of the spectrum, from which a mean source function was then 

 determined through the energy balance equation (1). The 

 characteristic +- distribution of the source function (cf. 

 Fig. 5) is due to the shift of the spectral peak towards 

 lower frequencies. The positive lobe corresponds to the 

 rapid wave growth on the forward face of the spectrum; after 

 the peak has passed to lower frequencies, the components on 

 the right of the peak then decay again before approaching a 

 quasi-stationary equilibrium value (cf. also Fig, 3). 



4. THE ENERGY TRANSFER DUE TO WAVE-WAVE INTERACTIONS 



This "overshoot" phenomenon has been observed indepen- 

 dently by several workers in field and laboratory experi- 

 ments ([6] [7] [36a, b] [37] [53]). Barnett and Sutherland 

 (1968) suggested nonlinear wave-wave interactions as a 

 possible explanation, a conjecture which later found some 

 support by Mltsuyasu's (I968) estimates of the nonlinear 

 transfer rates for a decaying spectrum in a wave tank. The 

 calculations were based on Barnett 's (1968) parametrlzation 

 of the Boltzmann scattering integrals computed in Hasselmann 

 [20c]. Exact computations of the nonlinear transfer 

 integrals for a number of JONSVJAP spectra ([5] [50]) have 

 confirmed that the principal features of the observed source 

 function can indeed be explained by resonant wave-wave 

 interactions: Fig. 5 shows that both the overshoot and the 



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