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

 From these equations one obtains 



hp = da, + -^ds-\- -^^ ^^ - a^ ^^ 



and thus 



dag. dcg^ J, ds de&^ ^ d'& dp dr 

 ^ ^ ~dz '^ 8s dz "^ ~dif dz ~ ~8& dz' 



In this expression for E the first term is usually the main one and the others are only 

 correction terms; the second term shows the effect of changes in salinity, the third 

 shows the effect of changes in temperature on the compressibility while the fourth 

 allows for the adiabatic temperature effect. Estimation of the order of magnitude of 

 these terms shows that they cannot be neglected; the effect of the temperature differ- 

 ence on the compressibility must already be taken into consideration for depths be- 

 low 100 m; in deeper layers also the adiabatic effect is of the same order of magni- 

 tude as the first term. In general only the effect of changes in salinity is mostly small. 

 The quantity do^jdz for itself thus cannot give a very precise measure of the stability. 



6. The Distribution of Stability in the Atlantic Ocean 



Schubert (1935) has carried out a detailed examination of stability conditions in the 

 Atlantic Ocean — in particular of regional stability differences in vertical sections and 

 on horizontal charts. Table 80 also gives mean values of E for the entire ocean calcu- 

 lated as means of all "Meteor" stations; the surface layer down to 200 m, i.e. the 

 zone of disturbance, has been omitted. Of the many irregularities in the vertical distri- 

 bution at individual stations, only two remain in the mean values, the most important 

 being that at 1000 m. This is a definite intermediate stabiHty maximum. From the 

 location of this rather strong interruption, or sometimes even reversal, of the normal 

 decrease of stability downwards, the decrease in stability is considerably larger than 

 before. This irregularity is present at about the same depth throughout the total 

 ocean in temperate and tropical latitudes, and is connected with the subantarctic 

 intermediate water. Its basic cause is the reversal in the salinity gradient. 



There is another secondary maximum imposed on the regular decrease of the E- 

 values at a depth of 2000-4000 m. In contrast to the more sudden change at 1000 m 

 a weak and more gradual increase in stability is characteristic. 



In the regional variability of the stability in particular, a strong decrease towards 

 higher latitudes stands out. The higher values of E disappear already beyond 50° 

 latitude; the greater uniformity and lower values indicate that only in higher latitudes 

 do favourable conditions for vertical displacements of water exist. Solely by this, higher 

 latitudes become the principal regions of origin for the deep-sea circulation of the 

 oceanic stratosphere. 



Characteristic stability conditions are found in the top layer down to 100 m or 

 occasionally to 200 m where frequently negative values occur. Apart from cases in 

 the upper 25 m, where they are very frequent, these negative stabilities were formerly 

 regarded as due to observational errors (especially in the salinity). However, variations 

 of 0-01%o are in fact quite sufficient to explain them (Helland-Hansen, 1910). 



