162 Thermohaline Features 



TRANSPORT CALCULATIONS FROM HYDROGRAPHIC 

 DATA FROM THE ATLANTIC OCEAN 



The rectangular ocean basin discussed in the above section is not meant to 

 be a true representation of any real ocean basin. It does, however, illustrate 

 certain dynamical principles which we wdll now try to use in a rough, 

 tentative way in a discussion of the circulation of the North Atlantic Ocean. 

 In order to make the discussion as quantitative as possible, the geostrophic 

 transports across various sections in the Atlantic are computed, and an 

 elementary method for shifting the level of no motion is introduced. There 

 is, of course, no positively certain way to determine the depth of no motion. 

 A variety of arguments wdll therefore be introduced in this connection, none 

 of which can be regarded as really satisfactory. 



In the past two years Worthington has embarked upon a program of 

 repeating all old (1930-1940) Atlantis hydrographic sections. Moreover, 

 he has made his casts to the very bottom, and thus there is now becoming 

 available, for the first time, a thorough deep network of sections across the 

 western North Atlantic from which an adequate study of the geostrophic 

 transport can be made. As Worthington has pointed out in a recent lecture 

 at Honolulu, the feature of principal dynamical interest which these new 

 sections reveal is that there are horizontal density gradients under the Gulf 

 Stream at all depths sufficient to introduce sizable contributions to geo- 

 strophic transports as determined by standard ' dynamical ' computation. 



As a result, the net geostrophic transport computed for each section 

 depends strongly upon the choice of the level of no motion, and a change of 

 this level — for example, from 2000 m. to the bottom at 5000 m. — makes a 

 very large change in the net transport. In other words, the suppositions 

 introduced in making the integrations leading to equation (4), namely, that 

 the horizontal pressure gradients and motions vanish for all depths beneath 

 a certain great depth, are not vahd. Nevertheless, in any particular section 

 there may be a depth at which (though not below which) horizontal pressure 

 gradients parallel to the section and motions normal to the section vanish. 



This uncertainty in the depth of no motion makes it much more difficult 

 to compare the results of theory and observation than might be inferred 

 from Isehn's analysis of early observations (1936, 1940). ThusMunk's (1950) 

 comparison of his own theoretically deduced Gulf Stream transport of 

 35 X 10® m.^/sec. with the 'observed' transport of 74 x 10® m.^/sec. is fairly 

 meaningless. The latter figure was computed by Iselin with an assumed 

 depth of no motion at 2000 m., neglecting counterflows above 2000 m. on 

 the sides of the main current, and also neglecting counterflows beneath the 

 Stream. An even worse discrepancy can be obtained, for example, by using 

 Worthington's new Gulf Stream sections, excluduig counter currents, and 



