678 The Stratospheric Circulation 



to mixing, one obtains a rather good insight into the mixing process going on in the 

 total space of spreading (with reference to these conditions see Pt. I, p. 212 and 

 following pages and particularly the Figs. 100-102). In general, the diagrams indicate 

 a uniform spread towards the north taking place over the whole cross-section almost 

 immediately after the sinking at the polar convergence, but further north there is a 

 preference for the western half of the ocean which must be due to the effect of the 

 Coriolis force. Here close to the South American continent the spread possesses 

 current character. The entire width of the layer across the total ocean gets its supply, 

 then from the western side by lateral turbulence and by occasional occurring large 

 intrusions but the salinity distribution shows only the final stage after lateral mixing 

 has been effective and does not give information about the nature and way with which 

 the lateral mixing process operates. 



Since a current is formed on the western side along the South American continent 

 these processes can be regarded as a case of free turbulence (Defant, 1936c) and the 

 ratio [exchange: velocity] can be determined along the entire spread of this water 

 type. This then gives some idea about the current character of the spread of the 

 Antarctic intermediate water. In order to find the pressure forces that give rise to this 

 water transport it is necessary to determine the dynamic topographies of the isobaric 

 surfaces at these depths. The absolute topography of the 800-decibar surface which 

 corresponds north of 40° S. closest to the core layer of the sub-Antarctic intermediate 

 water is shown in Fig. 319 for the region of 20° N. South of 40° S. the zonal course of 

 the dynamic isobaths indicate the downward extension of the large Antarctic Circum- 

 polar Current flowing eastward; but at this depth the meridional pressure gradient is 

 only half of that observed at the sea surface. Also, the broad high-pressure ridge in the 

 subtropical convergence region is present only with a somewhat diminished intensity 

 and in the convergence regions still vortical disturbances appear extending down to 

 these depths. 



North of the high pressure ridge the isobaths run also from east-north-east to west- 

 north-west, but beyond 25° W. they turn towards the north and finally run along the 

 South American continent as far as Cape San Roque. The pressure gradient here is 

 thus directed towards the east but this gradient does not extend very far out from the 

 coast; the broad area from about 20° S. to 20° N. as far as the African coast shows 

 almost no gradient. Already downward from 500 m the water movements in this large 

 region must be extremely weak and there is no indication whatsoever of a circulation. 

 The water displacement corresponding to the absolute topography (see Fig. 320) on 

 the northern side of the subtropical disturbance zone in the Southern Hemisphere is 

 directed first to the west-north-west and then to the north-west and finally extends 

 in a narrow band along the South American coast as far as the West Indies and con- 

 tinues into the Gulf Stream region. The velocities everywhere remain small, between 

 6 and 12 cm/sec in the core layer, falling rapidly to weak intensities towards the eastern 

 edge. 



An analysis of the salinity distribution in the Intermediate Current gives values for 

 the ratio [exchange :velocity] of 0-8 to 2-3 at the upper and lower edges, respectively. 

 This leads to exchange coefficients of about (5-10 g cm"^sec"i) which is in good 

 agreement with the order of magnitude found by other methods at such 

 depths. 



