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Estimation of the ocean Mean Dynamic Topography by optimal combination of a geoid model and along-track altimetric mean profiles

Philippe Schaeffer(1) and Marie-Hélène Rio(1)

(1) CLS, 8-10 rue hermes, 31526 Ramonville saint agne, France

Abstract

Sea Level Anomalies (SLA) are computed from altimetric measurements with a few centimetre accuracy using the repeat-track method. In order to compute from altimetric SLA the absolute value of the ocean dynamic topography, which is the signal of interest for oceanographers, the missing component is the Mean Dynamic Topography. The ocean Mean Dynamic Topography can be simply computed by directly subtracting a geoid model from an altimetric Mean Sea Surface (this is the so-called direct method). The altimetric Mean Sea Surface CLS01 is computed on a high resolution regular grid (1/30°) using along-track mean profiles from different altimetric satellites and the associated errors as input to an objective analysis. However, while altimetric Mean Sea Surfaces are known at the centimetric error level for spatial resolutions down to 10-20 km, up-to-date geoid models, based on GRACE data, reach this precision at scales no shorter than 400 km. This implies that both surfaces are correctly and consistently filtered before being combined. Practically, difficulties arise along the coasts or in strong subduction areas, where strong gradients present in the MSS (but not resolved in the geoid models) are not totally filtered by classical spatial filters resulting in unrealistic circulation features in the final Mean Dynamic Topography. Also, the errors associated to the geoid and the MSS are not taken into account in this direct method. A technique is presented here where both the geoid heights and the along-track altimetric mean profiles are combined through an objective analysis taking into account the variance and covariance error characteristics of both fields. The method is applied on the latest satellite-only geoid models based on GRACE data (EIGEN-GRACEGL04S) and the obtained MDT is compared to the direct MSS-Geoid solution.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry