Phytoplankton seasonal distribution from SeaWiFS data in the Agulhas Current system

The interocean conduit for warm Indian Ocean water into the Atlantic Ocean is the Agulhas system which plays an important role in maintaining the global thermohaline circulation. The frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is also a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a. We jointly analyzed three different satellite data sets to relate the remotely sensed distribution of phytoplankton (SeaWiFS ocean color data) to the dynamical environment (Topex/Poséidon-ERS sea level anomalies-SLA- and sea-surface temperature-SST-) to gain insight into the seasonal behavior of the Agulhas Current system. We used a wavelet analysis to retrieve the characteristic wavelengths of the ARC and STC associated to their meandering. By meridionally averaging (between 15- 45E) the two-dimensional power Hovmöller of each signal (Chla, SLA, and SST), we obtained a seasonal average variance for Chla, SLA and SST as a function of latitude. Within the double frontal Agulhas and Subtropical frontal system, an extended temporal maximum in chlorophyll a concentration is observed in spring-summer-fall and a well-marked minimum occurs in winter, in phase opposition with the southwest Indian Ocean subtropical gyre north of the frontal system. Seasonal changes in strength of cross-frontal mixing with the subtropical gyre, in density strength of the juxtaposed fronts, and in mixed-layer depth and light availability seem the most likely explanations for the observed spatial and seasonal variability of the chlorophyll a distribution

Using JGOFS in situ and ocean color data to compare biogeochemical models and estimate their parameters in the subtropical North Atlantic Ocean

How well do biogeochemical data sets serve to decide among models and model parameter values? Data at 21N, 31W from the French JGOFS EUMELI cruises and the SeaWiFS ocean color sensor were used to estimate parameters for three very different models of biological nitrogen flux in a water column. The three models are (1) an NPZD (Nutrients, Phytoplankton, Zooplankton and Detritus) model (Oschlies et al., 2000), (2) a seven-component model with two pools of dissolved organic matter and detritus with different remineralization and sinking rates (Dadou et al., 2001) and (3) a model of nutrients and phytoplankton including aggregates (Kriest and Evans, 1999). Parameters of the three models are estimated using the same sets of data within the same one-dimensional physical framework. A combination of local and nonlocal optimization methods is used. It is not easy to decide among candidate models based on their fit to the data. Parameters that mean the same thing in the three models, like the half-saturation concentration for nitrate uptake, were estimated at not very different values in different models. The model with dissolved organic matter, based on its primary production and sediment flux data time evolutions, seems to exhibit the more reasonable annual behavior. Large seasonal changes in deep nitrate data suggest an unexpected role of lateral advection and may vitiate the 1-D approach even at the EUMELI oligotrophic site. The small number of sediment trap measurements are very powerful for constraining the biological nitrogen. Ocean color data did not add extra constraining power

Effects of upwelling duration and phytoplankton growth regime on dissolved-oxygen levels in an idealized Iberian Peninsula upwelling system

Abstract. We apply a coupled modelling system composed of a state-of-the-art hydrodynamical model and a low-complexity biogeochemical model to an idealized Iberian Peninsula upwelling system to identify the main drivers of dissolved-oxygen variability and to study its response to changes in the duration of the upwelling season and in the phytoplankton growth regime. We find that the export of oxygenated waters by upwelling front turbulence is a major sink for nearshore dissolved oxygen. In our simulations of summer upwelling, when the phytoplankton population is generally dominated by diatoms whose growth is boosted by nutrient input, net primary production and air–sea exchange compensate dissolved-oxygen depletion by offshore export over the shelf. A shorter upwelling duration causes a relaxation of upwelling winds and a decrease in offshore export, resulting in a slight increase of net dissolved-oxygen enrichment in the coastal region as compared to longer upwelling durations. When phytoplankton is dominated by groups less sensitive to nutrient inputs, growth rates decrease, and the coastal region becomes net heterotrophic. Together with the physical sink, this lowers the net oxygenation rate of coastal waters, which remains positive only because of air–sea exchange. These findings help in disentangling the physical and biogeochemical controls of dissolved oxygen in upwelling systems and, together with projections of increased duration of upwelling seasons and phytoplankton community changes, suggest that the Iberian coastal upwelling region may become more vulnerable to hypoxia and deoxygenation. This research has been supported by the IDEX UNITI – University of Toulouse (TEASAO IDEX UNITI – Univer- sity of Toulouse)

Variability of the biological front south of Africa from SeaWiFS and a coupled physical-biological model

