Thermohaline Features 165 



In most sections the function S{z) is nearly unity, except in the vicinity 

 of the bottom of the section; thus a graph of ^(2, d) for any particular choice 

 of reference level z = d can be approximately transformed to the function 

 T{z, d') for another reference level z = d' by simply shifting the origin of 

 function by a constant amount, 



T{z,d')^Tiz,d)-T{d',d), (26) 



so that the level of no motion can be made to move up and down without 

 changing the shape of the curve. Thus, in fig. 79, the approximate con- 

 sequences of a change of reference level on the vertical distribution of 

 transport per unit depth can easily be visualized by shifting the origin of 

 the abscissa to left or right so that the curve will intersect the ordinate at 

 the desired reference level. 



In fig. 79, one of the sections at which we can most certainly assign a 

 reference level is section 9 across the Florida Straits. We know from the 

 low salinity of the water at the depth of 600-700 m. immediately north of 

 the Florida Straits that the reference level must be at the bottom, or below 

 it. Direct current measurements and the results of electrical-potential 

 recordings between Key West and Havana indicate that the mean flow is 

 nearly 26 x 10^ m.^/sec; hence, unless the level of no motion is just about at 

 700 m. (the bottom) a serious discrepancy would exist between this estimate 

 and the vertically integrated geostrophic transport through the section. 

 We therefore take the transport-per-unit-depth curve for section 9, shown 

 in fig. 79, as a starting point in our analysis of the geostrophic transport of 

 the Gulf Stream in particular and the Atlantic Ocean in general. If the 

 flow in the western North Atlantic is geostrophic, then the total mass trans- 

 port through sections 8 and 9 must be very nearly the same as that through 

 section 6 or section 7. So far as I can determine, this requirement is best 

 met by the choice of reference levels indicated in fig. 79. In order to demon- 

 strate the degree to which the total mass conservation is achieved by this 

 choice, I have prepared the more detailed graph shown here as fig. 80, using 

 some of Worthington's newest data. 



It is reassuring to note that not only do the total transports ^{d) balance 

 approximately, but also the transports at each depth T{z, d) balance, very 

 roughly, when the depth of no motion is chosen in the interval of 1200- 

 1600 m. Evidently we cannot expect precise agreement. The shifting of the 

 level of no motion uniformly over the whole section may be too crude; there 

 may be differences in the level of no motion at various points across each 

 section. In any case, the choice of the interval of 1200-1600 m. leads to 

 much smaller violations of mass continuity than the choice of the bottom 

 as reference level. 



We can extend the argument of total mass conservation to other 



