Bernard Gentili

Comparison of Numerical Models for Computing Underwater Light Fields

Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

Diffuse reflectance of oceanic waters: its dependence on Sun angle as influenced by the molecular scattering contribution.

A spectral model of the inherent optical properties (absorption and scattering coefficients a and b, respectively) of oceanic case 1 waters with varying chlorophyll concentrations C is operated. It provides the initial conditions for Monte Carlo simulations aimed at examining the diffuse reflectance directly beneath the surface R and its variations with the solar zenith angle zeta. In most oceanic waters, molecular scattering is not negligible, and molecular backscattering may largely exceed backscattering. The variable contributions (depending on C and wavelength) of water molecules and particles in the scattering process result in considerable variations in the shape of the volume-scattering function. R(zeta) is sensitive to this shape. From the simulations, R (which increases as zeta increases) appears to be linearly related to cos zeta, with a slope that is strongly dependent on eta(b), the ratio of molecular backscattering to particle backscattering. The value of the single-scattering albedo (?= b/a + b) has a negligible influence on the R(zeta) function provided that ? < 0.8, a condition that is always fulfilled when dealing with oceanic case 1 waters. Practical formulas for R(zeta) are proposed. They include the influence of the diffuse sky radiation. The history of each photon and the number of collisions it experiences before exiting have been recorded. These histories and also a probabilistic approach allow the variations of R with cos zeta, eta(b), and ? to be understood.

Diffuse reflectance of oceanic waters. III. Implication of bidirectionality for the remote-sensing problem.

The upwelling radiance field beneath the ocean surface and the emerging radiance field are not generally isotropic. Their bidirectional structure depends on the illumination conditions (the Suns position in particular) and on the optical properties of the water body. In oceanic case 1 waters, these properties can be related, for each wavelength λ, to the chlorophyll (Chl) concentration. We aim to quantify systematically the variations of spectral radiances that emerge from an ocean with varying Chl when we change the geometric conditions, namely, the zenith-Sun angle, the viewing angle, and the azimuth difference between the solar and observational vertical planes. The consequences of these important variations on the interpretation of marine signals, as detected by a satelliteborne ocean color sensor, are analyzed. In particular, the derivation of radiometric quantities, such as R (λ), the spectral reflectance, or [ L(w)(λ)](N), the normalized water-leaving radiance that is free from directional effects, is examined, as well as the retrieval of Chl. We propose a practical method that is based on the use of precomputed lookup tables to provide values of the f/Q ratio in all the necessary conditions[ f relates (R(λ) to the backscattering and absorption coefficients, whereas Q is the ratio of upwelling irradiance to any upwelling radiance]. The f/Q ratio, besides being dependent on the geometric configuration (the three angles mentioned above), also varies with λ and with the bio-optical state, conveniently depicted by Chl. Because Chl is one of the entries for the lookup table, it has to be derived at the beginning of the process, before the radiometric quantities R(λ) or [L(W)(λ)](N) can be produced. The determination of Chl can be made through an iterative process, computationally fast, using the information at two wavelengths. In this attempt to remove the bidirectional effect, the commonly accepted view relative to the data-processing strategy is somewhat modified, i.e., reversed, as the Chl index becomes a prerequisite parameter that must be identified prior to the derivation of the fundamental radiometric quantities at all wavelengths.

Bidirectional reflectance of oceanic waters: accounting for Raman emission and varying particle scattering phase function

The bidirectionality of the upward radiance field in oceanic case 1 waters has been reinvestigated by incorporation of revised parameterizations of inherent optical properties as a function of the chlorophyll concentration (Chl), considering Raman scattering and making the particle phase function shape (beta(rho)) continuously varying along with the Chl. Internal consistency is thus reached, as the decrease in backscattering probability (for increasing Chl) translates into a correlative change in beta(rho). The single particle phase function (previously used) precluded a realistic assessment of bidirectionality for waters with Chl > 1 mg m(-3). This limitation is now removed. For low Chl, Raman emissions significantly affect the radiance field. For moderate Chl (0.1-1 mg m(-3)), new and previous bidirectional parameters remain close. The ocean reflectance anisotropy has implications in ocean color remote-sensing problems, in derivation of coherent water-leaving radiances, in associated calibration-validation activities, and in the merging of data obtained under various geometrical configurations.

Climate-driven basin-scale decadal oscillations of oceanic phytoplankton.

Untangling the Web Chlorophyll-containing phytoplankton is at the core of the marine food web. Martinez et al. (p. 1253) combined satellite data about upper ocean chlorophyll and sea surface temperatures to demonstrate a clear connection between phytoplankton and sea surface temperatures on a multidecadal time scale. Basin-scale ocean dynamic processes such as the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation connect the physical, climate-related variability to changes in phytoplankton distribution and amount. Thus, improving the reliability of forecasts of large-scale ocean dynamics may help to improve predictions of changes in ocean community ecology. Satellite data show that upper ocean chlorophyll and sea surface temperatures are connected on a multidecadal time scale. Phytoplankton—the microalgae that populate the upper lit layers of the ocean—fuel the oceanic food web and affect oceanic and atmospheric carbon dioxide levels through photosynthetic carbon fixation. Here, we show that multidecadal changes in global phytoplankton abundances are related to basin-scale oscillations of the physical ocean, specifically the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. This relationship is revealed in ~20 years of satellite observations of chlorophyll and sea surface temperature. Interaction between the main pycnocline and the upper ocean seasonal mixed layer is one mechanism behind this correlation. Our findings provide a context for the interpretation of contemporary changes in global phytoplankton and should improve predictions of their future evolution with climate change.

