RHINCALANUS GIGAS 287 



Drift. Where it meets with warmer, more saHne sub-Antarctic water the Antarctic sur- 

 face water sinks along a well-defined line known as the Antarctic convergence (Deacon, 

 1933' PP- I9°~3)- From one side of the Antarctic convergence to the other there is a 

 pronounced difference in surface temperature which is more marked at some places 

 than at others. The Antarctic convergence (Figs. 1-3) is thus held to be the boundary 

 between the Antarctic and sub-Antarctic Zones. Beneath this northward- and eastward- 

 flowing Antarctic surface layer is a very much thicker layer of warmer water flowing 

 southwards from the Atlantic, Pacific or Indian Oceans, known as the warm deep water. 

 "A certain amount of mixing must always take place between the two layers across the 

 discontinuity layer which separates them, especially in winter. . . . Warm deep water has 

 never itself been found at the surface although it has been found with its maximum tem- 

 perature at a depth of only 100 m.: it is always covered with Antarctic surface water" 

 (Deacon, 1933, p. 180). Along the Antarctic continental shelf, and in certain other 

 places, warm deep water wells upwards towards the surface. In the Falkland Sector this 

 upwelling is most pronounced along the west coast of Graham Land and the South 

 Shetlands " and along that part of the ridge known as the Scotia Arc which joins Join- 

 ville Island to the South Orkney Islands and the South Sandwich Islands" (Deacon, 

 1933, p. 181). 



Thus throughout the whole of the area covered by the present report two layers of 

 water must be taken into consideration — the northward- and eastward-moving Antarctic 

 surface water and the southward-moving warm deep water below it. The loo-o m. net 

 was almost always towed entirely in the Antarctic surface layer, but, as Tables II a-c 

 show, the lower net usually fished partly in the Antarctic surface layer and partly in the 

 warm deep layer. At a few stations, however, mostly in the positions mentioned above, 

 that is on the continental shelf or on the Scotia Arc where warm deep water wells up- 

 ward to the surface, the deep net was towed entirely in the warm deep water. These 

 stations are marked with two asterisks in Table II o-c. Those marked with one are 

 stations at which the upper net fished entirely in Antarctic surface water and the lower 

 net partly in Antarctic surface water and partly in warm deep water. Those which 

 bear no asterisk are stations at which both nets fished in Antarctic surface water since 

 the discontinuity layer lay below the range of the hauls. The tables show that at most 

 of the stations in the northerly part of the Antarctic Zone, where the discontinuity 

 between the two layers lay deep down, the deep net was worked in Antarctic surface 

 water. During the circumpolar cruise the deep net was almost always towed partly 

 in both layers of water, but at certain stations it was wholly in the deep layer. These 

 stations were situated either on the continental shelf or on the boundary between the 

 East and West Wind Drift currents shortly to be described, where again warm water 

 wells upward towards the surface. 



In the Falkland Sector water passes through the Drake Passage in an easterly direc- 

 tion, the West Wind Drift current being here constricted by the peninsula of Graham 

 Land on the south and by South America on the north. The Antarctic convergence passes 

 through the Drake Passage (Figs, i, 2 and 3), and in about 55° S and 49° W it turns north 



