194 Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 

 60° 50° 40° 30° 20° 10° 



Fig. 90. Salinity distribution on the 25-5 o^-surface in the north-east Atlantic between 0° and 

 about 30° N. (according to Montgomery) (only decimals have been entered as salinity 



values). 



within such an isopycnic surface must by definition proceed without changes in the 

 potential density and thus without changes in the potential temperature and the 

 salinity (or in the oxygen content also). If the distribution of the temperature and 

 the salinity (or of the oxygen content) pictured on such a surface show signs of 

 change, these must be due to mixing, and it is therefore possible to investigate these 

 more closely and to follow the main direction of flow and the spreading of different 

 water types by means of isolines. 



Thereby it was assumed that the mixing in such "isentropic" surfaces occurs pre- 

 dominantly in horizontal direction (that is in the direction of the surface) and to a 

 much smaller extent in vertical direction (normal to the surface). This assumption is 

 not entirely justifiable and may be satisfied only in cases where the Cj-surface runs 

 just within the density tran ition layer, since here the exchange coefficient in vertical 

 direction is strongly reduced due to the great stability of the vertical stratification, 

 and lateral exchange is thus very much favoured. Outside the density transition layer, 

 however, there is no reason to assume that the effect of vertical mixing is less important 

 than that of lateral mixing, especially as the reduced magnitude of the vertical ex- 

 change coefficient is compensated for by rather pronounced vertical gradients of the 

 oceanographic factors, as was seen earlier. 



Montgomery (1938) has applied this method to determine the oceanic circulation of 

 the upper layers of the southern North Atlantic. The results of this investigation will 

 be discussed later in connection with the dynamics of ocean currents; here only the 

 method for the use of the o-^-chart will be presented. Figure 90 gives an example of such 



