According to this picture, the development of the wave 

 spectrum for finite fetches is governed primarily by the 

 energy balance between the atmospher input S. and the non- 

 linear transfer of energy S from the main part of the spec- 

 trum to lower and higher frequencies. Under stationary condi- 

 tions the low-frequency energy generated by wave-wave inter- 

 actions is convected away, V*VF % S. , whereas at high 

 frequencies the energy gain due to the nonlinear transfer and 

 the atmospheric input has to be balanced by some dissipative 

 mechanism. About 10% of the total momentum transferred across 

 the air-sea interface is convected away by the low-frequency 

 waves. This can be deduced from the increase of the total 

 momentum of the wave field with fetch, independent of the 

 source function analysis . Between 15 and 60% of t is 

 accounted for by the nonlinear transfer* of momentum from the 

 main part of the spectrum to shorter waves ( f> . 7 Hz in 

 Fig. 5), where it is converted to current momentum by dissipa- 

 tion. Again, this estimate is independent of details of the 

 energy balance, following alone from the nonlinear transfer 

 rates computed for the observed spectra. To balance these 

 momentum fluxes, about 50% ±30% of the momentum transferred 

 across the air-sea interface must be entering the wave 

 field in the central region of the spectrum. 



* The uncertainty reflects the sensitive dependence of these 

 computations on the spectrum. 



25-25 



