8 2 DISCOVERY REPORTS 



be expected in a southward movement : the high temperature and salinity alone, occur- 

 ring as they do between the lower values of the surface and bottom layers, are reliable 

 evidence of a southward movement in the layer. The movement has so far not been 

 confirmed by current measurements, but it has been clearly indicated by the southward 

 drift of several species of plankton. 



Since the warm deep layer lies between less saline and colder waters, the temperature 

 and salinity of the deep currents which feed it decrease in the direction of movement, 

 and an increase of salinity or temperature occurring in the path of a current shows that 

 it has been joined by deep water from another source. A chart showing the distributions 

 of salinity and temperature in the layer can therefore be used for an approximate 

 examination of the movements in the layer. 



The chart in Fig. 19 shows the distribution of salinity at the level of maximum 

 salinity in the warm deep layer. For the purpose of following the deep currents it is 

 more useful than a chart showing the salinity distribution at one particular depth, since 

 the currents have pronounced vertical as well as horizontal movements, and it is more 

 convenient to use than a large series of charts showing the distribution at small 

 intervals of depth. It is based principally on the data collected by the ships of the 

 Discovery Committee between 1926 and 1934, but it includes some observations made 

 by the ' Deutschland ' (Brennecke, 1921) and the 'Norvegia' (Mosby, 1934). 



In order to provide a means of checking the conclusions reached from an examination 

 of the salinity chart, others showing the temperature and oxygen content of the water at 

 the level of maximum salinity have also been prepared. In the temperature chart 

 (Fig. 20) the distribution of potential temperature is shown. Where the deep current 

 climbs towards the surface its actual temperature, apart from any changes caused by 

 mixing, will be lowered by the cooling which takes place as the water expands adiabati- 

 cally and a chart showing the actual temperature could not be used to follow the water 

 movements over a large range of depths, without making some allowance for any 

 change in the depth of the current. A chart showing the potential temperature of the 

 water — the temperature to which the water would be adiabatically cooled if it were 

 raised to the surface — avoids this difficulty. 



The oxygen chart in Fig. 21 is not so useful as the salinity and temperature charts 

 because the oxygen content is subject to a greater number of modifying influences ; it 

 decreases in the direction of the current owing to the oxygen being used up by the 

 plankton and in various oxidation processes, and also because of mixing with the poorly 

 oxygenated water in the upper stratum of the deep layer ; it may on the contrary increase 

 by mixing with the highly oxygenated bottom water. In several regions, however, 

 the chart gives useful confirmation of the deductions made from the salinity and 

 temperature data. 



The conclusions reached about the movements in the warm deep layer from the 

 distribution of salinity, temperature, and oxygen shown in Figs. 19-21 do not neces- 

 sarily apply to the whole of the layer, because the stratum of maximum salinity generally 

 lies in the lower part of the layer. To examine the movements in the warmer upper part 



