Isabelle Dadou

Assimilation of surface data in a one‐dimensional physical‐biogeochemical model of the surface ocean: 1. Method and preliminary results

A new method to transform satellite ocean color data into estimates of primary production and carbon fluxes is presented. A one-dimensional coupled physical-bio-geochemical model of the surface ocean is constrained to reproduce the seasonal evolution of surface chlorophyll concentration by adjusting the parameters of the 10-compartment trophic system using a variational technique. The method is applied to in situ surface chlorophyll data from station Papa, but averaged over the characteristic depth of ocean color measurements, and affected by errors compatible with those of satellite values. Using 35 measurements for the year 1976, it is found that five linear combinations of the trophic parameters can be adjusted. This adjustment is also valid for 1975 chlorophyll data. In general, the adjusted values of the trophic parameters are sensitive to their a priori values, but consistent results are found for the grazing rate (0.35 to 0.6 d−1), the phytoplankton mortality rate (very small), and the minimum concentration of zooplankton in winter (less than 0.16 mmolC m−3). Some carbon fluxes, namely photosynthetic carbon production in the euphotic layer (95 to 110 gC m−2 yr−1), its regeneration by grazing (60 % of the latter), and the recycling efficiency of nitrogen (60%) seem to be robustly constrained, though primary production is apparently underestimated compared to the most recent ones. The export flux amounts to 35 to 40% of primary production, but its value depends on the particle sinking rate, which is not adjustable from chlorophyll data. This study suggests that simplified biological models, compared to the model used here, would be sufficient to achieve this task.

Biomass changes and trophic amplification of plankton in a warmer ocean

Ocean warming can modify the ecophysiology and distribution of marine organisms, and relationships between species, with nonlinear interactions between ecosystem components potentially resulting in trophic amplification. Trophic amplification (or attenuation) describe the propagation of a hydroclimatic signal up the food web, causing magnification (or depression) of biomass values along one or more trophic pathways. We have employed 3-D coupled physical-biogeochemical models to explore ecosystem responses to climate change with a focus on trophic amplification. The response of phytoplankton and zooplankton to global climate-change projections, carried out with the IPSL Earth System Model by the end of the century, is analysed at global and regional basis, including European seas (NE Atlantic, Barents Sea, Baltic Sea, Black Sea, Bay of Biscay, Adriatic Sea, Aegean Sea) and the Eastern Boundary Upwelling System (Benguela). Results indicate that globally and in Atlantic Margin and North Sea, increased ocean stratification causes primary production and zooplankton biomass to decrease in response to a warming climate, whilst in the Barents, Baltic and Black Seas, primary production and zooplankton biomass increase. Projected warming characterized by an increase in sea surface temperature of 2.29 ± 0.05 °C leads to a reduction in zooplankton and phytoplankton biomasses of 11% and 6%, respectively. This suggests negative amplification of climate driven modifications of trophic level biomass through bottom-up control, leading to a reduced capacity of oceans to regulate climate through the biological carbon pump. Simulations suggest negative amplification is the dominant response across 47% of the ocean surface and prevails in the tropical oceans; whilst positive trophic amplification prevails in the Arctic and Antarctic oceans. Trophic attenuation is projected in temperate seas. Uncertainties in ocean plankton projections, associated to the use of single global and regional models, imply the need for caution when extending these considerations into higher trophic levels.

Spatial and temporal variability of the remotely sensed chlorophyll a signal associated with Rossby waves in the South Atlantic Ocean

The present study focuses on the spatial and temporal variability of interactions between physics and biogeochemistry during the Rossby wave passage in the South Atlantic Ocean. The Rossby wave signature in sea level anomalies (SLA) and surface chlorophyll a concentration anomalies (CHLA) is analyzed using remotely sensed data from 1997 to 2006. Wavelengths between 400 and 1100 km, with westward propagating speeds up to 7.5 cm.s(-1), are observed. Using a theoretical model, three processes (meridional advection of surface chlorophyll a concentrations, uplifting of subsurface chlorophyll a maximum, and upwelling of nutrients) are likely to explain the chlorophyll a Rossby wave signature. A statistical assumption allows quantifying the relative importance of each process. Three zones are identified. The Subtropical Gyre is the only area where the contribution of the uplifting process reaches 20%. North and south of this gyre, the meridional advection process is responsible for an important part (around 60%) of the observed chlorophyll a signals. The temporal variability of this dominant process is studied using the phase relationships between CHLA and SLA and the surface meridional chlorophyll a gradient. A seasonal meridional shift (4 degrees) is shown on both data sets on the area of negative meridional gradient. At 30 degrees S-31 degrees S, a clear seasonal cycle is observed in both data sets for the whole studied period, except in 2003 and 2004 where both data sets do not follow the usual seasonal cycle. These particular years can be related to anomalies in large scale atmospheric circulation over the South Atlantic Ocean.

Rossby wave and ocean color: The cells uplifting hypothesis in the South Atlantic Subtropical Convergence Zone

[1] Rossbywavessignaturesonfilteredoceancolordataare detected in the subtropical convergence zone of the South Atlantic ocean. We investigate whether these chlorophyll anomalies can be accounted for by the uplifting mechanism of phytoplanktonic cells associated with the passage of a Rossby wave. We consider vertical chlorophyll profiles exhibiting a subsurface chlorophyll maximum typical of the South Atlantic Subtropical Convergence Province. Chlorophyll remotely-sensed concentrations resulting from an academic uplifting of the three chlorophyll profiles are reconstructed with a radiative transfer model. Amplitude of the chlorophyll enrichments found with this hypothesis (0.06 mg chl.m 3 ) compares well with propagative ocean color anomalies detected in the SeaWiFS data. However, other processes such as the Rototiller effect [Siegel, 2001] or advection of chlorophyll gradients could also play a non negligible role. INDEX TERMS: 4560 Oceanography: Physical:Surface wavesandtides (1255);9325Information Related to Geographic Region: Atlantic Ocean; 4552 Oceanography:

Modeling δ 15 N evolution: First palaeoceanographic applications in a coastal upwelling system.

The delta(15)N Signal in marine sediments appears to be a good palaeoceanographic tracer. It records biological processes in the water column and is transferred to and preserved in the sediments. Changes in forcing factors in upwelling systems may be recorded by delta(15)N. These forcing conditions can be of a biogeochemical nature, such as the initial isotopic signal of the nutrients or the trophic structure, or of a physical nature, such as wind stress, insolation, temperature or dynamic recycling. A simple nitrogen-based trophic chain model was used to follow the development of the nitrogen isotopic signal in nutrients, phytoplankton, zooplankton and detritus. Detrital delta(15)N, influenced by the isotopic signature of the upwelled nutrients and isotopic fractionation along the trophic chain (photosynthesis and zooplankton excretion), was then compared to the sedimentary signal measured off Mauritania. In our model, the biological variables are transported at shallow depths by a simple circulation scheme perpendicular to the coast depicting a continental shelf recirculation cell. Because cell length depends on the extension of the continental shelf, modifications of the cell length mimic sea level changes. Long cell length (high sea lever) scenarios produce higher delta(15)N values whereas short cell length scenarios result in lower values as in the glacial low sea level periods. Despite changes in many climatic parameters throughout this period, our results show that changing the sea level is sufficient to reconstruct the main pattern of the sedimentary delta(15)N variations offshore of the Mauritanian upwelling, i.e. an increase from about 3 parts per thousand to 7 parts per thousand during the deglaciation, without invoking any change in nitrogen fixation or denitrification.

Interannual variability in the South‐East Atlantic Ocean, focusing on the Benguela Upwelling System: Remote versus local forcing

We investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the South-East Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000–2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interannual timescales it is dominated by remote equatorial forcing. At interannual timescales (13–20 months), remotely forced Coastal-Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 248S, with phase speeds ranging from 0.8 to 1.1 m.s. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model’s southern boundary (308S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forcing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (628C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.

