42 DISCOVERY REPORTS 



the depth is 200-300 m., in the deep basin east of the Cape Adare-Easter Island ridge 

 about 300 m., and in the Drake Passage 200-300 m. 



In the region of the prevailing east winds the slope of the layers of equal density to- 

 wards the south is often very steep, and near the continent the boundary between the 

 Antarctic and warm deep waters may lie as deep as 400 m. The boundary in this region 

 is, however, not well defined ; both layers flow westwards, and the mixing which takes 

 place between them while they are so long in contact gives rise to an extensive stratum 

 of mixed water. 



The observations made by Brennecke (1 921) between 73 and 74 S at the Deutschland 

 Sts. 120 and 138 show that the sharpest changes of temperature, salinity, and oxygen 

 content with depth are found at about 400 m. From certain shelf seas such as the Ross 

 Sea and that north of Vahsel Bay at the southern extremity of the Weddell Sea the 

 warm deep current is excluded, and in winter the whole water column is completely 

 mixed from surface to bottom ; even in summer there are only small changes with depth, 

 except in a very shallow stratum at the surface, and the whole water column is justly 

 regarded as highly saline Antarctic surface water. At 'Deutschland' St. 125 the tem- 

 perature, salinity and oxygen data indicate that the complete mixing during the winter 

 extended down to a depth of 685 m. The deepening of the Antarctic layer towards the 

 continent is shown by each of the sections which extend into the region of the east winds ; 

 it is also shown very clearly by the observations of the ' Gauss ' (Drygalski, 1926) in the 

 Indian Ocean and by those of the 'Belgica' (Arctowski and Mill, 1908) and 'William 

 Scoresby ' (Station List, 1932) in the Pacific Ocean. The layer reaches a depth of 300 m. 

 in the Indian Ocean and 400 m. south of Peter 1st Island in the Pacific Ocean. 



In addition to the two principal tendencies of the layer, the deepening towards the 

 north and south, the depth of the layer undergoes many other changes caused by the 

 varying slope of the surfaces of equal density in currents of varying direction and 

 strength. Such changes are shown very clearly by the observations in section 3 (Plate IV). 

 Owing to the existence of current differences and eddies between the Weddell Sea and 

 Bellingshausen Sea currents the depth of the layer varies from 100-150 m. at St. 1033 to 

 300 m. between Sts. 1031 and 1032; it then shallows to 150 m. at St. 1029 and deepens 

 to 250-300 m. near the convergence. Similar changes may also be observed in section 9 

 (Plates XVI-XVIII) near the Kerguelen-Gaussberg ridge ; where the northward current 

 of Antarctic water is strongest, the depth of the layer on the right of the current is only 

 about 100 m., but elsewhere it is more than 150 m. 



TEMPERATURE AND SALINITY 



In winter almost the whole of the Antarctic surface layer is homogeneous ; only near 

 the convergence in certain regions does the temperature or salinity change with depth, 

 and generally the first increase takes place in the discontinuity stratum which separates 

 the layer from the warm deep water. The temperature is usually between - i-8 and 

 — 1 -9° C. in the southern half of the zone, but it increases towards the north. The increase 

 depends very largely on the strength of the northward current, but in the main the 



