They are all reference surfaces representing a state of the ocean surface. Their oceanographic meaning is given below:
- The Mean Sea Surface (MSS) represents the mean state of the ocean and therefore includes permanent effects of global currents. MSS models are developed based on data provided by altimetry satellites. The MSS is not an equipotential surface and deviates from the Geoid. The MSS is a composite of the Geoid and sea surface topography.
- The Mean Sea Level (MSL) refers to a ‘level’ water surface, which you would get if the sea was shaped by the Earth’s gravity field only and perfectly at rest. As such it is an imaginary surface that doesn’t physically exist. It coincides with an equipotential surface, as for example the Geoid.
- The Geoid is the equipotential surface of the Earth’s gravity model that best fits the global mean sea surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest, and extended through the continents – approximated by Geoid Models as for example EGM96 and EGM2008.
- Dynamic Ocean Topography (DOT): the average over 12 year of the difference between the mean sea surface and the Geoid. It originates from the fact that the major ocean circulation has a (more or less) time-invariant non-zero component (i.e., a component that does not average to zero over time).
If the oceans were static and not affected by external influence as for example wind, temperature and air pressure, then Mean Sea Surface (MSS) and the Geoid would be the same surfaces. However, there are steady currents in the ocean, driven by winds and atmospheric heating and cooling, which give rise to differences in sea level around the world. These local differences between the Geoid and MSS are described by the Dynamic Ocean Topography (DOT). The DOT values range between (approximately) -2.5m and +1.2 m
An example plot of the DOT, calculated as the difference between the DTU10 MSS model and the EGM2008 Geoid model is available on the Danish National Space Institute web site, see:
Tides calculated relative to a Geoid model therefore are relative to Mean Sea Level (MSL). Tides calculated relative to the actual mean state of the local sea level are relative to Mean Sea Surface (MSS).