the shaded area of temperatures less than zero degrees C, 

 is well marked in this section. The northern limit of this 

 area occurs just south of the convergence, and serves as a 

 further indication of the location of the boundary zone. 



The three vertical sections in figure 16, sections C-l, 

 C-2, and C-3, all show a fairly well-defined boundary zone 

 at subsurface depths as indicated by the temperature gradients. 

 These sections were all taken approximately along the same 

 north-south line and serve not only to demonstrate the fluctu- 

 ations with time in the location of the convergence but also 

 show the seasonal development of the subsurface temperature 

 minimum. On vertical section C-l the structure a short 

 distance south of the convergence shows a nearly isothermal 

 surface layer with temperatures below zero degrees C. The 

 increase in temperature in the transition layer at greater 

 depths does not appear here because of the shallowness of the 

 section. Section C-2, taken 54 days later, indicates that the 

 surface heating has produced an area with a well-marked 

 subsurface minimum. The data in this section extended deep 

 enough to penetrate the region of positive gradient. Section 

 C-3 also shows this subsurface minimum and its northern 

 boundary, serving as an indication of the location of the bound- 

 ary zone. 



The southward displacement of the convergence in the 

 54-day period between section C-l and C-2 may be related 

 to an increase in the thermohaline circulation in the sub- 

 antarctic region, coupled with a decrease in the intensity of 

 the transverse circulation related to the prevailing westerlies 

 in the antarctic region. This displacement appears to be a 

 seasonal phenomenon with the convergence occurring farther 

 south in summer than in the colder months. In winter the 

 cold antarctic water extends farther north along the surface 

 before sinking to form the antarctic intermediate water at 

 the convergence. The northward displacement of the con- 

 vergence between sections C-2 and C-3 may indicate that the 

 southward seasonal trend has been reversed by late February 

 when section C-3 was obtained. 



31 



