RATIONALE

The purpose is to understand the ocean/atmosphere mechanism functioning, with all the medium and long term impacts on the climate evolution, on water and carbon cycles evolution or on sea level evolution, each of these impacts having major societal stakes.

Oceanologists have more and more sophisticated observation means: from remote sensing to in-situ measurement stations, as well as measurement from boats. These data are integrated to sophisticated models, through more and more complex assimilation techniques, which enable to extrapolate surface information to the whole water column, and thus, to be able to understand, to model or to forecast the ocean behaviour.
The needs of ocean observation by satellite range from the operational, i.e. continuous and homogen observations, sometimes in quasi realtime, with a mandatory maintain, to exploratory needs, with some relevant parameters that still cannot be measured by satellite.

Even if taking into account all the oceanic components is crucial, we will consider five themes for simplicity purpose: oceanic circulation, ocean state and ocean-atmosphere interfaces, sea ice, marine biogeochemistry and coastal environment. For each of these themes, this page sums up the stakes, the objectives, the main successes and the present limits.

OCEANIC CIRCULATION

The purpose is to measure the oceanic circulation at global and medium scale, as well as the water bodies formation and to follow the sea level evolution. The main relevant observables that need to be measured are surface topography, water bodies temperature and salinity, wind, sea ice evolution, etc.
Space observations which contributed to our understanding of these phenomena came from altimetry missions, Topex-Poseidon (Cnes-Nasa) series, Jason or ERS 1 and 2 series and Envisat (Esa). These missions with on board bi-frequency altimeters, radiometers and accurate positioning devices brought an exceptional spatio-temporal description of the ocean surface. Their data are distributed to thousands of users all over the world by Aviso. Moreover, they are integrated to models enabling to forecast several weeks in advance the tridimensional state of the ocean. This is the purpose of the Mercator-Ocean Center.
Among the altimetry successes, the dynamic topography measurement has enabled to represent the major ocean streams: the spatial and temporal variability monitoring and its relations with the major streams, the most typical phenomena monitoring, such as El Niño signature, the description and quantification of ocean tides or the wave propagation. The continuous measurements for over a decenium have also enabled to observe the sea level evolution, estimated to 2.5 mm per year, with an unprecedented accuracy.

Sea level evolution estimated by Topex-Poseidon, and Jason-1. The altimetry missions continuity enables an excellent accuracy. © A. Cazenave

These applications need the continuity of the two complementary altimetry missions. Indeed, only two complementary missions provide the adequate spatio-temporal sampling for the ocean variability observation, in the coastal domain as well as the deep-sea domain.
Moreover, a polar mission must be set-up, mission adapted to large ice masses monitoring, Groenland or Antarctic, because they potentially contribute to sea level variations. At last, new themes appear about continental altimetry, glaciology and also hydrology, which need a good spatio-temporal sampling. Beyond Jason-2, realized in cooperation between Cnes, Nasa, Noaa and Eumetsat, Cnes also contributes to the Saral mission, in cooperation with Isro (Indian Space Agency). The Envisat operational follow-on will be ensured by Sentinel3 mission, in the GMES frame of work.

The recent gravimetry missions, Champ (DLR) and Grace (Nasa), have hugely supported the altimetry missions contribution by independantly evaluating the mean topography linked to the geoid, which is contained in the topography measured by altimetry. Now we have a dynamic topography which enables a very good description of the ocean circulation in relation to streams.
The launch of the gravimetry satellite Goce (Esa), forecast in 2007, will supply a geoid with a better spatial resolution for a much finer description of the circulation.

On the other hand, no satellite sensor is yet able to measure the salinity, which is one of the ocean state parameters. Salinity controls the water density and thus has a role in the sea level as well as in the circulation. Two complementary sensors, Smos (Esa-Cnes-CDTI) and Aquarius (Nasa-Conae) will be launched soon. These sensors are based on the same concept which consists in having lower passive radiometry frequency to obtain the required sensibility to estimate the salinity. The expected effects are sea level variation interpretation, some phenomena forecast and ocean circulation modelling.

OCEAN STATE AND OCEAN-ATMOSPHERE INTERFACE

The objective is to forecast the waves height for the sea users or the atmospheric circulation models, to understand the waves physic and its impact on the atmosphere-ocean exchanges, to better grasp the wind and waves climatology at sea level and to monitor its evolution. The knowledge of the ocean state is also required to correct the hyperfrequency or optical observations made by satellite which are greatly affected by the different roughness scales.
The observables are the waves height, the wind speed and direction as well as the waves spectrum.

Wind direction and intensity measured by QuickScat scatterometer (Nasa) in North Atlantic.

