13 



DEPTH AT WHICH GREATEST OBLIQUITY OF ISOBARIC SURFACES 



OCCUR 



It is important to distinguish where the greatest obhquity of the 

 isobaric surfaces prevail in an ocean mass. D3'namic measurements 

 and pressures have been considered as being laid off from the surface 

 of the sea downwards on the assumption that the sea surface is always 

 level — an equipotential surface. This premise demands considerable 

 revision, as we shall see, in the light of the following facts : 



As a result of compiled oceanographic observations, it is well known 

 to-day that the greatest variations in temperature and salinity of the 

 water take place in the upper levels of the sea. In the North Atlantic, 

 for example, below depths of 3,000 meters there is little variation, as 

 we proceed from place to place, in the temperature or the salinity. 

 Now, if we regard two stations with widel}^ differing specific volumes, 

 we shall generally find that their difference decreases more or less 

 rapidly with an increase in depth, and gradually approaches a constant 

 or zero. Where the water is light we shall observe a relatively low 

 pressure in decibars at a certain dynamic depth, or conversely at a 

 given observed pressure in decibars, the dynamic depth will be least 

 where the water is heaviest. In view of this natural state of the 

 ocean, if the sea surface be level, then the obliquity of the isobaric 

 surfaces must increase downwards and the maximum of forces and 

 currents would be relegated to the greater depths, a condition which 

 we know is contrary to fact. It follows alternatively that at an 

 appreciable depth below the surface there will generally be a sheet 

 where motion most nearly approaches zero and where isobaric, 

 isosteric, and equipotential surfaces are parallel. It follows, further- 

 more, that above such a motionless plane, the water, over any 

 given horizontal extent, lies at the greatest height (the surface of 

 the sea highest) at that place where the water is the lightest — i. e., 

 the specific volume the greatest. 



We should endeavor to select from a group of observations indi- 

 cative of a surveyed area an isobaric surface which in itself has the 

 most nearly equal dynamic depths, thereby sounding out a level or 

 motionless plane and which as stated before generally will be found 

 to lie at a relatively great depth beneath the sin*f ace of the sea. When 

 employed as a " bench mark " this surface provides a means of measur- 

 ing the currents which usually are present in the upper levels. The 

 velocities are determined by a comparison of any two dynamic heights 

 measured upwards from the level, isobaric plane to the surface of the 

 sea. Figure 5, page 14, shows in exaggerated form the obliquity of 

 the sea surface, and also the other isobaric surfaces of observation as 

 they lay May 5-7, 1922, south of the Grand Banks, between stations 

 206 and 201. The state of relative obliquity is based upon the 

 assumption that the maximum depth of observation, the 750 declbar 



