Fluctuations 147 



The chief difficulties in utilizing tide-gauge data are: (i) it has been 

 impossible, thus far, to make the proper allowance for the slope of the 

 water surface produced by the direct stress of the local wind over the 

 continental shelf; (ii) the cross-stream slope is related only to surface 

 velocity, and, unless something definite is known about the vertical dis- 

 tribution of velocity, does not give an unambiguous value of the total 

 transport. 



One of the most striking features of fig. 75 is the small amount of 

 hydrographic data available, and, worse yet, the concentration of the data 

 toward the latter half of each year. The project was interrupted by the 

 Second World War and was never resumed. The broken-line curve is 

 drawn as a kind of fanciful interpolation, and the ordinate scales of the 

 tide-gauge data are chosen simply to make the curves look alike. I think 

 one is well advised, when confronted by a set of curves such as those in 

 fig. 75, to regard them as merely suggestive, rather than to imagine that 

 they prove anything. 



THEORETICAL STUDIES OF TRANSIENT CURRENTS 



Several attempts have been made to construct theoretical models that will 

 help in understanding the transient state of the ocean circulation brought 

 about by forced changes in the winds. For example, Ichiye (1951) has 

 obtained an interesting solution for a wind-driven circulation produced by 

 a periodic wind stress acting upon a homogeneous ocean within a rect- 

 angular ocean. He uses the frictional boundary layer for the Gulf Stream. 

 He finds that the dynamic topography on the western coast consists of a 

 series of 'waves' of dynamic height moving toward the shore with in- 

 creasing amplitude at the rate of one crest per year. Thus the annual 

 fluctuation of transport of the Gulf Stream is associated with a shift in 

 the Stream's position. Veronis and Morgan (1955) have obtained a result 

 very different from Ichiye's. The difference apparently arises from their 

 inclusion of the important time-variable term in the mass-continuity 

 equation (which Ichiye takes as vanishing). Veronis and Morgan treat a 

 homogeneous ocean, and find : that the mass transport of the Gulf Stream 

 responds to variable wind systems of more than three months' duration 

 with lags of the order of nine days ; and that there is no noticeable shift in 

 the position of the Stream throughout the year. 



Both these theoretical studies treat the ocean as though it were homo- 

 geneous, whereas, of course, the real ocean is stratified in layers of different 

 density. In the subtropical regions of the Atlantic Ocean, most of the 

 density change with depth occurs in the thermocline, as we have already 

 seen (Chapter IV), and we get some approximation to the true density 



