Directional decomposition of internal tides propagating from multiple generation sites

Abstract

Internal tides generated from multiple sites create complicated interference patterns, often making it difficult to identify the origin, direction of propagation, and dissipation of individual waves. To identify these constituent waves, we apply a directional Fourier filter (DFF) method to analyse multimodal, broad-wavenumber, internal wave fields found over varying topography. We apply the method to data derived from both a numerical model and satellite remote sensing observations. Using a series of two-dimensional (i.e. x–z) simulations, we first demonstrate the capability of the DFF method to separate incoming and reflected internal tides emanating from a supercritical slope. The results show the DFF method requires model sampling rates of Δt∕T<0.15 and horizontal cell sizes of Δx∕λ<0.2, where T is the wave period and λ is the wavelength. Remote internal tides can significantly affect the generation and propagation of local internal tides in regional ocean simulations. Using a gridded satellite altimetry product at three globally diverse regions, we demonstrate how the DFF method can be used to specify the internal tide boundary conditions at the open boundaries of regional ocean models. Lastly, we apply the DFF method to a three-dimensional hydrostatic numerical simulation of the southern Australian North West Shelf to estimate the energy decay characteristics of internal tides. The energy decay length scale, for offshore-propagating waves from the continental slope, in the model was estimated to be two mode-1 wavelengths (i.e. 240 km), in close agreement with an estimate derived from a satellite-derived sea surface height anomaly.