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



B 



>a • b 



P 



s+ds 



^+d4-dt 



z-¥dz 

 p + dp 

 s+ds 



4+di 



Fig. 91. Calculation of stability. 



by dr so that its temperature becomes & + d& 

 points b and a will thus be 



dr. The density difference between 



dp dp 



Pp,s+ds,&+d»-dT — Pp,s,9 ~ ^ ^^ ^ 'M. ^^^ ~ ^'^^■ 



The stability E is then given by the expression : 



dp ds dp id'd' 

 ds dz dd' \dz 



E = 



dr 

 Jz 



The geometrical changes in salinity and temperature ds\dz and ddjdz for the depth z 

 at a give station can thus be determined from the given values of T and S, and the 

 temperature gradient drjdz as well as dpjds and dpjdd' can be found from hydro- 

 graphic tables. 

 If the salinity is constant in vertical direction (dsjdz = 0) then 





d^ 



dr 

 d 



doe 

 1z 



This is in agreement with the previously given equilibrium condition for the potential 

 temperature. For a given vertical change in salinity its effect on E is so large that it 

 cannot be ignored. 



"Meteor" St. 310 (see Table 79) has been selected as an example for the vertical 

 stability distribution; the E distribution is given in Table 80. 



In the top layer down to 25 m there is a very weak negative stability and just below 

 the top layer E lises to very large values. This is the density transition layer where the 

 stratification of the water is extremely stable. Underneath the stability decreases some- 

 what to assume a value of about 100 at the boundary between the oceanic troposphere 

 and the stratosphere. Tt then decreases steadily approaching neutral equilibrium in the 

 greatest ocean depths. All tropical and subtropical stations show similar conditions. 

 Towards polar latitudes the large positive values of £" in the upper layers disappear and 

 are replaced by a more uniform, however, not espec'a!ly la ge stability; only the sur- 

 face layer can be disturbed to any extent by changes from season to season. 



The vertical stability at great depths in the deep-sea trenches is of particular interest. 

 Since in these the salinity is very largely constant the vertical stability conditions can 

 be estimated fairly accurately from the potential temperature (see p. 127). According 



