The impact of the new gravity field models on the Mean Dynamic Ocean Topography and the derived geostrophic velocities

The absolute Mean Dynamic ocean Topography (MDT) can be determined from an accurate geoid model and a Mean Sea Surface (MSS). The MSS is derived using long-term time series of sea surface heights from multi-mission satellite altimetry. Recently, data from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite has become available. Now, GOCE and GRACE satellite data can be combined to obtain a geoid with higher accuracy and spatial resolution than before. The improvement in the geoid accuracy and resolution implies improvements in the resolution of MDT. From only 6 months of GOCE data, oceanographic fields like mean dynamic topography and geostrophic velocities are given in a fine spatial scale that has been poorly resolved previously. This is especially true in the areas of strong currents like Agulhas, Gulf, Kuroshio and Antarctic Circumpolar Current. Geostrophic velocities derived from only satellite data show very good agreement with geostrophic velocities measured by drifters. In addition the assimilation of this data set allows us to obtain all surface and subsurface ocean variables consistent with new MDT, giving promising results in comparison to the free model

Assimilation of geodetic dynamic ocean topography with ensemble based Kalman filter

We use the geodetic method to obtain the dynamical ocean topography (DOT). This method combines the multi-mission-altimeter sea surface height and the GRACE/GOCE gravity field. Using the new global filtering approach, the spectral consistency of both fields is achieved by filtering the sea surface height and the geoid. The new global filtering approach reduces the artificats near the coast lines. Further, results of assimilation of multi-mission-altimeter data and the GRACE/GOCE gravity data into the finite element ocean model (FEOM) are investigated. By assimilating only absolute dynamical topography data using the ensemble Kalman filter and time varying observation error covariances, we were able to improve modeled fields. Results are closer to observations which were not used for assimilation and lie outside the area covered by altimetry in the Southern Ocean (e.g. temperature of surface drifters or deep temperatures in theWeddell Sea area.