Mean Dynamic Topography - MDT-CNES-CLS2022

The Mean Dynamic Topography MDT-CNES-CLS2022 is an estimate of the mean over the 1993-2012 period of the sea surface height above geoid. Use of the latest GOCO06S geoid model (based on the complete GOCE mission fully reprocessed and 14 years of GRACE data) and 30 years of altimetry and in-situ data (hydrologic, drifters and High Frequency radar on a limited area).

The Mean Dynamic Topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents.

This new combined MDT CNES-CLS22 defines better currents near the coast, estimated with in situ data High Frequency radars.

The MDT CNES-CLS calculation combines several steps: a) We first compute a first guess: a medium scale of Mean Dynamic Topography using only satellite data (gravimetry and altimetry). The raw difference of Mean Sea Surface with geoid model is filtered using an optimal filter to remove noise and residual geodetic signal. b) Beside we compute synthetic estimate of MDT and mean geostrophic current from oceanic in-situ data (T/S profiles and drifters).

• First, the data are processed to have the same physical content as altimetry: the drifters (SVP and Argo drifting at the surface) are processed to only extract the geostrophic component also deep baroclinic and barotropic components are added to dynamic height computed from T/S profiles.
• Then sea level anomalies are interpolated and subtracted from in-situ measurements of the ocean state to estimate a mean: MDT and the corresponding mean geostrophic surface velocities. c) Then, using a multivariate objective analysis, the synthetic estimates of mean heights and mean velocities are combined to improve the first guess of the MDT. d) Finally, a validation is done by comparing this version to other MDTs and independent in-situ data.