214 DISCOVERY REPORTS 



1° C. This is, however, a large seasonal variation of temperature for water at so great a 

 depth, and it is a result of the seasonal variation in the temperature of the Antarctic 

 surface water which sinks to that depth. The maximum temperature at a depth of 

 600 m. is only reached about two months after the maximum temperature at the surface. 

 This is because the Antarctic surface water, though it has its maximum temperature at 

 about the same time as sub-Antarctic surface water, takes two months to sink to 600 m. 



The temperature curves for the water at depths of 100 and 400 m. show that the water 

 at these depths has a maximum temperature at times intermediate between the times of 

 the surface and 600-m. maxima. The surface water has its minimum temperature at 

 the end of August and the water at 600 m. about two months afterwards. 



The curves in the lower half of the diagram show the changes in salinity of the water 

 at the surface and at a depth of 600 m., and the dotted line shows the seasonal change 

 in the average salinity of the whole water column from the surface down to 600 m. The 

 annual range at the surface is about o-i^°l ^^ and about 0-04 "/^^ at a depth of 600 m. 

 At 600 m. the salinity is at a minimum when the temperature is at its maximum, which 

 is what happens in the Antarctic surface water itself. At the surface, however, the 

 minimum salinity is only reached about two months after the maximum temperature, 

 and it appears that the minimum salinity is due to the mixing of Antarctic surface 

 water from below. 



The diagram also shows that the difference in temperature between the water at the 

 surface and that at a depth of 600 m. is least in September, when the layer is almost com- 

 pletely mixed, and greatest in February, when, especially in the surface 200 m., there are 

 fairly stable strata in the layer. The line showing the variation in the average salinity of 

 the upper 600 m. of water shows that the layer as a whole, or at least that part of it in the 

 region of intense mixing (which extends to about 100 miles north of the convergence) 

 has its minimum salinity at the end of April and its maximum salinity in November. 

 The curve is not quite symmetrical : the time taken for the water to be reduced from its 

 greatest to its least salinity is about 5^ months, whilst that taken to regain its maximum 

 salinity is about 6^ months. This would be expected if the speeding up of the currents 

 of Antarctic surface water in spring is quicker than the slowing down of the same 

 currents in the autumn. 



The nature of these seasonal changes helps to show that sub- Antarctic water has its 

 origin principally in the Antarctic water. The changes, except in surface temperature, 

 lag behind those of Antarctic water and the layer is less directly affected by Antarctic 

 weather conditions. 



The distribution of temperature at the surface of the sub-Antarctic Zone is shown in 

 Figs. 8 and 12 (pp. 183, 196). The surface water north of the 3° C. isotherm in the west, 

 and north of 3-5 ' C. in the east is sub-Antarctic. The isotherms follow the direction of the 

 surface currents. They run approximately south-west to north-east through the Drake 

 Passage and then turn towards the north. Part of the surface water flows between the 

 Falkland Islands and the Patagonian coast, but since the greatest depth in this channel 

 is less than 250 m. the movement is almost confined to the surface stratum. This water 



