General description of Satellite Altimetry

The radar altimeters on board of the satellites TOPEX/POSEIDON (T/P), ERS-1 and ERS-2 measure the topography of the sea surface and its variations in space and time. The Sea Surface Height changes are caused by multiple external forces (e.g., tides, atmospheric pressure effects, wind forcing on the ocean), and internal processes (e.g., oceanic waves, water masses salinity and heat content... ). The data are processed in order to provide the sea level variations associated with the ocean dynamical processes. The Sea Level Anomalies (SLA) obtained along the satellite passes correspond mainly to the ocean geostrophic circulation. Note that the non-variable part of this circulation has been subtracted with the geoid height removal. A complete description of how and why satellite altimetry has been developped is presented in the AVISO web site.

The orbital characteristics of T/P and ERS satellites are different. The high repetitivity (~10 days) of T/P allow a good monitoring of the ocean sea level changes. But its rather low spatial sampling (~270 km at 32°N, the black ground tracks on the right figure) does not allow a complete description of the ocean mesoscale circulation. At the opposite, ERS-1 and ERS-2 have a finer spatial sampling (~80 km at 32°N), as it is shown by the figure, where ERS and T/P ground tracks have been plotted between Madeira and the Canary Islands. But ERS repetitivity (35 days) is fairly low and the aliasing of the ocean temporal spectrum is larger

At a specific date, two-dimension, synoptic views of the ocean surface circulation can be obtained on a regular grid by mapping the SLA measured by altimeters along the satellite ground tracks. On the right figure, altimetric data measured at a 5-day lag from the 12th of July 1993 have been plotted in dashed blue along the satellite tracks (the corresponding cross-track geostrophic velocities are plotted in red). From this data, a map have been calculated by estimating on a regular grid, revealing highs (green) and lows (blue) of the geostrophic circulation (in cm). This estimation technique is based on an improved optimal interpolation method ( Hernandez et al., 1995 ; Le Traon et al., 1997) of the altimetric SLA. Such a technique allows to combine data from several satellites, taking into account the accuracy of each dataset.