region, with a temperature near freezing. Below this isothermal layer^ 

 which extends to about 300 ft. below the surface, there is a transition 

 region in which the temperature increases with depth. This' transition 

 layer lies between the surface layer and the antarctic circumpolar water. 

 This latter is a large, well-defined water mass, characterized by a 

 temperature maximum of slightly more than 2° c, at a depth of between 

 1,500 and 2,000 ft. In summer, during the period of increased radiation, 

 the surface layers are warmed. The resulting temperature increase 

 seldom extends below 150 ft. ; consequently, at a depth of from 200 to 

 500 ft. there remains a cold layer of winter-cooled water. North of the 

 Antarctic Convergence the temperature continuously decreases with 

 depth, and the typical minimum at 200 to 500 ft. is not developed. This 

 subsurface temperature structure characteristic of the Anta,rctic Con- 

 vergence is demonstrated by vertical cross-section plots of the five 

 crossings (see Figs. 4 and 5). On these cross-sections all areas below 

 zero degrees Centigrade have been shaded in order to indicate the location 

 of the subsurface minimum. 



In the vertical section shown in Fig. 4 (a) the temperature gradient 

 between 2 and 3-5° c. continues to mark the convergence as it slopes 

 downward to the north below the warmer subantarctic surface water. 

 Taking the 3° c. isotherm as indicative of the location of the boundary 

 between the two water masses at the convergence zone, it is seen that 

 this boundar}'" has a slope downwards towards the north of 3 x 10^^ 

 in the upper 400 ft. — that is, the cold Antarctic intermediate water sinks 

 below the warmer subantarctic surface water at the rate of 18 ft. for each 

 mile to the north. South of the Antarctic Convergence the typical area 

 of subsurface minimum temperature does not appear well developed. 



The vertical temperature cross-section shown in Fig. 4 (6) is parti- 

 cularly interesting since bathythermographs were taken at very short 

 intervals in the southward progress of the U.S.S. " Northwind." The 

 detail shown here clearly indicates that the usual cross-section, drawoi 

 from stations separated by much larger intervals, presents a very smoothed 

 picture of the vertical thermal structure. The apparent thermal structure 

 is very . complex, appearing more and more complicated with greater 

 observational detail. Internal waves may contribute considerably to the 

 complicated thermal structure shown here. 



The boundary zone in this vertical temperature section (Fig. 4 (b) ) is 

 fairly well marked at subsurface depths between the l"" c. and the 3° c. 

 isotherms. This zone has an average slope of about 4 X 10~* in the upper 

 300 ft. The region of subsurface temperature minimum, shown by the 

 shaded area of temperatures less than zero degrees Centigrade, is weU 

 marked in this section. The northern limit of this area occurs just north 

 of the convergence, and serves as a further indication of the location of 

 the boundary zone. 



The three vertical sections of Fig. 5, sections C-1, C-2, and C-3, aU 

 show a fairly well-defined boundary zone at subsurface depths, as in- 

 dicated by the temperature gradients. These sections were all taken 

 approximately along the same north-south line and serve not only tO' 

 demonstrate the fluctuations with time in the location of the convergence, 

 but also to show the seasonal development of the subsurface temperature 

 minimum. On vertical section C-1 the structure a short distance south 



294 



