Density of Water Masses in Ocean, Vertical and Horizontal Density Distribution 187 



50° N.; however, in regions close to the coasts seasonal displacements of different 

 types of water also cause large annual density variations (>20). 



2. Density Distribution at the Surface of the Ocean 



It is very characteristic of the density distribution at the surface of the ocean that in 

 spite of the extended strong salinity maximum in middle latitudes there is a rather 

 regular increase of density from the equatorial regions towards the poles in all oceans. 

 This already points towards a decisive influence of the temperature. Figure 86 shows 

 the distribution of density at the surface of the Atlantic Ocean according to Bohnecke 

 (1936). This picture illustrates the meridional increase from about 23-0 at 7°-8°N. 

 to a value somewhat larger than 27-0 in higher latitudes mentioned above. Table 77 

 gives mean values for successive latitude zones of 5 degrees width. The increase is not 

 entirely uniform in all these zones ; the regions of subtropical convergence stand out 

 as zones with a smaller density gradient and this gradient becomes larger again only 

 near the oceanic polar fronts. Beyond the extensive areas of maximum density in 

 subpolar and polar regions of maximum density the surface density seems again some- 

 what to decrease. 



Table 77. Mean meridional density distribution in the Atlantic (o-^) 



* Minimum; f Maximum 



For the Indian and the Pacific Oceans the surface density charts of Schott (1935) 

 give only summer conditions for each hemisphere. These charts show essentially the 

 same basic features as in the Atlantic. In the northern Indian Ocean only, conditions 

 are somewhat complicated due to the large annual variations in salinity. The large 

 differences in density between the Bay of Bengal with values of 22-0-1 8-0 and the 

 Arabian Sea with an increase to 23-0 or even to 24-0 should particularly be mentioned. 



3. Vertical Density Distribution and Horizontal Charts for Different Depths 



The density is equally expressed by the quantity a, for the deeper layers. In this 

 quantity the effect of pressure acting on the water mass is not taken into consideration 

 and it refers therefore to zero sea pressure. As a rough approximation, ct< can be taken 

 as the density which would occur in a water mass after displacement ofthe mass with its 

 in situ temperature and salinity from the depth to the surface (potential density); 

 thereby only the adiabatic temperature effect remains out of consideration. 



For a study ofthe vertical density stratification ofthe ocean it is necessary to go back 

 to the values of the density or the specific volume in situ. Table 78 contains values for 

 a standard sea at 0°C and 35%o salinity, the vertical distribution ofthe density 0-^,^,5, 

 and of the specific volume a^.^^p, and the corrections which must be applied to these 

 as,<,^ to obtain the distribution at 35%o for 10° and 20°C, respectively, or at 0°C for 

 32-5 and 37'5%o, respectively. 



