[1] Knowledge of the relative proportion between small-sized and larger particles in the surface ocean is essential to understand the ocean ecology and biogeochemistry, including particle dynamics and carbon cycling. We show that this information may be assessed qualitatively from satellite observations of ocean color. Such capability is based on the estimation of spectral dependence, g, of particulate backscattering coefficient, b bp , which is sensitive to particle size distribution. Our results obtained from satellite observations of the global ocean are supported by in situ measurements, and they demonstrate a general decrease of the spectral slope g from oligotrophic to eutrophic regimes, although significant regional differences are observed in the relationship between g and the chlorophyll a concentration, Chl. To first approximation, such a decrease in g is expected to be accompanied by an increased role of larger particles. This is consistent with our field data that show relatively high concentrations of submicron particles in very clear oceanic waters. Different seasonal patterns are also observed depending on the oceanic regions. The seasonal amplitude of g is generally higher than that of Chl and b bp in equatorial and tropical regions, and it is much lower at temperate latitudes. These spatio-temporal patterns are interpreted in terms of processes that modify the composition of particulate assemblages and physiology of phytoplankton in response to environmental forcing. The changes in g are clearly related to variations in the mixed layer depth and photosynthetic available radiation.

Monthly averaged level-3 SeaWiFS chlorophyll concentration data from 1998 to 2001 are globally analyzed using Fourier's analysis to determine the main patterns of temporal variability in all parts of the world ocean. In most regions, seasonal variability dominates over interannual variability, and the timing of the yearly bloom can generally be explained by the local cycle of solar energy. The studied period was influenced by the late consequences of the very strong El Niño of 1997-98. After this major event, the recovery to normal conditions followed different patterns at different locations. Right at the equator, chlorophyll concentration was abnormally high in 1998, and then decreased, while aside from the equator, it was low in 1998, and increased later when equatorial upwelled waters spread poleward.

Abstract-POLDER is a CNES instrument on board NASDA's ADEOS polar orbiting satellite, which was successfully launched in August 1996. On October 30, 1996, POLDER entered its nominal acquisition phase and worked perfectly until ADEOS's early end of service on June 30, 1997. POLDER is a multispectral imaging radiometer/polarimeter designed to collect global and repetitive observations of the solar radiation reflected by the earth/atmosphere system, with a wide field of view (2400 km) and a moderate geometric resolution (6 km). The instrument concept is based on telecentric optics, on a rotating wheel carrying 15 spectral filters and polarizers, and on a bidimensional charge coupled device (CCD) detector array. In addition to the classical measurement and mapping characteristics of a narrow-band imaging radiometer, POLDER has a unique ability to measure polarized reflectances using three polarizers (for three of its eight spectral bands, 443 to 910 nm) and to observe target reflectances from 13 different viewing directions during a single satellite pass.One of POLDER's original features is that its in-flight radiometric calibration does not rely on any on-board device. Many calibration methods using well-characterized calibration targets have been developed to achieve a very high calibration accuracy. This paper presents the various methods implemented in the in-flight calibration plan and the results obtained during the instrument calibration phase: absolute calibration over molecular scattering, interband calibration over sunglint and clouds, multiangular calibration over deserts and clouds, intercalibration with Ocean Color and Temperature Scanner (OCTS), and water vapor channels calibration over sunglint using meteorological analysis. A brief description of the algorithm and of the performances of each method is given.

[1] The geographic distribution and seasonal to interannual variability of the particulate backscattering coefficient, b bp , is described and compared with that of chlorophyll concentration, Chl. Both variables are obtained from satellite-derived ocean color and inverse modeling. In general, b bp exhibits larger seasonal variations than Chl, and the b bp values are more evenly distributed in space, especially at low and middle latitudes. A phase shift between the annual cycles of b bp and Chl is evidenced, and is attributed to the presence of an important pool of nonpigmented particles. Converting b bp to particulate organic carbon, POC, the annual mean surface POC pool between 60°S and 60°N is about 19 Tg C per meter.

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