ANTARCTIC SURFACE WATER: WEST WIND DRIFT 37 



There were two similar strata at St. 891 ; the mean temperature in the first 100 m. 

 was 3-09° C, whilst it was only 1-82° C. between 150 and 200 m., the high temperature 

 of the surface water again shows that the surface water south of the convergence was 

 mixed with sub-Antarctic water. The biological data, obtained from an examination of 

 nets fished between the surface and a depth of 260 m., show that both Antarctic and 

 sub-Antarctic species were present, but the greater proportion of the sub-Antarctic 

 species was found in the first 100 m. At St. 891, where the surface water was not so 

 warm as at St. 883, the sub-Antarctic species were fewer. 



Although the presence of a certain amount of sub-Antarctic water in the surface 

 stratum south of the convergence shows that there must be a current towards the south, 

 the sudden temperature change of i-8 and 2-0° C. where the main body of Antarctic 

 water sinks, proves that the current is not continuous. The warm water south of the 

 convergence is probably carried there by occasional movements in which tongues of 

 sub-Antarctic water are driven across the usual position of the convergence towards 

 the south-east. There is evidence of such a tongue of water at St. 891, where the surface 

 temperature was 0-5° C. higher than it was farther north, between the station and the 

 convergence. 



The southward movement in the neighbourhood of the convergence may be due to 

 the West Wind Drift being forced into a slightly higher latitude in order to pass south- 

 ward of Australia : it is also possible that the movement is only a seasonal feature. The 

 thermohaline differences between the south and north of the Antarctic zone tend to set 

 up a southward movement at the surface, but generally, owing to the influence of the 

 prevailing west wind, the current flows in the opposite direction towards the north 

 (see p. 13). In summer the northward movement is strengthened by the addition of 

 fresh water from melting ice and snow, but in winter, when this support is removed, 

 there is a greater possibility that the effect of the wind should sometimes be too weak 

 to prevent a southward movement. The fact that the convergence lies comparatively far 

 south, where the the west wind is beginning to weaken, also favours the effect of the 

 thermohaline differences. 



South of the Tasman Sea the convergence makes a small advance towards the north 

 and regains its sharp definition. In sections 12 and 13 (Plates XXV-XXX) it is marked 

 by temperature differences of 2-5 and 3-5° C. The temperature and salinity distribution 

 show that the water movements in the warm deep layer have lost the irregularity which 

 they exhibited south of Australia and the layer has the clearly defined steep slope which 

 is more typical of the Southern Ocean. 



South-east of New Zealand, where the convergence recedes once more towards the 

 south, the observations in section 14 (Plates XXXI-XXXIII) show that the warm deep 

 water makes only a gradual ascent over a weak bottom current. The distribution of 

 temperature and salinity again indicates that the Antarctic convergence is formed at the 

 point where the warm deep water lies deep enough for the Antarctic water to sink as 

 a whole below the sub-Antarctic water, and the convergence is not so well defined as 

 in regions where the bottom current is stronger and the warm deep layer climbs 



