CoAuthors

Marie-Hélène Rio (CLS, France); Yannice Faugere (CLS, France)

Altimetry gives information about the Sea Surface Height (SSH), but the interesting quantity for the study of the ocean dynamic is the SSH difference from the geoid height called the Absolute Dynamic Topography (ADT). As the geoid is not known with enough accuracy at high resolution, the ADT is classically computed by an indirect method. A Mean Sea Surface (MSS) is first removed to the SSH; then a Mean Dynamic Topography (MDT) is added. This method is thus impacted by MSS and MDT errors; the Arctic Ocean is particularly challenging because of the poor altimetry coverage of the area which is seasonally covered by sea ice.

Thanks to the Gravity Field and Steady-State Ocean Circulation (GOCE) mission, the geoid models have reached higher accuracy and a direct method can now be used to compute the ADT. For this propose, we use the EGM_DIR_R4 geoid model computed in the framework of ESA HPF (High Processing Facility) from two years of reprocessed GOCE data. The EGM_DIR_R4 geoid height model is subtracted to the Cryosat-2 SSH to compute along track ADT. Errors associated with along track ADT field are evaluated taking into account the omission and the commission geoid errors and also the errors from altimetry measurement. Then the ADT are combined through an optimal analysis to compute a regular gridded ADT map. With this method, we do not need to compute mean quantities (MSS and MDT).

In the study we compare these two methods, that have different error sources, to explore the potential of the direct method and to attempt to evaluate the error of the indirect method in the Arctic Ocean.