46 DISCOVERY REPORTS 



SUB-ANTARCTIC WATER 



At the Antarctic convergence the surface temperature increases suddenly towards the 

 north — generally about 2-5° C. Where the convergence is far north the change is from 

 1 to 3-5° C. in winter and from 3-5 to 6-o° C. in summer, and where it is far south from 

 o to 2-5° C. and 2-5 to 5-0° C. In the Pacific Ocean and the western part of the Atlantic 

 Ocean the rise of temperature is generally accompanied by a small increase of salinity — 

 from about 33-85 to 34-15 °/ 00 — but in the Indian Ocean and the eastern part of the 

 Atlantic Ocean the warm water just north of the convergence has practically the same 

 salinity as the Antarctic water just south of it. The warm water is a mixture of Antarctic 

 water with water from the north, and its mode of formation and properties suggest the 

 name sub-Antarctic water., Like the surface water in the Antarctic Zone it lies in a 

 poorly saline layer above the highly saline warm deep current, but owing to the much 

 greater depth of this current it is a much deeper layer. 



Observations made along the meridian of 30 W show that in 40-45 S the warm deep 

 current lies below 1 600-1 200 m., but south of 50 S — in the Antarctic Zone — it reaches 

 to within 250 m. of the surface. In the shallow space above it in the Antarctic Zone 

 there is only one outstanding water movement, a drift towards the north-east, but in the 

 much deeper layer of sub-Antarctic water there are at least three movements. The 

 general drift of the water is towards the east, but incorporated with this movement there 

 are northward currents of poorly saline water in the surface and deep strata, and a more 

 saline current towards the south in the subsurface stratum. The presence of such move- 

 ments is clearly indicated by the distributions of temperature, salinity and oxygen in 

 the layer; these are shown for 35-55 S in 30° W by Figs. 12-14. 



At the surface there is a well-mixed stratum, 60-80 m. deep, in which there are only 

 minor changes of temperature and salinity with depth. The direction of the current in 

 this water cannot be determined with certainty from the temperature and salinity dis- 

 tribution (see footnote, p. 48), but its uniformity suggests that it flows in more or less the 

 same direction as the surface current which has been determined directly by means of 

 current measurements and drift observations. 



Meyer's current chart of the Atlantic Ocean (1923) shows that the sub- Antarctic water 

 flows slightly north of east. Those of Michaelis (1923) indicate the same tendency for 

 the Indian Ocean, although in certain localities— north-west of Kerguelen, and south 

 of Australia and Tasmania— the current flows slightly south of east. In the Pacific Ocean 

 the preliminary chart drawn by Merz, and published by Wiist (1929, p. 41), shows that 

 the current is again generally slightly north of east. This general tendency is also in- 

 dicated by the drifts of icebergs and by the results of drift-bottle experiments. 



Kriimmel (191 1, II, pp. 677-8) gives a summary of such experiments. Bottles 

 liberated south of 40 S near the east coast of South America were recovered on the 

 south coast of Australia and the west coast of New Zealand. These drifts show that the 

 surface currents tend to flow northwards as well as to the east, but they indicate that 

 the northward movement is comparatively small. One record tells that a bottle liberated 



