Using Lagrangian Filtering to Remove Waves From the Ocean Surface Velocity Field
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AbstractThe Surface Water and Ocean Topography satellite will measure altimetry on scales down to about 15km: at these scales, the seasurfaceheight signature of inertiagravity waves, including barotropic tides and internal tides, will be visible. However, tides and inertiagravity waves have little impact on tracer transport. Recent work has shown that Lagrangian filtering can be used to isolate the inertiagravity wave part of the flow. This manuscript presents a recipe for removing barotropic motions and inertiagravity waves from the surface velocities and from the seasurface height (SSH), to estimate the nonwave part of the flow in the Agulhas region of a highresolution 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 highfrequency motions by the lowfrequency 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 superinertial 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 nonwave part of the velocity field that is preserved more effectively by Lagrangian filtering includes convergent motions near regions of frontogenesis.
