252 research outputs found
Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems
Eastern Boundary Upwelling Systems (EBUS) are characterized by a high
productivity of plankton associated with large commercial fisheries, thus
playing key biological and socio-economical roles. The aim of this work is to
make a comparative study of these four upwelling systems focussing on their
surface stirring, using the Finite Size Lyapunov Exponents (FSLEs), and their
biological activity, based on satellite data. First, the spatial distribution
of horizontal mixing is analysed from time averages and from probability
density functions of FSLEs. Then we studied the temporal variability of surface
stirring focussing on the annual and seasonal cycle. There is a global negative
correlation between surface horizontal mixing and chlorophyll standing stocks
over the four areas. To try to better understand this inverse relationship, we
consider the vertical dimension by looking at the Ekman-transport and vertical
velocities. We suggest the possibility of a changing response of the
phytoplankton to sub/mesoscale turbulence, from a negative effect in the very
productive coastal areas to a positive one in the open ocean.Comment: 12 pages. NPG Special Issue on "Nonlinear processes in oceanic and
atmospheric flows". Open Access paper, available also at the publisher site:
http://www.nonlin-processes-geophys.net/16/557/2009
Sensitivity of ecosystem parameters to simulated satellite ocean colour data using a coupled physical-biological model of the North Atlantic
A means of assimilating simulated satellite ocean color data with a coupled physical-biological model of the North Atlantic Ocean is implemented, allowing the relative sensitivities of different biological parameters to those data to be investigated. The model consists of an eddy-permitting general circulation model derived from the WOCE Community Modeling Effort and a nitrogen-based, four-compartment NPZD marine ecosystem model. Many of the parameters in marine ecosystem models are poorly known and via assimilation, we hope to better constrain their values. The control parameters chosen for the variational assimilation are the model parameters involved in parameterizations of recycling as these are the most poorly known. Simulated observations are taken while following several floats seeded in varying dynamical biogeochemical provinces of the North Atlantic model domain over a six-month period. Twin experimental results show that, for the given functional forms of growth, mortality and grazing, the following parameters can be successfully recovered from simulated satellite ocean color data: nitrate and detrital recycling parameters in the trade wind domain, zooplankton parameters at higher latitudes (westerly wind and polar domains), and the phytoplankton mortality rate in all regions. By simultaneously assimilating ocean color data in different biological provinces, it becomes possible to successfully constrain all ecosystem parameters at once
Multidisciplinary Observing in the World Ocean\u27s Oxygen Minimum Zone Regions: From Climate to Fish - The VOICE Initiative
Multidisciplinary ocean observing activities provide critical ocean information to satisfy ever-changing socioeconomic needs and require coordinated implementation. The upper oxycline (transition between high and low oxygen waters) is fundamentally important for the ecosystem structure and can be a useful proxy for multiple observing objectives connected to eastern boundary systems (EBSs) that neighbor oxygen minimum zones (OMZs). The variability of the oxycline and its impact on the ecosystem (VOICE) initiative demonstrates how societal benefits drive the need for integration and optimization of biological, biogeochemical, and physical components of regional ocean observing related to EBS. In liaison with the Global Ocean Oxygen Network, VOICE creates a roadmap toward observation-model syntheses for a comprehensive understanding of selected oxycline-dependent objectives. Local to global effects, such as habitat compression or deoxygenation trends, prompt for comprehensive observing of the oxycline on various space and time scales, and for an increased awareness of its impact on ecosystem services. Building on the Framework for Ocean Observing (FOO), we present a first readiness level assessment for ocean observing of the oxycline in EBS. This was to determine current ocean observing design and future needs in EBS regions (e.g., the California Current System, the Equatorial Eastern Pacific off Ecuador, the Peru-Chile Current system, the Northern Benguela off Namibia, etc.) building on the FOO strategy. We choose regional champions to assess the ocean observing design elements proposed in the FOO, namely, requirement processes, coordination of observational elements, and data management and information products and the related best practices. The readiness level for the FOO elements was derived for each EBS through a similar and very general ad hoc questionnaire. Despite some weaknesses in the questionnaire design and its completion, an assessment was achievable. We found that fisheries and ecosystem management are a societal requirement for all regions, but maturity levels of observational elements and data management and information products differ substantially. Identification of relevant stakeholders, developing strategies for readiness level improvements, and building and sustaining infrastructure capacity to implement these strategies are fundamental milestones for the VOICE initiative over the next 2-5 years and beyond
