TOO OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 



at which the pressure was observed could be joined by lines of precise 

 leveling, but this is obviously impossible. 



On the other hand, it is possible to determine the field of mass in the 

 sea, since the density or the specific volume of the water can be obtained 

 with great accuracy from observations of temperature and salinity. 

 If the distribution of mass is known, the relative topography of the isobaric 

 surfaces can be derived by using the hydrostatic equation, and the cor- 

 responding relative currents can be computed. 



The Field of Mass. The field of mass in the ocean is generally 

 described by means of the specific volume (p. 12) : 



Ois,i},p = OiZB,0,p + 5. (VI, 19) 



The field of the specific volume can be considered as composed of two 

 fields — the field of 0:35,0,^ and the field of 8. The former field is of a 

 simple character. The surfaces of 0:35,0,:? coincide with the isobaric 

 surfaces, the deviations of which from level surfaces are so small that for 

 practical purposes the surfaces of 0:35.0,^ can be considered as coinciding 

 with level surfaces or with surfaces of equal geometric depth. The field 

 of aso.o.p can therefore be fully described by means of tables gi^^ing 

 0:35.0,^ as a function of pressure and giving the average relationships 

 between pressure, geopotential, and geometric depths. This field can 

 be considered a constant one, and the field of mass is therefore completely 

 described by means of the anomaly of the specific volume, d, the deter- 

 mination of which was discussed on p. 12. 



The field of mass can be represented by means of the topography of 

 anomaly surfaces or by means of horizontal charts or vertical sections 

 in which curves of 5 = constant are entered. The latter method is the 

 most common. 



The density of the ocean waters generally increases with depth, and, 

 therefore, the stratification is stable. The degree of stability, or, 

 briefly, the stability, is evidently related to the change of density with 

 depth and can be expressed as 



E = l'£, (VI, 20) 



where dp represents the difference in density between a small mass of 

 water and its surroundings after the mass is moved adiabatically through 

 the vertical distance dz. In the upper layers, E = dat/dz, approximately. 

 With regard to the field of mass, it must be added that equipotential 

 surfaces do not exist in the sea — that is, no surfaces exist along which 

 masses of water may be moved without altering the potential energy of 

 the system. The at surfaces are approximately equipotential surfaces, 

 and in recent years considerable evidence has been accumulated to show 

 that in the ocean lateral mixing takes place along at surfaces and that 

 the direction of flow is parallel to at surfaces. 



