146 



Fluctuations 



Hatteras, all show a pronounced maximum in early summer. The broken 

 lines represent combined annual and semiannual components obtained by 

 harmonic analysis. 



Two other types of data have been advanced to support the idea that 

 there is a seasonal fluctuation in the Gulf Stream. Iselin (1940) computed 

 the volume transport of the Gulf Stream from thirteen hydrographic 

 sections made in the period 1937-1940, and beUeved that these showed an 

 annual variation of about 15 x 10® m.^/sec. in the transport of the Stream. 

 These data are plotted in heavy black dots in fig. 75. 



+15 



cm. 



+10 



+ 5 



II 



iV 



VI 



VII VIII IX 



XI 



r 



-5 

 -10 



-15 



Fig. 76. Annual variations of differences of sea level reduced to normal 

 atmospheric pressure. The solid line is used for the Bermuda-Charleston 

 section, the broken line for that from Key West to Miami. From Montgomery 

 (19386, fig. 71). 



The monthly mean sea level from tide-gauge records should also be an 

 indication of changes in transport. 



The geostrophic equation requires that the transverse slope of the free 

 surface and surface velocity be proportional. A decrease of mean sea level 

 at a station along the United States coast suggests an increase in Gulf 

 Stream transport, but does not prove it. Montgomery (19386) has pointed 

 out a number of corrections (such as allowance for seasonal variations in 

 mean atmospheric pressure) which must be applied to the tide-gauge data. 



Records from Miami and Charleston are also plotted in fig. 75. Miami 

 and Charleston are on the same side of the Stream, and hence the records 

 from these two places do not, strictly speaking, serve as an indication of 

 changes in the cross-stream slope. Montgomery (19386) has plotted a 

 curve for changes in the cross-stream slope between Bermuda and 

 Charleston, reproduced here as fig. 76. 



