From the altimetric sea level measurement to the ocean absolute dynamic topography: Mean Sea Surface, Geoid, Mean Dynamic Topography, a three-component challenge
Philippe Schaeffer(1) , Marie-Helene RIO(1) , Fabrice Hernandez(2) , and Jean-Michel Lemoine(3)
8-10 rue Hermes,
31526 Ramonville St-Agne,
(2) IRD-MERCATOR OCEAN, 8-10 rue Hermes, 31526 Ramonville Ste Agne, France
(3) CNES, 18 avenue Edouard Belin, 31401 Toulouse, France
With the launch of the altimetric satellites ERS-1 and T/P in the 1990’s it first became possible to measure with a precision of few centimetres the sea level above a reference ellipsoid. In order to extract from the altimetric measurement the absolute dynamic topography, (the sea level above the marine geoid, which is the signal of interest for oceanographers), the accurate knowledge of the marine geoid above the same ellipsoid is required. Huge improvements have been made in our knowledge of the geoid since the time when only the longest wavelengths could be retrieved applying the theory of perturbations to geodesic satellites (Starlet, Lageos, Etalon). Nowadays, with the succession of dedicated gravimetric missions (CHAMP, GRACE), the geoid can be estimated with centimetric accuracy at a 400 km resolution. However this resolution is still too coarse to fully resolve the entire spectra of oceanographic phenomena.
To compensate for the lack of an accurate geoid, the altimetric Mean Sea Surface is commonly subtracted from each single altimetric measurement to obtain an estimate of the variable part of the dynamic topography (or Sea Level Anomaly, SLA). Thanks to the global, continuous and repetitive set of altimetric observations allowed by the successive launches of ERS-2, Jason-1, Envisat, GFO, the ocean MSS is now calculated with a sub-centimetric precision for a spatial resolution lower than thirty km.
The reference field needed to reconstruct the ocean absolute dynamic topography from the SLA is the ocean Mean Dynamic Topography (MDT). It can be obtained subtracting the geoid from the MSS (the so-called direct method) but its accuracy and resolution is limited by the geoid accuracy. Other methods have thus been developed to estimate the shortest scales of the MDT, based on Ocean General Circulation models, inverse modelling, in-situ data, or a combination of different datasets.
The aim of this paper is to review the fundamental improvements realized since the launch of ERS-1 to precisely determine the MSS, the geoid and the MDT, all three key components of the altimetric measurement for oceanographic applications, as well as to investigate the contribution and limitation of upcoming altimetric (Altika, WSOA, Icesat) and gravimetric (GOCE) missions.