ON INTERNAL WAVES GENERATED BY TRAVELING WIND

Oceanic internal waves forced by a latitude-independent wind field tra velling eastward at speed U is investigated, extending the hydrostatic f-plane model of Kundu & Thomson (1985). The ocean has a well-mixed s urface layer overlying a stratified interior with a depth-dependent bu oyancy frequency N(z), and f can vary with latitude. Solutions are fou nd by decomposition into vertical normal modes. Problems discussed are (i) the response to a slowly moving line front, and (ii) the response in a variable-f ocean. For the slowly moving line front assuming a de pth-independent N, the trailing waves are found to have large frequenc ies, and the vertical acceleration partial derivative w/partial deriva tive t is important (that is the dynamics are non-hydrostatic) if the frequency omega is larger than a few times (Nf)1/2. The wake contains waves associated with all vertical modes, in contrast to hydrostatic d ynamics in which slowly moving line fronts do not generate trailing wa ves of low-order modes. It is argued that slowly moving wind fields ca n provide an explanation for the frequently observed broad peak in the spectrum of vertical motion at a frequency somewhat smaller than N, a nd of the vertical coherence of the associated waves in the upper ocea n. To study lower-frequency internal waves, the hydrostatic constant-f model of Kundu & Thomson is extended to variable f. Various sections through such a flow clearly illustrate the development of a meridional wavelength lambda(y) = 2pi/betat as predicted by D'Asaro (1989), in a ddition to the zonal wavelength lambda(x) due to translation of the wi nd. The two effects combine to cause a greater horizontal inhomogeneit y, so that energy from the surface layer descends quickly, travelling equatorward and downward. Since waves at any point arrive from differe nt latitudes, spectra no longer consist of discrete peaks but are more continuous and broader than those in the constant-f model. The waves are more intermittent because of the larger spectral width, and vertic ally less correlated in the thermocline because of a larger bandwidth of vertical modes. The vertical correlation in the deep ocean, however , is still high because the response is dominated by one or two low-or der modes after 30 days of integration. As U decreases, the larger ban dwidth of frequency increases the intermittency, and the larger bandwi dth of vertical wavenumber decreases the vertical correlation. A super position of travelling wind events intensifies the high-frequency end of the spectrum; a month-long travelling series of realistic strength can generate waves with amplitudes of order 4 cm/s in the deep ocean. It is suggested that propagating winds and linear dynamics are respons ible for the generation of a large fraction of internal waves in the o cean at all depths. The main effect of nonlinearity and mean flow may be to shape the internal wave spectra to a omega-2 form.