220 DISCOVERY REPORTS 



such growth as can exist on the small amounts of phosphate which are obtained from 

 > the sub-tropical under-layer. 



EXTENT OF THE SUB-TROPICAL AND TROPICAL ZONES 

 The tropical convergence, between tropical and sub-tropical waters, is not so well 

 defined as the other convergences. It is found at the surface where the increase in sur- 

 face temperature from south to north becomes more rapid, and where the surface 

 stratum becomes shut off from sub-tropical water by a sharp discontinuity. In the 

 South Atlantic Ocean it corresponds in summer with the 23° C. isotherm. 



The tropical Zone extends to about 28° S in the western half of the ocean where the 

 Brazil current carries tropical water southwards, but only to 10-15° S in ^^^ ^^^ where 

 the Benguela current carries sub-tropical water northwards. The boundary between 

 it and the sub-tropical Zone lies between St Helena and Ascension. Hydrologically 

 St Helena is sub-tropical, and Ascension tropical. The water off the Cape of Good 

 Hope, and ofT the west coast of Africa as far as 10-15° S, is sub-tropical; but close 

 inshore, particularly south of 20° S, it is also influenced by upwelling water from the 

 Antarctic intermediate layer. 



THE DEEP WATERS OF THE SOUTH ATLANTIC OCEAN 



There are three principal deep layers in the South Atlantic Ocean. They are: the 

 Antarctic intermediate layer, the warm deep layer, the Antarctic bottom layer. 



It has already been shown that the Antarctic surface water which sinks at the Ant- 

 arctic convergence mixes with sub-Antarctic water in a region of intense vertical mixing 

 just north of the Antarctic convergence. The mixture of water then sinks downwards 

 towards the north, and spreads over the whole of the South Atlantic Ocean. It gives 

 rise to a layer of water which is both Antarctic and sub-Antarctic in origin, and it is 

 found farther north between layers of sub-tropical water, and warm deep water, both 

 of which are of sub-tropical origin. The layer has therefore been called the Antarctic 

 intermediate layer, and the water Antarctic intermediate water. 



Below the Antarctic surface water in the Antarctic Zone, below the sub-Antarctic 

 water in the sub-Antarctic Zone, and below the Antarctic intermediate layer farther 

 north there is a layer of water which has been called the warm deep layer. It is most 

 probable that all the water in the warm deep layer is not of the same origin. Brennecke 

 (1921) and WiJst (1928) have shown that a stream of North Atlantic deep water flows 

 southwards in the South Atlantic from its origin in the sub-tropical regions of the North 

 Atlantic. Clowes (1933) has shown that south of 46° S the water in the layer is derived 

 from the Pacific Ocean. In the Antarctic Zone the maximum temperature in a vertical 

 column of water is found in the warm deep layer, but north of the Antarctic convergence 

 there is only a secondary temperature maximum in the layer because there are warmer 

 layers of water nearer the surface. 



Below the warm deep layer there is a heavy type of Antarctic water which has been 

 formed by the cooling of warm deep water without appreciable dilution. It sinks near 



