  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
$U_c+U_t \sin(2\pi f_t t)$, where $U_c$ is the mean, $U_t$ is the tidal
component and $f_t$ 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 $f_{s,c}$. The relative frequency $f^*=f_{s,c}/f_t$
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 ($U_t=0$) 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 K\'arm\'an 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 is linked to
changes in flow separation and near-wake vortices.
