Variations in sea level
Satellite altimetry is used to measure precisely (at centimetre level), globally and almost instantaneously (at the scale of ocean dynamics) sea level variations. Ocean surface topography is variable on several scales of time and space, reflecting a large number of phenomena:
Constant deviation with regards to the reference ellipsoid (approximately 100 meters) is mainly attributed to the geographical structure of the Earth geoid, i.e. the uneven distribution of mass inside our planet.
Sea surface deviation with regards to the earth geoid (that can be determined independently using gravimetric satellites such as CHAMP and GRACE), with amplitudes of the order of one meter, is known as "dynamic topography". Such deformations on the sea's surface are related to global oceanic circulation. In a similar manner to atmospheric pressure maps used for meteorology, ocean surface currents follow level curves with a speed proportional to its local slope. We can thus map the main sea currents, such as the Gulf Stream or the Kuroshio.
Temporal variations in surface topography are also used to observe and monitor ocean variability (vortex, Rossby waves, etc.), tides, seasonal and/or climatic phenomena, such as El Niño.
Finally, in the long term it is possible to monitor the average sea level. Since the beginning of the TOPEX/POSEIDON mission in 1992, an average global increase in sea levels of around 3 mm has been observed, with strong spatial variability (up to ± 20 mm/yr according to the region). This increase is an indicator of global warming, and in this respect sustaining the continuity and precision of these measurements is a major challenge for altimetry missions.
Products derived from altimetry measurements
In addition to surface topography, the signal recorded by altimeters is used to measure two other very useful parameters for marine meteorology: the significant wave height (SWH: average wave height over the footprint of the altimeter) and the surface wind speed. Available in almost real-time, these measurements are used for meteorological forecasts.
Altimetry on the continents
Although designed to measure the height of ocean waters (of which the "radar signature" is correctly identified), altimeters also have the capacity to obtain observations above continents, particularly on any water expanse that is large enough to be detected. This capacity has opened new perspectives for continental hydrology. Using altimetry satellites thus makes it possible to monitor seasonal variations in lake levels and certain major rivers. These applications are particularly important in remote and/or poorly instrumented areas, such as the Amazon basin.
Combination of altimetry and in-situ measurements: operational oceanography
Data assimilation consists of combining observations and models to make precise predictions about evolutions in complex systems. Classicly used for meteorological forecasts, this technique can be transposed to operational oceanography. In 2003 was initiated the international GODAE (Global Ocean Data Assimilation Experiment) experiment, the first "full scale" international operational oceanography experiment. It aimed to demonstrate that it is possible to observe, model and predict the global ocean in three dimensions, routinely and in real-time.
Pilot systems from the GODAE programme, the Public Interest Group Mercator Ocean, created in April 2002, implemented a system used to describe the state of the ocean, an essential component of our environment, at any time and from any corner of our blue planet.
The Mercator system is "fed" through inputs consisting of observations of the ocean measured by satellites (altimetry, and also surface temperature and salinity) as well as in situ measurements (drifting buoys, sensors and temperature, salinity and current profilers). These measurements are "ingested" (assimilated) by the analysis and prediction model. Assimilating observation data in a model is thus used to describe and predict the ocean over periods of up to 14 days. Since October 2005, Mercator has been operating a global oceanographic prediction model with ¼° resolution, i.e. approximately 28 kilometres from the equator.