Functioning of the Mediterranean Sea: past and present changes related to freshwater input and climate changes

Abstract Mediterranean functioning is linked to continental climate and more particularly to freshwater budget across its surface. In the sediment of the eastern basin, the sapropel layers are signatures of variations in hydrology and climate that have occurred over the last half million years. They may be used to investigate the probable evolutions following global climatic change, and particularly the sensitivity of marine dynamics to freshwater input variations. At the present time, in the water column of the eastern and western basins, there are changes in hydrology originating in evolutions in heat and water budgets across the sea surface. With respect to the water budget of the Mediterranean, some evolutions have occurred as a result of the anthropogenic use of freshwater, and consequently, environmental driving forces must also be considered alongside climatic factors. Marine variations may be used as constraints for the quantification of probable evolutions in external driving forces at the scale of a whole basin. As an example, the previously developed 20-box model provides an explanation for the increasing trends of temperature and salinity observed in the western deep water over the 1960–1996 period by accounting for changes in freshwater and heat budgets, reaching 0.1 m yr−1 and 1.5 W m−2, respectively, in 1995 over the Mediterranean.

Warming and freshwater budget change in the Mediterranean since the 1940s, their possible relation to the greenhouse effect

Temperature and salinity of Mediterranean deep water have been increasing, based on observations extending back to 1959. Over the 1940–1995 period, the corresponding changes in heat and water budgets across the sea surface have been estimated to be 1.74 Wm−2 for the greenhouse effect change, 0.5°C and 0.4°C for mean air and sea-surface temperature increases, respectively, and an increase of 0.1 m (or 11%) in the freshwater deficit, which is due partly to human activities and partly to climatic changes.

Remote sensing of sea surface Sun-induced chlorophyll fluorescence: consequences of natural variations in the optical characteristics of phytoplankton and the quantum yield of chlorophyll a fluorescence

Abstract The rate of Sun-induced chlorophyll a fluorescence (SICF) observed at sea surface is determined by chlorophyll a concentration, incident irradiance, and optical and fluorescence properties of the phytoplanktonic population. In this study, the impact of natural variations in the two latter on the use of remotely sensed SICF to determine ocean surface chlorophyll a concentration, is assessed using a simple parameterization of phytoplankton optical properties and a model describing the variations in the quantum yield of chlorophyll a fluorescence as a function of environmental factors, such as excess irradiance and nutrient limitations. It is shown that (1) variations in the optical properties of phytoplankton are the main cause of non-linearity in the relationship between SICF and chlorophyll a concentration, (2) the extent of spatial variations in the rate of fluorescence per unit chlorophyll a concentration and irradiance, at the level of a typical sensor scene, prevents the use of a linear relat...

The Mediterranean Sea, coastal and deep-sea signatures of climatic and environmental changes

At great scales of time and space, the dynamics of the Mediterranean Sea, a concentration basin, are mainly linked to its freshwater budget. This budget is subject to evolutions due to mans use of freshwater and to climatic changes affecting precipitation and/or evaporation. Marine dynamics and Atlantic, atmospheric and terrestrial inputs are strong constraints for the geochemical behaviour of the Mediterranean Sea. From measurements made during the last decades in the deep western water, it appeared that temperature, salinity, nutrients and trace metal concentrations were changing with time. In spite of its depth, the Mediterranean Sea looks like a coastal ocean, according to its coast length, watershed and number of inhabitants and to its fast response to climatic and environmental changes. The changes discovered in deep homogeneous waters are signatures of evolutions occurred in the surface layer. But in this layer and particularly in coastal waters, climatic and/or environmental trends may be masked by seasonal and interannual variabilities of not only physical and chemical characteristics but also climatic forcing or anthropic inputs. Analyses of river runoff, atmospheric inputs or climatic trends together with marine evolutions indicate constraints concerning probable changes in the coastal sea and/or in the surface water and processes involved at the interfaces. Moreover, changes observed in coastal or deep-water constitute new constraints for the modelling of the marine circulation and the transfer of matter.

Bidirectional reflectance of oceanic waters: A comparison of modeled and measured upward radiance fields

The bidirectional reflectance of oceanic waters is conveniently described in a normalized way by forming the ratio of the up welling irradiance Eu to any up welling radiance Lu(θ′, φ). This ratio, Q[θ′, θ0, (φ0 − φ)], where θ′, φ are the nadir and azimuth angles for the upward radiance and θ0, φ0 are the zenith and azimuth angles of the Sun, has been determined from measurements at sea and computed via Monte Carlo simulations using the inherent optical properties measured in the field and appropriate boundary conditions (clear sky, no wind, varying Sun angle). Experimental and computed Q values are in excellent agreement. This successful comparison confirms the importance of the bidirectional character of ocean reflectance, already pointed out from a purely numerical approach without field validation, and corroborates the extended range of the Q variations. The later point is of importance when interpreting the marine signals detected by an ocean color satellite-borne sensor. The validation is extended by considering the historical data for the radiance distributions in Lake Pend Oreille determined at various depths. The closure issue in ocean optics is examined by solving the direct problem of radiative transfer and through a model-data comparison in terms of radiance field.