108 OCEAN CURRENTS RELATED TO THE DISTRIBUTION OF MASS 



greatly changed. Thus, in latitude 33°40'N, longitude 120°W, the 

 computed surface current on the first cruise was 36 cm/sec toward 

 N 80°E, whereas on the second cruise it was 14 cm/sec toward S 30°E. 

 In the time interval between the two cruises the average local acceleration 

 was therefore 2.7 X 10~^ cm/sec^ toward S 57° W, but, compared to the 

 acting forces, this is a small quantity. On the first cruise the numerical 

 value of the geopotential slope of the surface was 294 X 10~^ cm/sec^, 

 and on the second it was 117 X 10~^ cm/sec^. The total acceleration is 

 generally of the same order of magnitude as the local acceleration and 

 therefore was also small. Even in these cases the stream lines of the 

 flow coincided approximately with the contour lines, although the distri- 

 bution of mass was changing continuously and although the combination 

 of observations taken during ten days led to a somewhat distorted 

 picture of the topography. 



As a rule, it can be stated that the smaller the area the more nearly 

 simultaneous must observations be in order to permit conclusions as to 

 currents. On the other hand, when dealing with ocean-wide conditions, 

 observations from different years can be combined. 



The second assumption, that the motion is not accelerated, is evi- 

 dently not fulfilled when dealing with a smaller area within which con- 

 ditions change rapidly, but according to the above numerical example no 

 serious errors are introduced if one is satisfied with an approximate 

 value of velocity. The assumption will be more closely correct when 

 large-scale conditions are considered. 



The third assumption, that the frictional forces can be disregarded, 

 must also be approximately correct, as is evident from agreement obtained 

 between computed surface currents and surface currents that are derived 

 from ships' logs or from the results of experimentation with drift bottles. 

 The fourth assumption is nearly correct when the accelerations are 

 small. It can be stated, therefore, that the computations that have been 

 outlined can be expected to render an approximately correct picture of 

 the relative velocities which are associated with the distribution of mass. 



Slope Currents. If a slope field (p. 103) exists, corresponding 

 slope currents will be present and will stand in the same relation to the 

 slope field as the ''relative " currents to the relative field. The important 

 difference between the two types of currents is that a slope current is 

 uniform from the sea surface to the bottom, whereas a relative current 

 changes with depth. 



Existence of slope fields and corresponding slope currents has been 

 demonstrated in such land-locked bodies of water as the Baltic Sea, and 

 results of precise leveling along the North American east coast suggest 

 that slope currents may be present there also. In the open oceans, 

 slope currents probably do not exist, except as transitory phenomena 

 caused by changing winds (p. 133). 



