106 OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 

 the conditions 



^Vy . dVy ^ 



must be fulfilled. Examination of these conditions shows that the 

 motion can be stationary only if the geopotential contour lines are 

 directed east-west and are spaced at equal intervals. These conditions 

 are never fulfilled if a larger area is considered, for which reason the motion 

 derived from the geopotential topographies of isobaric surfaces is, as a 

 rule, not stationary. Thus the contour lines represent approximately 

 stream lines but not trajectories. It should, furthermore, be observed 

 that a current represented by equation (VI, 17) and flowing east-west is 

 free from horizontal divergence: 



^4-^ = 

 dx "^ dy ""^ 



but a current that has a component toward the north or the south is not 

 free from horizontal divergence and must be accompanied by vertical 

 motion. 



The computation of the geopotential distances between isobaric sur- 

 faces has already been discussed, and the whole problem of computing 

 ocean currents would therefore be very simple if (1) simultaneous obser- 

 vations of temperature and salinity at different depths were available 

 from a number of stations so that relative topographies could be con- 

 structed, (2) accelerations ccadd be neglected, (3) frictional forces could 

 be neglected, and (4) periodic changes in the distribution of mass as 

 related to internal waves were negligible. 



Simultaneous observations from a number of stations are never 

 available, and the question therefore arises as to whether charts based on 

 stations that have been occupied within a certain time interval can be 

 considered as approximately representing a synoptic situation. This 

 question can be examined by repeated surveys of the same area. Such 

 surveys have shown that conditions vary in time, but so slowly that the 

 main features of a certain topography are represented correctly by 

 nonsimultaneous observations that have been taken within a reasonably 

 short time interval. Results of repeated surveys are found in the publi- 

 cations of the U. S. Coast Guard presenting the work of the International 

 Ice Patrol off the Grand Banks of Newfoundland, where a small area has 

 been covered in less than a week and where cruises have been repeated at 

 intervals of three to four weeks. In these intervals of time the details 

 of the relative topography have changed greatly, but the main features 

 have changed much more slowly. Similar surveys have been conducted 

 by the Scripps Institution of Oceanography off southern California. 