The spatio-temporal variability of the biological front in the Agulhas Current system is investigated by comparing SeaWiFS chlorophyll a data and modeled chlorophyll fields over the October 1997–October 2001 period. The latter fields are simulated using a regional eddy-permitting (1/3° × 1/3°) coupled physical (AGAPE)-biological model forced by the monthly atmospheric NCEP/NCAR reanalysis. The annual cycle of the observed chlorophyll within the Agulhas Current system biogeochemical provinces is quite well reproduced by the model. The modeled phase of the seasonality in the SWSIG (South Western Subtropical Indian Gyre) is opposite to that of the SCZ (Subtropical Convergence Zone encompassing the Agulhas Front-AF, the Subtropical Front-STF and the Subantarctic Front-SAF), in agreement with observations. In the SWSIG, the switch from nitrates limitation to light control for the modeled phytoplankton growth shifts southward from winter to summer. In the SCZ, light availability modulates growth throughout the year. The wavelet average variance of the SeaWiFS data is slightly underestimated by the modeled chlorophyll variance over the four-year period within the 36 –45S and 15–45E domain. This might originate in the interannual monthly NCEP forcing which does not include the high frequency information of the atmospheric fluxes. The model coarse resolution precludes a proper simulation of vertical motions produced by submesoscale flows thereby underestimating biological variability. Interestingly, the modeled chlorophyll distribution mimicks the strong early retroflection of the Agulhas Current in summer 2001 which induces a southward displacement of the STF/SAF double front

Analytical Form of the Deuteron Wave Function Calculated within the Dispersion Approach

We present a convenient analytical parametrization of the deuteron wave function calculated within dispersion approach as a discrete superposition of Yukawa-type functions, in both configuration and momentum spaces.Comment: 3 pages, 2 figure; several minor corrections adde

Sources of short-lived bromocarbons in the Iberian upwelling system

Seawater concentrations of the four brominated trace gases dibromomethane (CH2Br2), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3) were measured at different depths of the water column in the Iberian upwelling off Portugal during summer 2007. Bromocarbon concentrations showed elevated values in recently upwelled and aged upwelled waters (mean values of 30 pmol L−1 for CHBr3), while values in the open ocean were significantly lower (7.4 pmol L−1 for CHBr3). Correlations with biological variables and marker pigments indicated that phytoplankton could be identified as a weak bromocarbon source in the open ocean. In upwelled water masses along the coast, halocarbons were not correlated to Chl-a, indicating an external source, overlapping the possible internal production by phytoplankton. We showed that the tidal frequency had a significant influence on halocarbon concentrations in the upwelling and we linked those findings to a strong intertidal coastal source, as well as to a transport of those halocarbon enriched coastal waters by westward surface upwelling currents. Coastal sources and transport can be accounted for maximum values of up to 185.1 pmol L−1 CHBr3 in the upwelling. Comparison with other productive marine areas revealed that the Iberian upwelling had stronger halocarbon sources than the phytoplankton dominated sources in the Mauritanian upwelling. However, the concentrations off the Iberian Peninsula were still much lower than those of coastal macroalgal influenced waters or those of polar regions dominated by cold water adapted diatom

Influence of Rossby waves on primary production from a coupled physical-biogeochemical model in the North Atlantic Ocean

Rossby waves appear to have a clear signature on surface chlorophyll concentrations which can be explained by a combination of vertical and horizontal mechanisms. In this study, we investigate the role of the different physical processes in the north Atlantic to explain the surface chlorophyll signatures and the consequences on primary production, using a 3-D coupled physical/biogeochemical model for the year 1998. <br><br> The analysis at 20 given latitudes, mainly located in the subtropical gyre, where Rossby waves are strongly correlated with a surface chlorophyll signature, shows the important contribution of horizontal advection and of vertical advection and diffusion of inorganic dissolved nitrogen. The main control mechanism differs according to the biogeochemical background conditions of the area. <br><br> The surface chlorophyll anomalies, induced by these physical mechanisms, have an impact on primary production. We estimate that Rossby waves induce, locally in space and time, increases (generally associated with the chlorophyll wave crest) and decreases (generally associated with the chlorophyll wave trough) in primary production, ~±20% of the estimated background primary production. This symmetrical situation suggests a net weak effect of Rossby waves on primary production

Generalized parton distributions in the deuteron

We introduce generalized quark and gluon distributions in the deuteron, which can be measured in exclusive processes like deeply virtual Compton scattering and meson electroproduction. We discuss the basic properties of these distributions, and point out how they probe the interplay of nucleon and parton degrees of freedom in the deuteron wave function

Measurement of the Analyzing Power in p⃗d→(pp)n\vec{p}d \to (pp)n \\with a Fast Forward 1S0^1S_0--Diproton

A measurement of the analyzing power $A_y$ of the $\vec{p}d \to (pp) + n$ reaction was carried out at beam energies of 0.5 and 0.8 GeV by detection of a fast forward proton pair of small excitation energy $E_{pp} < 3$ MeV. The kinematically complete experiment made use of the ANKE spectrometer at the internal beam of COSY and a deuterium cluster--jet target. For the first time the $S$--wave dominance in the fast diproton is experimentally demonstrated in this reaction. While at $T_p=0.8$ GeV the measured analyzing power $A_y$ vanishes, it reaches almost unity at $T_p=0.5$ GeV for neutrons scattered at $\theta_n^{c.m.}=167^\circ$. The results are compared with a model taking into account one--nucleon exchange, single scattering and $\Delta$ (1232) excitation in the intermediate state. The model describes fairly well the unpolarized cross section obtained earlier by us and the analyzing power at 0.8 GeV, it fails to reproduce the angular dependence of $A_y$ at 0.5 GeV.Comment: 4 pages, 4 figures, 1 tabl

The HPS electromagnetic calorimeter

The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called “heavy photon.” Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015–2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier