Contribution of the Geoid to the Error Budget of Computed Geostrophic Currents
Bruinsma, Sean¹; Balmino, Georges¹; Rio, Marie-Helene²; Mulet, Sandrine²; Förste, Christoph³; Knudsen, Per⁴; Andersen, Ole^4
1CNES, FRANCE; ²CLS, FRANCE; ³GFZ, GERMANY; ⁴DTU Space, DENMARK

When global gravity field models are used to compute the geoid over the ocean, which is then subtracted from an altimetric Mean Sea Surface (MSS), the ocean Mean Dynamic Topography (MDT) is obtained. The error characteristics of the mean geostrophic currents associated with the MDT have their sources in the MSS, i.e. the altimeter measurement system, and the geoid. While the first error source is very complicated to calculate, the second one can be computed relatively easily using the variance-covariance matrix of the gravity field solution. The resulting errors can then be visualized as error ellipses for any location over the ocean, as was shown by Balmino (2009). This study aims at describing the error introduced by the geoid model on the derived geostrophic currents. Signal-to-noise ratios can be calculated as the computed current-to-current error ratios. Error ellipses and signal-to-noise ratios over the entire ocean are thus analyzed, and in particular at the locations of the major current systems. The errors are put into context by comparing them to ocean mean geostrophic currents measured by a dataset of SVP buoy velocities. The EIGEN-6C3 combined gravity field model and the fourth release of the GOCE satellite-only gravity field model for ESA, EGM-DIR4, are used in this analysis.