The ocean exerts a major influence on the Earth's meteorology and climate through its interaction with the atmosphere. Understanding the transfer of moisture and energy between ocean and atmosphere is therefore a scientific priority. Better observations are needed, to improve the accuracy of weather forecasts of marine conditions and the assessment of climatic change.
Earth observation satellites have revolutionised the study of the ocean. They now provide detailed repetitive measurements over remote areas of the world, where previously there were only a limited number of (isolated) observations from ships and buoys. Microwave instruments, including SARs and radar altimeters, have a remarkable sensitivity to the roughness and height of the ocean surface, enabling the detection of ocean currents, fronts and internal waves, oil slicks and ships, as well as accurate measurement of sea level changes, wave height and wind speed. Optical instruments provide measurements of ocean colour and temperature, which are important indicators of phytoplankton, yellow substance and suspended sediments.
ENVISAT, by including advanced SAR, radar altimeter, ocean colour and ocean temperature instruments together on the same platform, offers particularly exciting opportunities for synergetic measurements over the oceans. It provides an improvement in measurement capability compared with ERS, together with possibilities for many new geophysical measurements. The simultaneous recording of MERIS ocean colour measurements with both AATSR sea surface temperature, and ASAR sea surface roughness offers particularly exciting possibilities.
Ocean Biophysical Properties
There remain major uncertainties about the amount of carbon stored in the ocean and the biosphere, and about the fluxes between these reservoirs and the atmosphere. In particular, there is an important need for better information on the spatial distribution of biological activity in the upper ocean and its temporal variability, especially in the case of oceanic phytoplankton biomass, which has an important role in fixing CO2 through photosynthesis. In the upper layers of the open ocean, chlorophyll concentration is the most convenient index for phytoplankton abundance and this can be measured using the visible part of the spectrum.
"The remote measurement which has caused the greatest interest within the JGOFS (Joint Global Ocean Flux Study) is the estimation of basin and global-scale variability in the concentration of chlorophyll in the upper ocean. The images of the global distribution of these pigments, derived from data taken by the coastal zone colour scanner (CZCS) onboard the United States' Nimbus-7 spacecraft, have revolutionised the way biological oceanographers view the oceans. For the first time, the blooming of the ocean basins in spring has been observed, as has the extent of the enriched areas associated with the coastal ocean." (International Geosphere-Biosphere Programme [IGBP] A study of Global Change, Report No. 12, 1990).
Although CZCS, launched in 1978, was intended as a one-year proof-of-concept mission, the sensor continued to transmit data over selected oceanic test sites until early 1986. The figures below show examples of CZCS chlorophyll maps of the Earth and the Mediterranean Sea.
Remotely sensed information about global ocean colour is once again available; firstly from the OCTS and POLDER instrument on the Japanese ADEOS mission, from the NASA SeaWiFS satellite launched in August 1997, and from the MOS instrument on IRS-3. MERIS provides data continuity with improved spectral and spatial performance. This results from the use of several near-infrared channels to perform atmospheric corrections, and several narrow visible channels to compute radiance values.
Phytoplankton abundance varies from less than 0.03 mg m-3 in oligotrophic waters (i.e., waters poor in nutrients and therefore in phytoplankton), up to about 30 mg m-3 in eutrophic waters (i.e., in nutrient rich waters, supporting high biomass). Ocean colour responds in a non-linear way to these large changes in chlorophyll content. It is conveniently depicted by the ratio of blue-to-green radiation backscattered by the ocean, with the ratio that is most sensitive based on wavelengths of 445 and 565 nm. It varies within a range of 1 to 20 for the types of pigments considered, and decreases, almost linearly, with the logarithm of the concentration.
The coastal regions are the most populated areas in the world and coastal waters are highly affected by human activities. Marine ecosystems are affected by the influx of large amounts of agricultural and industrial pollutants and sewage from rivers which may inhibit or stimulate marine productivity.
Continuous long-term observation of coastal waters, which cover more than three million square kilometres, is most important for regional climate impact studies and for environmental monitoring. Remote sensing measurements from satellite are the only available means of monitoring such large areas of water.
The major water constituents, which determine the marine and estuarine ecology and the bio-geochemical budget and whose concentration and distribution can be determined by optical remote sensing, are suspended matter, phytoplankton and Gelbstoff.
Figure 1.10 - Simulated multispectral radiances for a spectral resolution of 5 nm just above the water surface
Suspended matter is defined as a combination of:
· inorganic particles and detritus, present due to re-sedimentation and advection processes
· atmospheric inputs
· dead material of plankton
Gelbstoff consists of various polymerised dissolved organic molecules which are formed by the degradation products of organisms. These originate in brackish and underground water as well as in extraordinary plankton blooms. All these constituents have different optical properties, but there are similarities in their spectral scattering and absorption coefficients.
The upward radiance at any visible wavelength is composed of contributions from all these substances. Figure 1.9 above shows simulated multispectral radiances for different ocean waters. Suspended matter usually enhances the upward radiances through reflection within the visible spectrum, while Gelbstoff reduces these radiances mainly in the blue.
To convert from the optical properties of the water constituents, used in the radiative transfer model, to pigment or suspended matter concentration units, robust algorithms have been developed with global applicability. The accuracy of derived oceanic properties depends strongly on the precision of the atmospheric correction procedure.
The development of inverse modelling techniques for the interpretation of MERIS measurements is an ongoing process. For monitoring coastal regions world wide, precise multispectral radiances, with contemporary optical and concentration measurements of the water constituents, are needed. As well as the chlorophyll concentration and several atmospheric parameters, planned geophysical products include total suspended matter and yellow substance concentration.