Nonlinear anisotropic diffusive filtering applied to the ocean's mean dynamic topography

Despite the improvement in global geoid determination that the Gravity Recovery and Climate Experiment (GRACE) satellite mission has provided, it remains necessary to filter the ocean's mean dynamic topography (MDT) that is derived from it to remove residual noise. A standard approach is to use isotropic spatial smoothing. Unfortunately, this also attenuates MDT gradients associated with surface currents. Here an alternative, anisotropic approach taken from the field of image processing that preferentially filters along rather than across gradients is presented. Applying the method to the MDT of the North Atlantic, it is found that attenuation of MDT gradients corresponding to important currents such as the Gulf Stream and Labrador Current is much reduced. Along the axis of the Gulf Stream, this translates in places to more than a doubling in the estimated current speed compared to a conventionally filtered MDT.