288 DEACON [chap. 12 



water mixes with a large proportion of deep water in the Weddell Sea and 

 with more on its way round the continent. The result is water with a very 

 complex history. 



C. Antarctic Surface Water 



Conditions that produce water with a salinity less than 34.51 % and mixtures 

 light enough to float above the warm deep layer occur all round the continent. 

 The southward movement in the warm deep layer is, as far as we know, the 

 only flow towards the continent, and this, mixed with the surface water, cooled 

 by radiation and exchanges with the atmosphere, and diluted by melting ice 

 and snow and drainage from the land, produces cold, poorly saline water which 

 lies in a shallow well-defined layer above the warm deep water. It has a depth 

 of 150 to 250 m, deepest near the Antarctic convergence and near the con- 

 tinent, and shallowest in the divergence region between the west- and east- 

 wind drifts. It is separated from the warm deep layer by a sharp discontinuity 

 in which the temperature and salinity increase with depth and the dissolved 

 oxygen content decreases. In winter it is almost uniform with depth, near its 

 freezing point in the south, and only 1° or 2°C warmer in the north ; its salinity 

 is highest in winter. In summer it generally has a warmed and diluted surface 

 stratum but its lower half is still for the most part well below 0°C and not 

 warmed very much above its winter condition. The seasonal changes seem to 

 be least in the divergence region, presumably because the formation of the warm 

 dilute surface strata found farther south as well as farther north in summer is 

 hindered by up welling and some south and north transport at the surface. 

 The great volume of poorly saline water that spreads northwards well beyond 

 the equator in all oceans and can be fed only from the Antarctic surface currents 

 is unmistakable evidence of their large overall flow away from the Antarctic. 

 We have little information on which to base an estimate of the relative strengths 

 of the circumpolar and northward transports, but it seems likely that most of 

 the water in the surface layer spends several years in high latitudes before it 

 escapes to the north. 



D. The Antarctic Convergence 



The latitude where the convergence occurs is marked almost as clearly by 

 changes in level of the deep and bottom currents as by the sinking of the 

 Antarctic surface water below the warmer sub-Antarctic water. The isotherms 

 and isohalines at great depths slope as steeply as those near the surface. Condi- 

 tions in the ocean seem to be paralleled to some extent in the atmosphere, for, 

 although the data are scanty, there are enough to indicate that the meridional 

 atmospheric pressure gradient and the west wind are strongest in the same 

 latitude. It might be argued that the atmospheric circulation and the wind 

 determine the latitude of the frontal region in the ocean, but it can also be 

 maintained that the latitude of the north-to -south gradients in the atmosphere 

 is determined by some kind of balance between the deep and bottom layers 

 of the ocean. The convergence plotted in Fig. 3 is not everywhere in the same 



