Ocean Circulation Estimations using GOCE Gravity Field Models
RIO, Marie-Helene1; Mulet, Sandrine1; Andersen, Ole2; Knudsen, Per2; Bruinsma, Sean3; Marty, Jean-Charles3; Förste, Crhistoph4; Abrikosov, Oleg4
1CLS, FRANCE; 2DTU, DENMARK; 3CNES/GRGS, FRANCE; 4GFZ, GERMANY
The European GOCE Gravity consortium (EGGc), under ESA contract, computes gravity field models that only use GOCE and GRACE plus GOCE satellite data, respectively. Gravity field models that are constructed using both satellite and surface (gravimetry and mean sea surface) data are called combined models. The different geoid models (GRACE, GOCE, GRACE+GOCE, and combined models) are used together with an altimetric mean sea surface to compute the ocean Mean Dynamic Topography (MDT) and the associated mean geostrophic currents. This is done for different spatial resolutions by applying a filter to the MDTs. The mean currents obtained with the different geoid models are compared to the ocean mean geostrophic currents measured by a dataset of SVP buoy velocities available from 1993 to 2010 from which the Ekman currents are removed as well as the temporal variability measured by altimetry. Currents derived from existing combined MDT as the one by (Maximenko et al, 2009) or the latest CNES-CLS MDT are also used for comparison. In the construction of a combined gravity field model, surface and satellite data are mixed at the normal equation level in which the satellite part exclusively provides the low to medium degree coefficients (i.e. spatial resolution) of the model, whereas the surface data contain the information for the high degree coefficients. The model coefficients are computed using both data types in a transition band, which is selected based on the accuracy as a function of spatial scale of the data types. However, the scale at which the data types are mixed has a large impact on the smoothness of the model over the ocean, and therefore on the MDT and the derived currents. The differences between the mean geostrophic currents derived from gravity field models and inferred from drifter data or the combined MDT models are analyzed as a function of resolution and location, and notably for all of the ocean’s major current systems. Such an analysis allows quantifying the gain in accuracy compellingly: 1) from GRACE to GOCE models, 2) due to assimilating more and more GOCE data, 3) and to the inclusion of surface data in the models.