2i2 DISCOVERY REPORTS 



to 5-5° C. is normal; in winter a rise from i° to 3-5° C. occurs. The salinity in this region is about 

 33*93 %o- When the surface temperature change is not sharply marked, the position of the Antarctic 

 Convergence is indicated by the sinking of the coldest part of Antarctic Surface Water below 200 m. 

 For example, in March 1937 ' Discovery II ', crossed the convergence in 49 ° 50' S. north of St. 2022, 

 and although no very sharp change in the surface temperatures between Sts. 2022 and 2023 is apparent, 

 experience shows that the prominent northerly bend of the 2-00° C. isotherm between 200 and 300 m. 

 in this area (Text-fig. 19) indicates the position of the convergence. 



In August 1938, January 1939 and May 1936 the Antarctic Convergence lay in the positions 

 indicated in the temperature and salinity sections for these months (Text-figs. 15-16, 17-18, 21-22). 

 In April 1931, Deacon (1933) found it to occur along the meridian of 30 W., just north of St. 666; in 

 March 1938 its position lay between Sts. 2292 and 2293 (Mackintosh, 1946). 



(b) Sub-Antarctic Surface Water extends to greater depths than the Antarctic Surface layer. From 

 south to north, the surface temperatures increase from approximately 3 to 11-5° C. in winter, and 

 from 5-5° to 15-0° C. in summer; the salinity is low varying between 33-85 to 34-65 % . No dis- 

 continuity layers are set up and vertical mixing takes place. In consequence, it is not possible to 

 define any boundary between the Sub-Antarctic Surface Water and the underlying Antarctic Inter- 

 mediate layer. 



In the upper layer of Sub-Antarctic Surface Water movement is a little to the north of east con- 

 tinuously around the continent. In the middle layers the principal component of movement is to the 

 south, and below this to the north again (Text-fig. 14). Between the Falkland Islands and South 

 Georgia the position of the Antarctic Convergence is less sharply defined than elsewhere because Sub- 

 Antarctic Surface water moves southwards and rather confused conditions exist. South and west of 

 the Cape of Good Hope also, confused conditions are found because of an admixture of sub-tropical 

 water from the opposed Agulhas current. Bohnecke (1938) distinguished a secondary polar front in 

 the Sub-Antarctic Zone, which Deacon (1945) has confirmed from Discovery observations south of 

 the Agulhas current. Along the Greenwich Meridian this boundary lies approximately in latitude 

 45 S. and separates water of mixed origin in the southern half of the Sub-Antarctic Zone from more 

 stably stratified water in the northern half. It has a marked effect on the distribution of Pelagobia 

 longicirrata. 



The northern boundary of the Sub-Antarctic Zone is at the Sub-Tropical Convergence where Sub- 

 Antarctic Surface Water sinks but does not move to the north. It either mixes with Sub-Tropical 

 Water moving south or wells up to the surface. At the Sub-Tropical Convergence a much sharper 

 change of temperature and salinity takes place than at the Antarctic Convergence and it has a much 

 greater influence on the distribution of pelagic polychaetes. In Table 13, the hydrological details 

 and positions of the stations adjacent to the Sub-Tropical Convergence have been listed. 



Deacon (1933) has given an analysis of the conditions prevailing in April 1931 and further details 

 of the lines of stations made since 1936 are shown in Text-figs. 17-22. 



In the vicinity of Tristan da Cunha a variation of 6° of latitude in the position of the Sub-Tropical 

 Convergence is possible (Deacon, 1945), and St. 4, made in January 1925, with a surface temperature 

 °f I 4'59° C. and a salinity of 34-85 % , very close to the Sub-Tropical Convergence, is of uncertain 

 hydrological position. No details are available for St. 247 also near Tristan da Cunha, but at St. 254, 

 about io° to the west and slightly south, the surface temperature was 13-5° C. and salinity 35-i4% , 

 which in June, when these readings were made, indicates the station is north of the Sub-Tropical 

 Convergence. 



It will be noted from these hydrological readings that increase in temperature is the best indicator 

 of the position of the Sub-Tropical Convergence, salinity increase not always being marked. 



