Using Lagrangian Filtering to Remove Waves From the Ocean Surface Velocity Field

The Surface Water and Ocean Topography satellite will measure altimetry on scales down to about 15 km: at these scales, the sea-surface-height signature of inertia-gravity waves, including barotropic tides and internal tides, will be visible. However, tides and inertia-gravity waves have little impact on tracer transport. Recent work has shown that Lagrangian filtering can be used to isolate the inertia-gravity wave part of the flow. This manuscript presents a recipe for removing barotropic motions and inertia-gravity waves from the surface velocities and from the sea-surface height (SSH), to estimate the non-wave part of the flow in the Agulhas region of a high-resolution ocean model (LLC4320). First, two methods for removing the barotropic component of SSH variability are presented. Then Lagrangian filtering, a method that accounts for Doppler shifting of high-frequency motions by the low-frequency velocity field, is applied to both the SSH and the ocean surface velocity field. The results of Lagrangian filtering are presented in spectral space. Lagrangian filtering preserves motions that appear super-inertial in the reference frame of the Earth, while other methods do not preserve these motions as effectively. In some locations most of the energy at high frequencies comes from these Doppler shifted balanced motions. We show that the non-wave part of the velocity field that is preserved more effectively by Lagrangian filtering includes convergent motions near regions of frontogenesis.