Vertical Temperature Section. The convergence should be 

 marked at subsurface depths by a continuation of the sharp 

 gradients found at the surface. Another important indication 

 of the northern limit of the antarctic region, hence of the 

 location of the Antarctic Convergence, is the position of the 

 northern boundary of the subsurface temperature minimum 

 which develops in summer. The development of this typical 

 summer thermal structure in the antarctic region was dis- 

 cussed earlier. North of the Antarctic Convergence the tem- 

 perature continuously decreases with depth, and the typical 

 minimum at 200 to 500 feet is not developed. This subsurface 

 temperature structure characteristic of the Antarctic Con- 

 vergence is demonstrated by vertical cross section plots of 

 the five crossings (see figs. 15 and 16). On these cross 

 sections all areas below zero degrees C. have been shaded 

 in order to indicate the location of the subsurface minimum. 

 In vertical section, figure 15A, the temperature gradient 

 between 2 and 3.5 degrees C. continues to mark the con- 

 vergence as it slopes downward to the north below the warmer 

 subantarctic surface water. Taking the 3-degree C. isotherm 

 as indicative of the location of the boundary between the two 

 water masses at the convergence zone, it is seen that this 

 boundary has a slope downward toward the north of 3 x 10"^ 

 in the upper 400 feet; that is the cold antarctic intermediate 

 water sinks below the warmer subantarctic surface water at 

 the rate of eighteen feet 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 figure 15B 

 is particularly interesting since bathythermographs were 

 taken at very short intervals in the southward progress of 

 the USS NORTHWIND. The detail shown here points out that 

 the usual cross section drawn from stations spread in much 

 larger intervals presents a very smoothed picture of the 

 vertical thermal structure. The apparent thermal structure 

 is very complicated indeed, appearing more and more com- 

 plicated the greater the detail of the observations. Internal 

 waves, which are discussed later, may contribute greatly to 

 the complicated thermal structure shown here. 



The boundary zone in this vertical temperature section 

 (fig. 15B) is fairly well marked at subsurface depths between 

 the 1-degree C. and the 3-degree C. isotherms. This zone 

 has an average slope of about 4 x 10"^ in the upper 300 feet. 

 The region of subsurface temperature minimum, shown by 



29 



