COUPLED ACOUSTIC MODE PROPAGATION THROUGH CONTINENTAL-SHELF INTERNAL SOLITARY WAVES

Three techniques are used to investigate mode coupling as acoustic ene rgy passes through continental-shelf internal solitary waves (ISW's), Results from all techniques agree, The waves considered here are singl e downward undulations of a thermocline layer separating upper and low er well-mixed layers, Two techniques are numerical: parabolic equation (PE) solution and a sudden approximation joining range-invariant regi ons at sharp vertical interfaces, The third technique Is an analytic d erivation of ISW scale lengths separating adiabatic (at large scale) a nd coupled-mode propagation, Results show that energy is exchanged bet ween modes as ISW's are traversed, The sharp interface solutions help explain this in terms of spatially confined coupling and modal phase i nterference, Three regimes are observed: 1) for short ISW's, coupling upon wave entrance is reversed upon exit, with no net coupling; 2) for ISW scales of 75-200 m, modal phase alteration averts the exit revers al, giving net coupling; transparent resonances yielding no net coupli ng are also observed in this regime; and 3) for long ISW's, adiabatici ty is probable but not universal, Mode refraction analysis for nonpara llel acoustic-ISW alignment suggests that these two-dimensional techni ques remain valid for 0 degrees (parallel) to 65 degrees (oblique) inc idence, with an accordant ISW stretching.