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Impact of geoid definition on the simulated strength of the equatorial undercurrent in a global data assimilation system.

Dr Femke Vossepoel(1), Dr Johannes Bouman(2) , Dr Radboud Koop(2) , Marc Naeije(3) , and Dr Peter Jan Van Leeuwen(4)

(1) IMAU/SRON, Princetonplein 5, 3584 CC Utrecht, Netherlands
(2) SRON, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands
(3) DEOS, Kluyverweg 1, 2629 HS Delft, Netherlands
(4) IMAU, Princetonplein 5, 3584 CC Utrecht, Netherlands


The availability of gravity data from recent satellite missions (CHAMP, GRACE) and the future GOCE mission will improve our current knowledge of the Earth's geoid. An accurate description of the geoid is essential for a proper interpretation of altimetric sea level data, which can help us to better understand the ocean circulation. This study evaluates the sensitivity of an estimated ocean circulation to the definition of the geoid, using a global ocean circulation model (OGCM) and an advanced data assimilation technique.

In the analysis system, Topex/Poseidon altimetric sea level heights are related to modeled sea level. Instead of using sea level anomalies with respect to a long-term mean, which is the most common usage of altimetry, the absolute sea level minus the geoid estimate is assimilated. In the current study, we adopt TOPEX/Poseidon altimetry and incorporate the most recent geoid solutions based on satellite data (GRIM5-S1, EIGEN-2, GRACE).

The analysis system consists of the OPA OGCM, in which observations are being assimilated. The assimilation technique, the sequential importance resampling (SIR), is an ensemble-based method, that determines the optimal model trajectory by statistical evaluation of a number of perturbed model simulations. The implementation of this technique is especially efficient in a parallel computing environment.

Specific attention is being paid to the equatorial undercurrent in the Atlantic, Indian and Pacific Oceans. We evaluate the impact of the geoid definition on its strength and its variability, and discuss the implications of improving geoid accuracies for the estimation of ocean circulation.


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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