THE SUBTROPICAL CONVERGENCE 57 



is available so far, though not conclusive, shows that the sub-Antarctic water generally 

 has a movement north of east, whilst the subtropical current has a southward com- 

 ponent. Since the sub-Antarctic water is the heavier of the two currents it is more likely 

 to sink at the convergence. 



In 30 W (Figs. 12-14, pp. 47, 48), the boundary between the two waters was crossed 

 between 43 and 42 S, where the temperature at the surface increased suddenly from 

 9-5 to 13-5° C. and the salinity from 34-4 to 34-9 °/ 00 . The continuous surface 

 temperature record showed that there were minor fluctuations extending 2 or 3 farther 

 north, but the sudden change in 43-42 S evidently marked the principal boundary. 



The nature of the water movements in the neighbourhood of the convergence seems 

 to be indicated most clearly by the salinity distribution shown in Fig. 13. One of the 

 most positive indications given by the section is that there is an unbroken current to- 

 wards the south in the subsurface stratum, between 80 and 200 m. The current starts 

 from the subtropical region and can be followed almost to the Antarctic convergence. 

 Owing to the presence of this current any sub-Antarctic water sinking at the subtropical 

 convergence will be turned back towards the south between 80 and 200 m., and the 

 properties of the subsurface current indicate very clearly that it is partly composed of 

 such water (p. 63). 



Owing to its lesser density the subtropical water is less likely than the sub-Antarctic 

 water to sink at the convergence, and the observations suggest that it accumulates at the 

 surface north of the convergence until its increasing volume or the weakening of the 

 sub-Antarctic current causes it to advance towards the south above the sub-Antarctic 

 water, thus driving the convergence towards the south. The subtropical water at St. 

 1 165 in section 7 (Plates X-XII) may belong to such a movement. Between the surface 

 and a depth of 150 m. there is a stratum of highly saline subtropical water, but at 200 m. 

 above a well developed subsurface current, the water has the usual low salinity of 

 northward-flowing sub-Antarctic water. 



An increase in the strength of the sub-Antarctic current or a removal of the factors 

 which caused the subtropical water to advance towards the south will cause the con- 

 vergence to retreat towards the north. There are frequent indications that this has taken 

 place in such a way that some subtropical water has been left behind, and being isolated 

 from the main current this water has become mixed with sub-Antarctic water. The first 

 50 m. of water at St. 72 (Station List, 1929) in 41 43' S, 42 21' W, has a salinity of 

 34-67-34-69 °/ 00 . This is an abnormally high salinity for sub-Antarctic water and it sug- 

 gests that the surface water is mixed, to the extent of about 50 per cent, with subtropical 

 water which has previously made an advance towards the south and has been left behind 

 when the convergence retreated northwards. Below 50 m. the water has the usual low 

 salinity 34-43-34-42 °/ 00 , which suggests that the water at this depth has not been dis- 

 turbed by the southward advance of the subtropical water. 



Where the subtropical water is known to have a strong movement in opposition to the 

 sub-Antarctic current the position of the convergence appears to be subject to frequent 

 fluctuations, and the isolation of southward salients of subtropical water leads to the 



