Zealand. 



Sub-Antarctic surface water is formed from the northward flowing Antarctic water 

 which has sunk below the surface at the Antarctic convergence and from subtropical 

 water which enters the sub-Antarctic Zone from the north as a subsurface current. 

 Its depth increases from south to north and it is about five times as thick as the Antarctic 

 surface layer. Its strongest movement is towards the east, but it generally has a slow 

 movement to the north as well. The subsurface stratum, which flows towards the south, 

 is a mixture of more saline water from the subtropical Zone and of sub-Antarctic 

 surface water itself that has sunk below the surface at the subtropical convergence. 

 In the most southerly part of the sub-Antarctic Zone, for about 100 miles north of the 

 Antarctic convergence, there is a region of intense vertical mixing where this warmer, 

 more saline, water from the north is mixed with water from the surface of the Antarctic 

 Zone. From this region the current known as the Antarctic intermediate current sinks 

 and flows towards the north. 



Below the northward flowing Antarctic intermediate current in the sub-Antarctic 

 Zone is the southward flowing warm deep current. It climbs rapidly towards the surface 

 at the Antarctic convergence and flows south beneath the surface layer in the Antarctic 

 Zone. In the Atlantic and Indian Ocean sectors it comes from water which has sunk 

 below the surface in tropical and subtropical regions ; it is present in the Pacific Ocean 



'■ The flow of Antarctic water to the north is strongest in summer when ice and snow are melting in the 

 south. When new ice is being formed in winter it is considerably reduced. Our observations near the 

 Antarctic convergence in winter suggest that the slowing up of the current leads to a southern movement 

 of sub-Antarctic water at the surface ; north of the convergence was purely sub- Antarctic water, but the 

 Antarctic water south of it was mixed with sub-Antarctic water. At such times sub-Antarctic animals (e.g. 

 E. vallentini, see p. 214) may be found south of the convergence. 



