STOCHASTIC WAVE RADIATION BY THE GULF-STREAM - NUMERICAL EXPERIMENTS

The present work combines two ideas: (i) that transient Rossby waves a re excited by meander growth and decay and radiate into the far field; (ii) that the stochastic nature of meander growth, lifetimes and deca ys is responsible for the observed high values of eddy variances. A si mple barotropic model was used analytically through an asymptomatic th eory and numerically to explore the fully nonlinear regime. Three ques tions are addressed. First, in the fully nonlinear limit can a Rossby wave radiation field be excited for which the predicted covariance dis tributions compare well with the observed ones? Second, can the diverg ence of the associated eddy Vorticity fluxes induce a mean recirculati on similar to the deep recirculation gyre observed north of the stream ? Third, is the variable thickness of the deep layer due to the main t hermocline depth change across the stream and the sloping bottom topog raphy, crucial in driving the deep recirculation gyre? From the result s obtained the following was concluded. Over a flat bottom, both theor y and the numerical experiments predict a series of circulation cells produced by the nonlinear interactions between the forced and free wav e response of the flow. Realistic eddy variances due to Rossby wave ra diation into the far field are predicted. However, the induced mean re circulation is much too weak. A bowl-shaped topography that mimics the variable thickness of the deep layer across the stream front was then introduced. In this case, the mean cyclonic circulation follows the c losed potential vorticity contours and compares well with that observe d. However, this equivalent topography produces a very rapid energy de cay away from the southern boundary by inhibiting wave radiation. The fact that one or the other of the two results were achieved, but not b oth simultaneously, is a clear indication of the limitations of the mo del used in which the meandering stream is idealized as a southern bou ndary forcing. A final, interesting result of these simulations is tha t topographic Rossby waves are preferentially forced in regions where the stream meanders over the sloping relief, corresponding to the regi ons upstream of the Grand Banks and downstream of Cape Hatteras.