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The Ocean covers 70% of the Planet and plays a key role in regulating the global climate. The ocean is the main reservoir for heat as well as a powerful vehicle to transport warm water masses poleward. It has the capacity to intake (but also reject) significant amounts of carbon dioxide, one of the greenhouse gases. It also supports a cost-effective type of transportation of goods, is a milieu where to discover new oil fields, is a feeding ground for fish and sea-food to nourish the ever-growing human population. The Oceanographic mission objectives of Envisat are derived from the ERS results. To resolve low frequency signals in the ocean spectrum and to further understand oceanic processes a longer time series is however needed. The Oceanographic mission objectives of Envisat Altimetry are dynamic topography monitoring, mesoscale variability, seasonal and interannual variability, mean global and regional sea level trends, marine geophysics -especially in polar oceans even covered with sea-ice-, sea-state monitoring. The objectives are to be met with data products available either in near real time (3 hours), in quasi near real time (2-3 days) or with the highest precision off-line products (50 days).

 Seasonal and interannual variability has an important impact on climate. Planetary waves propagate from months to seasons across basins to adjust the ocean in response to wind forcing. Interannual variations of the seasonal or annual cycles have a direct and sometimes dramatic impact of the global climate, well illustrated by the El Niño-Southern Oscillation (ENSO). The data serve mainly to tune and evolve global ocean and atmosphere models, to better understand the ocean-atmosphere interaction and the underlying processes.

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Series of Sea Level Anomaly (cm) in the Tropical Pacific. Each row is one year -'97, '98, '99, '00- with one sample of the sea level anomaly field at each season, by column: March, June, September and December. The strong El Niño Event of late '97 followed by a La Niña event is clearly visible. A film of such 3D vignettes helps the researcher "visualize" the wave propagation involved in such events. Each weekly field can be assimilated in an ocean model. (SLA data processed by R. Scharroo, DEOS, NL, graphics processed at ESA/ESRIN.)

The ocean is vast and hosts a full spectrum of signals. One orbiting satellite alone cannot pretend to cover that spectrum. There are significant advantages in merging the data from two or more Altimetry missions sampling the Earth with different orbital patterns. A good illustration is the enrichment in space resolution of the mesoscale variability field computed with merged data from ERS and Topex-Poseidon which are in the same orbital configuration as Envisat and Jason will be.