EUMELI oligotrophic site: response of an upper ocean model to climatological and ECMWF atmospheric forcing

Abstract Within the frame of the EUMELI program—component of FRANCE-JGOFS—in the Northeast tropical Atlantic ocean, we investigate the potential of a one-dimensional eddy-kinetic-energy model (Gaspar et al., 1990, GGL) to characterize the vertical dynamics of the oceanic mixed layer (ML) at the EUMELI oligotrophic site (21°N, 31°W) north of the Cape Verde Frontal Zone. The atmospheric forcings used are derived from two different sources: the operational Atmospheric General Circulation Model of ECMWF (over two 12-month periods: August 1985–July 1986 and the full year 1990) and climatologies (Esbensen and Kushnir, EK, 1981; Hsiung, H, 1986; Oort, 1983). At the site, depending on the data base, the annual mean of the total energy flux at the ocean-atmosphere interface differs in sign and intensity and its monthly evolution presents significant variation both in amplitude and timing of the maximum. The monthly wind stress evolution due to the regular north-east trade winds prevailing in this region is quite consistent as derived by the different data sources. In our area, a net evaporation rate occurs throughout the year. The simulated ML depth, based on GGLs ML depth definition, is always shallower than climatological observations of ML depth, whatever the surface atmospherical forcing used, the exception being the simulation performed with the atypical ECMWF85–86 forcing. The simulated SSTs using H forcing compare rather well (within 1°C) with the observed SSTs of the climatologies of Lamb and EK. Sampling experiments on the surface boundary conditions showed that simulated evolutions of the ML depth and SST differ quite significantly due to differences in data bases rather than differences in forcing frequencies. An error analysis on the ocean surface energy fluxes and the prescription of evaporation and precipitation rates under various forms demonstrate the crucial need for heat, momentum and freshwater fluxes estimates as accurate as possible. From the distributions of the turbulent kinetic energy (TKE) budget on different time scales, it is found that most often shear production and viscous dissipation dominate in the ML. Gravitational production or destruction, turbulent diffusion and storage of TKE are of second-order. The use of a daily instead of a 3-hour forcing creates an underestimation of 19% of the total annual energy produced by the shear. When taking into account freshwater fluxes, gravitational production becomes of first order during fall and winter and intervenes in the balance between shear production and viscous dissipation.

Reproduire la circulation thermohaline à échelle réduite et comprendre son rôle dans le climat

Matériel nécessaire : un aquarium rectangulaire de 15 litres, des colorants alimentaires rouge et bleu (ce dernier sous forme de bleu de méthylène ou de grains de sable colorés en bleu), un sac plastique rempli de glaçons (un demi-litre environ), deux pinces à linge, une plaque en carton ou mieux en plastique de la largeur de l’aquarium, un tuyau flexible transparent d’un mètre de long, une bouilloire, une paire de gants (en latex souple), une spatule, éventuellement un chronomètre et une seringue.

High‐resolution modeling of the Eastern Tropical Pacific oxygen minimum zone: Sensitivity to the tropical oceanic circulation

The connection between the equatorial mean circulation and the oxygen minimum zone (OMZ) in the Eastern Tropical Pacific is investigated through sensitivity experiments with a high-resolution coupled physical-biogeochemical model. A validation against in situ observations indicates a realistic simulation of the vertical and horizontal oxygen distribution by the model. Two sets of climatological open-boundary conditions for the physical variables, which differ slightly with respect to the intensity and vertical structure of the Equatorial Current System, are shown to lead to contrasting characteristics of the simulated OMZ dynamics. From a Lagrangian perspective, the mean differences near the coast originate to a large extent from the different transport of deoxygenated waters by the secondary Tsuchiya Jet (secondary Southern Subsurface Countercurrent, sSSCC). The O2 budget further indicates a large difference in the balance between tendency terms, with advection exhibiting the largest difference between both simulations, which is shown to result from both linear and nonlinear advection. At regional scale, we also find that the variability of the physical contribution to the rate of O2 change is one order of magnitude larger than the variability associated with the biogeochemical contribution, which originates from internal high-frequency variability. Overall our study illustrates the large sensitivity of the OMZ dynamics to the equatorial circulation.

Climatically-active gases in the Eastern Boundary Upwelling and Oxygen Minimum Zone (OMZ) systems

The EBUS (Eastern Boundary Upwelling Systems) and OMZs (Oxygen Minimum Zone) contribute very significantly to the gas exchange between the ocean and the atmosphere, notably with respect to the greenhouse gases (hereafter GHG). From in-situ ocean measurements, the uncertainty of the net global ocean-atmosphere CO2 fluxes is between 20 and 30%, and could be much higher in the EBUS-OMZ. Off Peru, very few in-situ data are available presently, which justifies alternative approaches for assessing these fluxes. In this contribution we introduce.