Chiral effective theory predictions for deuteron form factor ratios at low Q^2
We use chiral effective theory to predict the deuteron form factor ratio
G_C/G_Q as well as ratios of deuteron to nucleon form factors. These ratios are
calculated to next-to-next-to-leading order. At this order the chiral expansion
for the NN isoscalar charge operator (including consistently calculated 1/M
corrections) is a parameter-free prediction of the effective theory. Use of
this operator in conjunction with NLO and NNLO chiral effective theory wave
functions produces results that are consistent with extant experimental data
for Q^2 < 0.35 GeV^2. These wave functions predict a deuteron quadrupole moment
G_Q(Q^2=0)=0.278-0.282 fm^2-with the variation arising from short-distance
contributions to this quantity. The variation is of the same size as the
discrepancy between the theoretical result and the experimental value. This
motivates the renormalization of G_Q via a two-nucleon operator that couples to
quadrupole photons. After that renormalization we obtain a robust prediction
for the shape of G_C/G_Q at Q^2 < 0.3 GeV^2. This allows us to make precise,
model-independent predictions for the values of this ratio that will be
measured at the lower end of the kinematic range explored at BLAST. We also
present results for the ratio G_C/G_M.Comment: 31 pages, 7 figure
Top marine predators track Lagrangian coherent structures
Meso- and submesoscales (fronts, eddies, filaments) in surface ocean flow
have a crucial influence on marine ecosystems. Their dynamics partly control
the foraging behaviour and the displacement of marine top predators (tuna,
birds, turtles, and cetaceans). In this work we focus on the role of
submesoscale structures in the Mozambique Channel on the distribution of a
marine predator, the Great Frigatebird. Using a newly developed dynamical
concept, namely the Finite-Size Lyapunov Exponent (FSLE), we have identified
Lagrangian coherent structures (LCSs) present in the surface flow in the
Channel over a 2-month observation period (August and September 2003). By
comparing seabirds' satellite positions with LCSs locations, we demonstrate
that frigatebirds track precisely these structures in the Mozambique Channel,
providing the first evidence that a top predator is able to track these FSLE
ridges to locate food patches. After comparing bird positions during long and
short trips, and different parts of these trips, we propose several hypotheses
to understand how frigatebirds can follow these LCSs. The birds might use
visual and/or olfactory cues and/or atmospheric current changes over the
structures to move along these biological corridors. The birds being often
associated to tuna schools around foraging areas, a thorough comprehension of
their foraging behaviour and movement during the breeding season is crucial not
only to seabirds' ecology but also to an appropriate ecosystemic approach of
fisheries in the Channel
Two-fermion relativistic bound states in Light-Front Dynamics
In the Light-Front Dynamics, the wave function equations and their numerical
solutions, for two fermion bound systems, are presented. Analytical expressions
for the ladder one-boson exchange interaction kernels corresponding to scalar,
pseudoscalar, pseudovector and vector exchanges are given. Different couplings
are analyzed separately and each of them is found to exhibit special features.
The results are compared with the non relativistic solutions.Comment: 40 pages, to be published in Phys. Rev. C, .tar.gz fil
Measurement of Unpolarized and Polarized Cross Sections for Deeply Virtual Compton Scattering on the Proton at Jefferson Laboratory with CLAS
This paper reports the measurement of polarized and unpolarized cross sections for the epâeâČpâČÎł reaction, which is composed of deeply virtual Compton scattering (DVCS) and Bethe-Heitler (BH) processes, at an electron beam energy of 5.88 GeV at the Thomas Jefferson National Accelerator Facility using the Large Acceptance Spectrometer CLAS. The unpolarized cross sections and polarized cross section differences have been measured over broad kinematics, 0.10 \u3c xB \u3c 0.58, 1.0 \u3c Q2 \u3c4.8 GeV2 and 0.09\u3cât \u3c2.00 GeV2. The results are found to be consistent with previous CLAS data, and these new data are discussed in the framework of the generalized parton distribution approach. Calculations with two widely used phenomenological models are approximately compatible with the experimental results over a large portion of the kinematic range of the data
Deuteron-Proton Elastic Scattering at Intermediate Energies
The deuteron-proton elastic scattering has been studied in the multiple
scattering expansion formalism. The essential attention has been given to such
relativistic problem as a deuteron wave function in a moving frame and
transformation of spin states due to Wigner rotation. Parameterization of the
nucleon-nucleon -matrix has been used to take the off-energy shell effects
into account. The vector, and tensor, , analyzing powers of the
deuteron have been calculated at two deuteron kinetic energies: 395 MeV and
1200 MeV. The obtained results are compared with the experimental data
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