MSS :overview


SPACE OCEANOGRAPHY MEAN SEA SURFACE
Overview Method Validation Data products Bibliography Links

A Mean Sea Surface (MSS) corresponds to the permanent sea level relative to a reference ellipsoid, as measured by satellite altimetry. The MSS contains the marine geoid undulations (±120 meters ) plus the permanent sea level elevation (1-2 meters). The MSS are usually dedicated to geodesy and geophysics studies (e.g., Rapp et al., 1995), based on repeat-orbit and geodetic altimetric dataset. They provide a high spatial resolution, but a low accuracy (10-20 cm) subject to the time-varying ocean signal (e.g., the 3-year T/P mean profile compared to OSUMSS95 show 4-5 cm rms discrepancies). Such surfaces are too crude and inhomogeneous for ocean studies: these MSS can not be used for calculating Sea Level Anomalies.

Thus, the objectives are to estimate a new MSS, dedicated to ocean studies, focusing on the accuracy of the mean sea height along the satellite ground tracks and the homogeneity of the surface. Topex/Poseidon (T/P), GEOSAT, and ERS-1 improved altimetric dataset are used to estimate this mean sea surface.

The 3-years T/P (accuracy of 1.2 cm rms) mean profile is processed to reference the MSS. A two-year ERS-1 mean profile is calculated by merging phase C and G dataset, with a 1.6 cm rms accuracy. The two-year GEOSAT mean profile is 2 cm rms accurate.
These two mean profiles are adjusted (at crossover points) to T/P, in order to reduce the systematic biases and large scale errors.

The data of the two 168-day ERS-1 geodetic cycles are also processed and adjusted to T/P, to provide a global 8 km resolution to the MSS with a 6.5 cm rms accuracy.

Note that for all the ERS-1 individual passes, the ocean variability is removed from the the sea surface height by subtracting the T/P Sea Level Anomalies (SLA), then the orbit error are also reduced by spline-fitting with T/P arcs (Le Traon et al., 1998a). Thus, geodetic data crossover differences with the T/P mean profile are reduced from 12 to 6.5 cm rms. A suboptimal inverse technique is applied to estimate the MSS from these data, on a 1/16 grid. This technique take into account the long wavelength biases on altimetric arcs and also the oceanic variability noise. The inverse technique allow to calculate the estimation error.

The MSS is validate first by comparison with the mean profiles (1.3 cm rms and 0.11 cm/km rms with the T/P mean profile).

The second validation is based on the differencies with other MSS (OSU95 and GRGS). The MSS differencies standard deviation are less than 11 cm. The maps of differences show that the CLS_SHOM v. 98.2 MSS have strongly reduced the trackiness effects. However, at really short scales, some so called "orange skin" effects are still observable.

OBJECTIVES

The CLS_SHOM MSS must be accurate along and close to the satellite ground tracks of the oceanic dedicated altimetric missions (T/P, ERS, GEOSAT.... JASON, ENVISAT, GFO).
The CLS_SHOM MSS must contain the short wavelength of the geoid undulations at the vicinity of the satellite ground tracks.
The CLS_SHOM MSS should be the reference for calculating and merging homogeneously Sea Level Anomalies.

PERSPECTIVES

The CLS_SHOM MSS will be used for merging present altimeter data and futur data of Jason, ENVISAT, and GFO.
The MSS should be determined over a longer period by a reference to 5 year TOPEX/Poseidon Mean Profile.
Our processing and gridding methode will be adapted for gravity anomalie and vertical deflection calculations.