Disentangling discrete and continuous spectra of tidally forced internal waves in shear flow

Abstract: Generation of internal waves driven by barotropic tides over seafloor topography is a central issue in developing mixing and wave drag parameterizations. Traditional analytical methods for calculating the energy conversion rate from barotropic tides to internal waves rely on expanding a wave field into discrete eigenmodes. However, this modal expansion becomes inadequate when a sheared background current is present, as singular solutions accompanied by critical levels emerge. To uncover the distinct roles of regular eigenmodes and singular solutions in tidal energy conversion, this study analytically investigates wave generation over a localized bottom obstacle in the presence of shear flow under a basic irrotational condition. Using horizontal Fourier and temporal Laplace transforms, we identify regions in the topographic wave number and forcing frequency space where unbounded energy growth occurs. These regions coincide with the spectrum of an operator governing free wave propagation and comprise the discrete and continuous parts, which correspond to regular eigenmodes and singular solutions, respectively. Asymptotic evaluation of the Fourier integral reveals that the flow field far from the obstacle consists of standing wave trains associated with the discrete spectrum and evolving wave packets associated with the continuous spectrum. Away from the obstacle, the velocity amplitudes of wave packets decay, but their vertical gradients grow in the horizontal direction--in contrast to a single-wave number solution attracted towards a particular critical level. Finally, we derive a formula for the net barotropic-to-baroclinic energy conversion rate, extending the classical one by incorporating the contributions from both the discrete and continuous spectra.