22 Pierre-Yves Le Traon
influenced by) the ocean and atmosphere circulation. The heat fluxes at the ocean!
atmosphere interface are also strongly linked to SST. This parameter is operationally measured by the NOAA satellites (AVHRR), and useful data sets are already
available. Geostationary satellites (e.g. GOES, METEOSAT) provide a much better temporal sampling but with a degraded spatial resolution and accuracy. SST
anomalies are key indicators of changes in the environment (e.g. El Nifio events).
At smaller scales, SST measurements can provide useful information on the mesoscale ocean flow field and position of the main current system (e.g. Gulf Stream and
ocean color measurements) and regions ofupwelling. Surf ace currents may also be
derived from the analysis of successive SST images (e.g. Kelly and Strub, 1992).
The usefulness of ocean color measurements was demonstrated by the CZCS
instrument on board the Nimbus-7 satellite (1978-1986). There are now several
ocean color missions flying, and more are scheduled for the years ahead. Ocean
color provides an indirect way of measuring phytoplankton concentration (chlorophyll-a pigment, for the so-called Case 1 waters) but is also sensitive to the influence of sediments, particulate and dissolved organic matter (so-called Case 2
waters found mainly in coastal areas) (see Bricaud, 1995). Phytoplankton is the primary food and energy source for the ocean ecosystem, so it is very important for
ecosystem studies. It also plays a significant role in the carbon cycle since phytoplankton dissolves CO 2 through photosynthesis into organic components (biological primary production). Global mean chlorophyll pigment concentration derived
from CZCS data shows, for example, regions of high phytoplankton concentration
in the upwellings and at high latitudes. Near the coast, the signal cannot be interpreted in terms ofphytoplankton only. From the chlorophyll pigment concentration
maps, we can derive maps of primary production (production of organic carbon
through photosynthesis). Quantitative estimation of phytoplankton and primary
production via ocean color measurements is difficult, however, as the retrieved signal is not always dominated by chlorophyll (dissolved organic matter, other pigments). The absorption coefficient ofphytoplankton is also variable (according to
how it is aggregated). To estimate primary production, we must assume a vertical
distribution of chlorophyll and primary production also depends on species, environmental and physiological conditions, etc. There is clearly a need for better sensors and sampling, more wave1engths (SEAWIFS, OCTS, POLDER) but also
comprehensive calibration, validation, and new bio-optical algorithms before
ocean color measurements can be used quantitative1y. These data sets are nevertheless very useful for verifying coupled ecosystem models. Chlorophyll concentration observation is also a useful parameter to be assimilated in these models.
2.1.3 Space oceanography missions
Tables below provide an overview of existing and approved satellite missions for
altimetry, SST, scatterometry and ocean color (courtesy of A. Ratier) for the next
decade [before and during the Global Data Assimilation Experiment (GODAE),
Smith and Lefebvre (1997)] (Ratier, 1999). As can be seen, there are a substantial
number of missions although only SST could be described as operational. There is
influenced by) the ocean and atmosphere circulation. The heat fluxes at the ocean!
atmosphere interface are also strongly linked to SST. This parameter is operationally measured by the NOAA satellites (AVHRR), and useful data sets are already
available. Geostationary satellites (e.g. GOES, METEOSAT) provide a much better temporal sampling but with a degraded spatial resolution and accuracy. SST
anomalies are key indicators of changes in the environment (e.g. El Nifio events).
At smaller scales, SST measurements can provide useful information on the mesoscale ocean flow field and position of the main current system (e.g. Gulf Stream and
ocean color measurements) and regions ofupwelling. Surf ace currents may also be
derived from the analysis of successive SST images (e.g. Kelly and Strub, 1992).
The usefulness of ocean color measurements was demonstrated by the CZCS
instrument on board the Nimbus-7 satellite (1978-1986). There are now several
ocean color missions flying, and more are scheduled for the years ahead. Ocean
color provides an indirect way of measuring phytoplankton concentration (chlorophyll-a pigment, for the so-called Case 1 waters) but is also sensitive to the influence of sediments, particulate and dissolved organic matter (so-called Case 2
waters found mainly in coastal areas) (see Bricaud, 1995). Phytoplankton is the primary food and energy source for the ocean ecosystem, so it is very important for
ecosystem studies. It also plays a significant role in the carbon cycle since phytoplankton dissolves CO 2 through photosynthesis into organic components (biological primary production). Global mean chlorophyll pigment concentration derived
from CZCS data shows, for example, regions of high phytoplankton concentration
in the upwellings and at high latitudes. Near the coast, the signal cannot be interpreted in terms ofphytoplankton only. From the chlorophyll pigment concentration
maps, we can derive maps of primary production (production of organic carbon
through photosynthesis). Quantitative estimation of phytoplankton and primary
production via ocean color measurements is difficult, however, as the retrieved signal is not always dominated by chlorophyll (dissolved organic matter, other pigments). The absorption coefficient ofphytoplankton is also variable (according to
how it is aggregated). To estimate primary production, we must assume a vertical
distribution of chlorophyll and primary production also depends on species, environmental and physiological conditions, etc. There is clearly a need for better sensors and sampling, more wave1engths (SEAWIFS, OCTS, POLDER) but also
comprehensive calibration, validation, and new bio-optical algorithms before
ocean color measurements can be used quantitative1y. These data sets are nevertheless very useful for verifying coupled ecosystem models. Chlorophyll concentration observation is also a useful parameter to be assimilated in these models.
2.1.3 Space oceanography missions
Tables below provide an overview of existing and approved satellite missions for
altimetry, SST, scatterometry and ocean color (courtesy of A. Ratier) for the next
decade [before and during the Global Data Assimilation Experiment (GODAE),
Smith and Lefebvre (1997)] (Ratier, 1999). As can be seen, there are a substantial
number of missions although only SST could be described as operational. There is