2IO DISCOVERY REPORTS 



and 200 m., where there is a movement southwards. The temperature starts to fall 

 rapidly immediately below the surface stratum, but its decrease is arrested some- 

 what, and occasionally is changed to an increase between 80 and 200 m. Below this 

 stratum the temperature and salinity both decrease rapidly until more uniform water is 

 reached near the level of minimum salinity between 400 and 600 m. Their decrease is then 

 less rapid. Below the level of minimum salinity the salinity increases rapidly, but the 

 temperature continues to decrease slowly until the level of minimum temperature is 

 reached, between 800 and 1200 m. In this level, too, the change of saHnity with depth 

 becomes slightly less. Below the level of minimum temperature both the temperature 

 and salinity increase, the temperature to a secondary maximum value in the upper stratum 

 of the warm deep layer, and the salinity to a maximum deeper down in that layer. 

 The origins of sub-Antarctic water can be summarized as follows : 

 (i) In Antarctic surface water which sinks below the surface at the Antarctic con- 

 vergence; and, to a much smaller extent, in Antarctic surface water which mixes 

 across the Antarctic convergence at the surface. 



(2) From the heavy precipitation in the sub-Antarctic Zone which exceeds the 

 amount of evaporation by about 700 mm. per annum, and also from coastal 

 waters. 



(3) In additions from the sub-tropical surface layer between 80 and 200 m., and 

 also by mixing across the sub-tropical convergence at the surface. 



The effect of mixing across the sub-tropical convergence will be described in the next 

 section. 



EXTENT OF THE SUB-ANTARCTIC ZONE 



The sub-tropical convergence, which is the northern boundary of the sub-Antarctic 

 Zone and the convergence of sub-Antarctic and sub-tropical waters, is not so well known 

 as the Antarctic convergence. It is, however, usually a much sharper convergence, and 

 is marked by a sudden change of surface temperature of at least 4° C, and a change of 

 salinity of at least 0-50 7oo- A continuous temperature record showing the sudden fall 

 in surface temperature on crossing the subtropical convergence is shown on the right- 

 hand side of Fig. 10 (p. 190). At the convergence sub- Antarctic water sinks below the 

 surface. It does not sink downwards to the north because between 80 and 200 m. sub- 

 tropical water is moving southwards into sub-Antarctic water. Instead, it must mix with 

 the water in this movement, and then, on its way southwards it either mixes again with 

 the deeper water moving northwards, or wells up towards the surface. 



To explain the sharp convergence at the surface that is almost always encountered, 

 there must be a much greater movement northwards in the surface of sub-Antarctic 

 water than there is in sub-tropical water. The sub-tropical convergence is still within 

 the region of westerly winds, but since it is found in about 40° S the winds are much 

 weaker north of it. The convergence can therefore be explained as a result of the greater 

 transport of water towards the north in the surface stratum of sub-Antarctic water ; but 

 this reason is not sufficient to explain the sharpness of the convergence, since the falling 

 off of wind strength is gradual. 