Among the successes, one should mention altimetry, able to estimate the waves height, the scatterometer designed to measure the wind intensity and direction and also the synthetic aperture radar - SAR - which enables in some cases to estimate the swell, notably in coastal oceanography.

On the other hand, waves climatology is still incomplete. There is still no access to waves directional spectrum and the spatio-temporal cover is still insufficient.
Swimsat mission, supported by Cnes since 1993 and proposed in international cooperation is designed to answer these needs. The concept, based on the observation of a nadir beam and rotating beams, should enable to estimate the waves directional spectrum and thus, to significantly improve their forecast. Swimsat should supply statistics on roughness ranges as well as a complete wave climatology and should enable a better understanding of the influence of the ocean state on the fluxes. At last, it should describe the margin areas of sea ice.

The ocean surface temperature is nowaday measured routinely by either geostationary or low orbit meteorological satellites. The Metop satellite (Eumetsat) will be one of the major European contributors to the ocean temperature measurement, on board of which is Iasi instrument (Cnes). Complementary measurements are also done by Earth observation satellites, such as Envisat. Oceans being the larger heat reservoir of our planet, the measurement of their temperature is crucial for the climate phenomena study.

SEA ICE

Sea ice influences ocean circulation, through its effect on water stratification, as well as the ocean-atmosphere interface, by insulating the sea. Through its very high albedo and retroaction effects, it also plays a role on the climate and constitutes a sensitive climate indicator.

Among the observables, the extend, the formation, the age or the drifting of sea ice are measured with a good accuracy. For example, temporal series of passive hyperfrequency radiometer enable to estimate the decrease of Arctic sea ice extend of about 3 to 4 % per decenium during summer and of 7 to 8 % per decenium during winter.

However, their depth still cannot be measured from space. This was the major objective of the European satellite Cryosat, which was unfortunately lost during its launch in october 2005. Its successor, Cryosat2, should be launched at the beginning of 2009. In addition to sea ice thickness, it should supply the accurate topography of some continental glaciers.

MARINE BIOGEOCHEMISTRY

The purpose is to understand the role of marine biology on carbon dioxide flux regulation, to monitor global and decenial changes of phytoplankton biomass, to understand the coupling between ocean circulation and primary production, with the objective of insuring a global monitoring of phytoplankton biomass.

Ocean primary production. © Morel and Antoine

The observables - chlorophyll concentration, sediments in suspension, toxic blooming or primary production - are included in the spectral signature of the sea in the visible range, usually called "water color". Thus this information has to be inversed to derive the different observable parameters required.

Among the successes, one should mention Seawifs (Nasa), Polder (Cnes) or Meris (Esa) sensors aptitude to quantify phytoplankton biomass with the needed accuracy as well as the new vision of the biogeochemistry that they provide at global ocean scale. Carbon fixation by ocean can be quantified and is estimated to 50 Gt/an.

The use of these sensors is becoming operational. However, even with the numerous in orbit missions, very few are operational and properly calibrated. The near future will be covered by american (Npoess) as well as european (Sentinel3) operational missions. Their spatio-temporal cover is however still inadequate for the study of fast phenomena, for which an instrument with a geostationary orbit would be mandatory. This instrument could be envisioned before the end of the next decenium.

It can be noted that after the loss of Polder-2 on Adeos-2, Cnes tries to continue the water color observation with Parasol micro-satellite.

COASTAL ENVIRONMENT

Coastal environments have two major specificities. The first being its own stakes and objectives, such as the necessity to know the depth and the circulation, the sediments in suspension, the stilting up issues, the phytoplankton or toxic algas blooms for example.

Gironde plume illustrating the simultaneity and complexity of the phenomena. Salinity measurement (a), phytoplankton bloom (b) and matter in suspension (c). © Dehouck

It also represents small size areas, between a few kilometres and a few dozen kilometres, and is extremely variable. The spatio-temporal sampling needs are thus very great. For example, a very high salinity gradient, phytoplankton bloom and matter in suspension can be associated to a flood for a few days. A sampling enabling to catch this event, to describe it with accuracy and to be able to decorrelate all the observables is thus required.

Among the successes, one should mention water color sensors as well as synthetic aperture radars which enable, in some cases, to measure the swell, its direction, its wavelength and its amplitude. Optic sensors enable to map dune fields or coastline and thus, to monitor its evolution, as well as to estimate the depth with an accuracy of about 20%.

Several studies are still needed to improve the interpretation of water color images or depth evaluation. Moreover, pertinent observables, such as salinity or surface streams, are not yet measurable in the coastal domain, and will not be available in the next years.

At last, the more crucial point is probably the repetitivity and accurate observation need in optic as well as in radar; this need could be fulfilled by Orfeo (combination of the italian radar Cosmo-Skymed and the french imager Pleiades).