34 DISCOVERY REPORTS 



other observations made in the same region show that it is generally sharp enough to be 

 recognized. 



The conditions near the convergence north of South Georgia are somewhat similar to 

 those east of the Falkland Islands ; there is a tendency, although a smaller one, for the 

 sub-Antarctic water to be carried southwards over the Antarctic water. The same is 

 true of the region between 20 and 25 ° W ; the data are not fully available at present, but 

 those at hand indicate that the convergence may lie as much as 100-150 miles south of 

 the latitude where the main body of the Antarctic water sinks. 



The movement of sub-Antarctic water across the Antarctic water in these regions, 

 especially east of the Falkland Islands, is no doubt facilitated by the trend of the 

 Antarctic convergence towards the north, across the path of the prevailing wind. 



Except for a small bend towards the south in the region north-east of the South 

 Sandwich Islands, the isotherms between the Falkland Sector and 0-10 W run slightly 

 north of east. The Antarctic convergence has been crossed at only a few points in this 

 area, but as far as can be seen at present it follows approximately the same course as the 

 isotherms; north-east of the South Sandwich Islands it lies in 50-50I S, and then it 

 advances to about 47J S in o-io° W; farther east, between o and 30 E, it recedes 

 gradually southwards. The trend of the isotherms and convergence suggests that the 

 principal movement of the Antarctic water in the northern part of the zone is towards 

 the east, but it also points to the existence of a northward movement. Without such a 

 movement the isotherms would recede towards the south as the surface water on its way 

 eastward became warmer and warmer. The advance towards the north in 0-10 W does 

 not necessarily suggest that the northward movement has a greater velocity in this 

 region, since the trend of the convergence, and the distance to which the Antarctic 

 water can flow northwards as a surface current, are determined primarily by the move- 

 ments of the bottom water. 



In 30 E the position of the convergence appears to be determined by a submarine 

 ridge. The temperature and salinity distribution in section 8 (Plates XIII-XIV) shows 

 that the northward flow of Antarctic bottom water is greatly restricted by a steep 

 narrow ridge in about 50 S. The least sounding obtained on the ridge was 2952 m., 

 but owing to a defect in the machine, the section 50 miles north of this point was not 

 sounded, and the actual depth of the water on the summit of the ridge may be much 

 less. The warm deep water climbs steeply over the ridge and the large volume of bottom 

 water which is pent up to the south of it, whilst the Antarctic surface water sinks steeply 

 in the opposite direction. The sudden sinking of the Antarctic water gives rise to a sharp 

 convergence above the ridge. 



The isotherms between 47 and 55 S in the western half of the Indian Ocean (Fig. 

 8) have been drawn with the help of those by Schott (1902, atlas) and Drygalski (1926, 

 pi. 5). They bend northwards across the Marion Island-Crozet Islands ridge, south- 

 wards across the Kerguelen gap between the Crozet Islands and Kerguelen, and north- 

 wards again in the neighbourhood of the Kerguelen-Gaussberg ridge. These fluctuations 

 are in all probability caused by the effect of the shallowing and deepening of the sea on 



