When are HF-radar observed wave heights modulated by periodic tidal and inertial currents?

Two recent sets of HF radar observations in the Atlantic by SeaSondes have observed unexpected periodic modulations of significant wave height at the M2 tidal and inertial frequencies. These are examined and related to the underlying, strong currents. They are interpreted in terms of kinematic advection influences that change the first-order wave characteristics due to frequency dispersion.