284 DEACON [CHAP. 12 



currents. Water which sinks there, joined by some from the Mediterranean Sea, 

 spreads southwards at depths of 1500 to 3000 m across the equator into high 

 southern latitudes. Its course is more or less horizontal as far as about 50°S, 

 where it slopes upwards above colder water formed in the Antarctic that 

 spreads northwards along the bottom in the opposite direction. This Antarctic 

 bottom water can be traced well into the Northern Hemisphere. Above the 

 warm deep water at the southern end of the section there is a relatively shallow 

 layer of poorly saline surface water. Except for a thin surface stratum that may 

 be warmed to 2 or 3°C in summer, it is colder than the deep water, but it 

 remains above it because of its low salinity. The action of the wind makes it 

 move north as well as east, and its relatively high density compared with that 

 of the warmer surface waters farther north must also make it drain slowly to 

 the north above the more saline deep water. Temperature and salinity observa- 

 tions all round the continent show that it sinks rather abruptly below warmer 

 surface water as soon as it passes the latitude where the deep layer climbs in 

 the opposite direction. 



A. The Warm Deep Water 



The presence of a warm layer between the cold and bottom layers all round 

 the continent is evidence of a general southward transport in this layer, and 

 isohalines in vertical sections running southwards from the Atlantic, Indian 

 and Pacific Oceans indicate that it is supplied from the deep layers of these 

 oceans. The source of highly saline water in the North Atlantic Ocean seems to 

 play a predominant part. A layer of highly saline water can be seen extending 

 southwards from the North Atlantic Ocean and Straits of Gibraltar, eastwards 

 round the Southern Ocean and northwards in the Indian and Pacific Oceans, 

 getting less saline and deeper all the way. There is another and apparently much 

 smaller source of highly saline deep water in the Arabian Sea. Its high salinity, 

 temperature and low oxygen content distinguish it clearly as far as 20°S ; south 

 of this there is a second kind of highly saline water some 2°C colder and 2 cc/1. 

 richer in dissolved oxygen which must be derived largely from the Atlantic 

 Ocean. 



In the Pacific Ocean the deep and bottom layers appear everywhere to be 

 separated from highly saline surface or sub-surface water by a layer with 

 relatively low salinity derived from surface waters that sink in the Antarctic 

 and Arctic regions ; there seems to be no supply of highly saline water to the 

 deep and bottom layers in addition to that derived from the eastward flow 

 south of Australia and New Zealand. Further work may reveal some enrichment 

 of the deep layer from a source within the complicated system of island ridges 

 and basins north-west of New Zealand, but it seems rather unlikely. 



The warm deep layer between the cold Antarctic surface and bottom waters 

 is not, however, filled with water straight from the North Atlantic Ocean. Much 

 of it must be formed by mixing between northward and southward movements 

 in low and middle latitudes all round the circumpolar ocean and in the ocean 

 itself, especially perhaps where the levels of the deep and bottom currents 



