Fluctuations 151 



wind-induced circulation. Curve 2 gives the actual tide-gauge data 

 averaged for seventeen years and corrected for atmospheric pressure. The 

 difference between curve 1 and curve 2 is plotted as curve 3. 



It is natural to inquire whether the difference (curve 3) can be due to the 

 directly wind-induced changes of oceanic circulation. In order to make a 

 rough, tentative order-of-magnitude calculation, I computed the mean 

 monthly curl of the square of the zonal winds centered at 32°5 N. latitude 

 across the Atlantic. This quantity should be directly proportional to the 

 mean monthly curl of the wind stress in the latitude of Bermuda. Ac- 

 cording to the theory of Veronis and Morgan (1955), the meridional 

 transport in the central oceanic areas should be given by the relation 



jj _ mean monthly transport _ mean monthly curl of squared zonal winds ,_, 

 mean yearly transport mean yearly curl of squared zonal winds ' ^ ' 



We shall call this quantity N. I was quite surprised to note that N varies 

 from 0-6 in September and October to a maximum of 1-6 in December. 

 This suggests very much larger variations in oceanic transport than those 

 indicated by Iselin (1940) on the basis of hydrographic-station data. It is 

 important to remember that the changes of transport which we are com- 

 puting cannot appear in hydrographic data, since they are associated with 

 the barotropic mode. The variation d in sea level due to these monthly 

 variations in transport is given by the formula 



d = ^{N-l), (6) 



where T is the mean yearly transport in the central oceanic area of the 

 North Atlantic at 32?5 N. latitude, and D is the total mean depth of the 

 ocean. In plotting curve 4 I assumed that /= 0-7 x 10~^/sec., T = 60x 

 10^ m.^/sec, and D = 4 km. The agreement of this very crude calculation 

 with curve 3 is, to my mind, rather good, and suggests a verification of 

 the theory of transient currents. However, a truly careful discussion of the 

 problem must await the detailed results of Dr Pattullo's studies of the 

 ocean-wide distribution of changes in sea level. There are other physical 

 obstacles to a convincing demonstration. For example, we do not know 

 the ways in which the wind stress depends upon the air-sea temperature 

 difference, and we do not have an adequate theory of the thermohahne 

 circulation in central oceanic areas to use as a companion piece along with 

 the theory of wind-driven circulation. These speculations about the data 

 and about the applicability of the theoretical models will remain incon- 

 clusive until really adequate data are available. 



Systematic long-duration series of oceanographic measurements are not 

 obtained on a routine basis. The Western Union cable measurements are a 



