Data Processing and Computation Laboratory of New York University. 

 The directional spectra and computational algorithm used for these 

 calculations are discussed in the following section. The results of 

 two surface drift fields are presented and discussed in the section 

 on surface drift results. These prove to be of considerable signifi- 

 cance not only because their magnitudes may at times be comparable 

 to Ekman current velocities but because they apparently cannot be 

 adequately explained by any of the classical pure wind-driven or 

 thermohaline theories for ocean currents. In order to gain a deeper 

 insight into the ramifications of these particle motions, a long-crested 

 nonlinear model is considered. It is shown that under a series of 

 not unreasonable assumptions, these motions may easily be mistaken 

 for thermal undulations in a weakly stratified fluid. 



CALCULATION OF THE MEAN STOKES' VELOCITY IN A SHORT- 

 CRESTED RANDOM SEA 



The directional spectra used for the calculation of U were es- 

 sentially those described by Pierson and Tick (1964) and Cote et al. 

 (i960) except for the recent inclusion of a modified Miles -Phillips 

 growth mechanism by Inoue (1967). At each of 519 grid points of a 

 Baer grid (Baer, 1962) in the North Atlantic, the directional spectrum 

 can be thought of as 180 contributions to the total variance distributed 

 among 15 frequency and 12 directional bands. Figure 1 is an attempt 

 by the authors to represent schematically one of these spectra in 

 three dimensions. The horizontal plane is a polar plot with angular 

 frequency increasing radially outward from the central grid point and 

 direction increasing clockwise from due north (0°). Each of the 

 directional bands is a constant 30°. The frequency bandwidths are 

 listed in Table I. Work in progress will double the number of 

 angular bands. The vertical coordinate is a measure of variance 

 in units of (ft) 2. Within each d-f band, the estimate of variance, 

 Ej^(cj, 9), is assumed constant. 



For completeness, this particular form of the spectrum for a 

 fully developed wind sea may be expressed as 



S(cu,e)=^e"^^"o/-)^[F(,.ep] (3) 



u 



_3 



where a = 8. 1 X 10 



p = 0.74 



"o = g/''l9.5 



v,q c = wind speed at 19-5 meters above the sea surface 



and 4 , 4 , 



1 -(oj/ojj /4 -(co/oo„) /4 



F(w,e) = -!^[l + (0.50 + 0.82e ^ )cos2e + 0.32e ° cos 40] 



as an example, where 9 = wind direction. S(oj, 9) is defined for 



400 



