SECT. 3] THE SOUTHERN OCEAN 291 



Current. At the surface, the transport, still well within the region of prevailing 

 west wind, must be north as well as east, but higher salinities in the sub- 

 surface layer indicate southward flow. This is the first sign of a southward 

 penetration of near-surface water into the Antarctic regime. It must contribute 

 fairly largely to the mixed water north of the Antarctic convergence. It seems 

 to be particularly strong in the Indian Ocean. There is generally a sharpening 

 of the rise in surface temperature where the increase of salinity becomes 

 noticeable. 



F. The Subtropical Convergence 



Some 10 degrees north of the Antarctic convergence there is another sharp 

 rise of surface temperature towards the north — from about 10° to 14°C in 

 winter and 14° to 18°C in summer. There is a sharp rise of salinity from about 

 34.3% to 34.9% . Details of the water circulation can only be inferred from 

 the temperature and salinity distribution, but the sharp change seems to be 

 caused by convergence of sub -Antarctic water moving northwards with sub- 

 tropical water moving southwards. The convergence is farthest south on the 

 western sides of the oceans, where warm water is carried southwards in the 

 Brazil Current, the Agulhas Current and the East Australian Current. Its 

 position, indicated in Fig. 2, is much more variable than that of the Antarctic 

 convergence, especially on the eastern side of the oceans, and it probably 

 depends largely on wind transport. It may be more appropriate to think of it as 

 a fairly wide region over which sharp frontal conditions move. South of the 

 Brazil, Agulhas and East Australian Currents there are generally isolated 

 patches of eddies of warm water separated from the main currents. The German 

 research vessel Meteor reported a very well defined crossing of the convergence 

 in 41°S, 22°E : the boundary appeared as a line of current disturbance at the sur- 

 face and the temperature rose 5.6°C in 1 mile and a total of 9.1°C in 5 miles. 



5. Climatology and Zoogeography 



D. D. John gave a graphic description of the significance of the Antarctic 

 convergence in a lecture to the Royal Geographical Society in 1934. After the 

 Discovery II had made observations all round the Antarctic continent, he said : 

 "It is a physical boundary very easily and precisely detected with a thermo- 

 meter by the sharp change in temperature as one passes from one zone to 

 another. It can be detected as easily if not so precisely by a zoologist with a 

 tow-net, because each of the two waters has a distinctive fauna of floating 

 animal life. The zoologist need only know the species of prawns of the genus 

 Euphausia to which E. superba, whale food, belongs. They are so numerous in 

 the surface that his net will always catch some. If, in the neighbourhood of the 

 convergence, he takes E. vallentini or E. longirostris he is in sub-Antarctic 

 water. He will have crossed the convergence and be in the Antarctic when his 

 net brings back E. frigida and not vallentini nor longirostris. But we, whether 

 sailors or scientists, know and will remember the convergence best in another 



