Wake vortices in tidally modulated currents past a conical hill in a stratified fluid are investigated using large‐eddy‐simulation. The vortex shedding frequency is altered from its natural steady‐current value leading to synchronization of wake vortices with the tide. The relative frequency (f*), defined as the ratio of natural shedding frequency (fs,c) in a current without tides to the tidal frequency (ft), is varied to expose different regimes of tidal synchronization. When f* increases and approaches 0.25, vortex shedding at the body changes from a classical asymmetric Kármán vortex street. The wake evolves downstream to restore the Kármán vortex‐street asymmetry but the discrete spectral peak, associated with wake vortices, is found to differ from both ft and fs,c, a novel result. The spectral peak occurs at the first subharmonic of the tidal frequency when 0.5 ≤ f* < 1 and at the second subharmonic when 0.25 ≤ f* < 0.5.

there is direct wave breaking at the ridge with enhanced turbulence dissipation at the seafloor while at topographies with shallow slopes like Brazil Basin, wave-wave interactions cascade energy to high wave numbers, and ultimately to turbulence (e.g., St. Laurent & Garrett, 2002). Recent measurements of MacKinnon et al. (2019) andVoet et al. (2020) show significant energy loss associated with vertically sheared wake eddies at the northern end of the Palau island

Large eddy simulations (LES) are employed to investigate the role of time-varying currents on the form drag and vortex dynamics of submerged 3D topography in a stratified rotating environment. The current is of the form Uc + Utsin(2πft), where Uc is the mean, Ut is the mean, Ut is the ideal component and ft is its frequency. A conical obstacle is considered in the regime of low Froude number. When tides are absent, eddies are shed at the natural shedding frequency fs,c. The relative frequency f* = fs,c/ ft is varied in a parametric study which reveals states of high time-averaged form drag coefficient. There is a two-fold amplification of the form drag coefficient relative to the no-tide (Ut = 0) case when f*lies between 0.5 and 1. The spatial organization of the near-wake vortices in the high drag states is different from a Kármán vortex street. For instance, the vortex shedding from the obstacle is symmetric when f* = 5/12 and strongly asymmetric when f* = 5/6. The increase in form drag with increasing f* stems from bottom intensification of the pressure in the obstacle lee which we link to changes in flow separation and near-wake vortices.

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