3.2.2.5.3. Mean Sea surface

The Mean Sea Surface represents the sea level due to constant phenomena. It is the addition of the geoid and the Mean Dynamic Topography (that includes the permanent stationary component of the ocean dynamic topography).

A Mean Sea Surface based on 7 years of Topex/Poseidon data, 5 years of ERS-1/2 data, 2 years of Geosat data and ERS-1 geodesic data
(Credits CLS)

A Mean Sea Surface shows the mean shape of the sea surface during the period covered by the altimetry data. It corresponds to the sum of geoid + mean ocean circulation (it is corrected from seasonal variations). The relief of this surface reveal:

on a scale of thousands metres, undulations of hundred of metres due to differences in densities into the mantle;
on medium wavelengths, currents and water masses of varying densities, leading to sea level variations of several metres;
on short wavelengths, the heterogeneous distribution of matter concerning earth's surface layer and ocean's bottom topography, that implies geoid undulations of several metres.

A Mean Sea Surface (MSS) represents the position of the ocean surface averaged over an appropriate time period to remove annual, semi-annual, seasonal, and spurious sea surface height signals. A MSS is given as a grid with spacing consistent with the altimeter and other data used in the generation of the grid values. The MSS grid can be useful for data editing purposes; for the calculation of along track and cross track geoid gradients; for the calculation of gridded gravity anomalies, for geophysical studies; for a reference surface to which sea surface height data from different altimeter missions can be reduced, etc.

Longer time spans of data that become available in the future, along with improved data handling techniques could improve the current MSS models. Care must be given to the retention of high frequency signal and the reduction of high frequency noise.