OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 101 



The Field of Pressure. If the field of mass is known, the relative 

 field of pressure can be determined from the hydrostatic equation in one 

 of the forms 



dp = pdD or dD — adp. 



In oceanography the latter form has been found to be the more practical, 

 but all reasoning applies equally well to results deduced from the former. 

 Integration of the latter form gives 



J 'pa 

 da.^.pdp. 

 pi 



Because of (VI, 19), one can write 



(Z)i - D2). + AD = P' as,,o,pdp + p 8dp, (VI, 21) 



where 



(Di - D2)s = P as,,o,^dp (VI, 22) 



is called the standard geopotential distance between the isobaric surfaces 

 Vi and P2, and where 



AD = fj^' bdp (VI, 23) 



is called the anomaly of the geopotential distance between the isobaric 

 surfaces pi and p2 or, abbreviated, the geopotential anomaly. 



Equation (VI, 21) can be interpreted as stating that the relative 

 field of pressure is composed of two fields — the standard field and the field 

 of anomalies. The standard field can be determined once and for all, 

 because the standard geopotential distance between isobaric surfaces 

 represents the distance if the salinity of the sea water is constant at 

 35°/oo and if the temperature is constant at 0°C. It follows that a chart 

 showing the topography of one isobaric surface relative to another by 

 means of the geopotential anomalies is equivalent to a chart showing the 

 actual geopotential topography of one isobaric surface relative to another. 

 The practical determination of the relative field of pressure is therefore 

 reduced to computation and representation of the geopotential anomalies, 

 but the absolute pressure field can be found only if one can determine 

 independently the absolute topography of one isobaric surface. 



In order to evaluate equation (VI, 23) it is necessary to know the 

 anomaly, 5, as a function of absolute pressure. The anomaly is computed 

 from observations of temperature and salinity, but oceanographic 

 observations give information about the temperature and the salinity at 

 known geometrical depths below the actual sea surface, and not at known 

 pressures. This difficulty can fortunately be overcome by means of an 

 artificial substitution, because at any given depth the numerical value 

 of the absolute pressure expressed in decibars is nearly the same as the 



