924 research outputs found
Inertia-induced accumulation of flotsam in the subtropical gyres
Recent surveys of marine plastic debris density have revealed high levels in
the center of the subtropical gyres. Earlier studies have argued that the
formation of great garbage patches is due to Ekman convergence in such regions.
In this work we report a tendency so far overlooked of drogued and undrogued
drifters to accumulate distinctly over the subtropical gyres, with undrogued
drifters accumulating in the same areas where plastic debris accumulate. We
show that the observed accumulation is too fast for Ekman convergence to
explain it. We demonstrate that the accumulation is controlled by finite-size
and buoyancy (i.e., inertial) effects on undrogued drifter motion subjected to
ocean current and wind drags. We infer that the motion of flotsam in general is
constrained by similar effects. This is done by using a newly proposed
Maxey--Riley equation which models the submerged (surfaced) drifter portion as
a sphere of the fractional volume that is submerged (surfaced).Comment: Submitted to Geophys. Res. Letter
Extracting quasi-steady Lagrangian transport patterns from the ocean circulation: An application to the Gulf of Mexico
We construct a climatology of Lagrangian coherent structures (LCSs), the
concealed skeleton that shapes transport, with a twelve-year-long
data-assimilative simulation of the sea-surface circulation in the Gulf of
Mexico (GoM). Computed as time-mean Cauchy-Green strain tensorlines of the
climatological velocity, the climatological LCSs (cLCSs) unveil recurrent
Lagrangian circulation patterns. cLCSs strongly constrain the ensemble-mean
Lagrangian circulation of the instantaneous model velocity, thus we show that a
climatological velocity may preserve meaningful transport information. Also,
the climatological transport patterns we report agree well with GoM kinematics
and dynamics, as described in several previous observational and numerical
studies. For example, cLCSs identify regions of persistent isolation, and
suggest that coastal regions previously identified as high-risk for pollution
impact, are regions of maximal attraction. Also, we show examples where cLCSs
are remarkably similar to transport patterns observed during the Deepwater
Horizon and Ixtoc oil spills, and during the Grand LAgrangian Deployment (GLAD)
experiment. Thus, it is shown that cLCSs are an efficient way of synthesizing
vast amounts of Lagrangian information. The cLCS method confirms previous GoM
studies, and contributes to our understanding by revealing the persistent
nature of the dynamics and kinematics treated therein.Comment: To be submitte
Travel time stability in weakly range-dependent sound channels
Travel time stability is investigated in environments consisting of a
range-independent background sound-speed profile on which a highly structured
range-dependent perturbation is superimposed. The stability of both
unconstrained and constrained (eigenray) travel times are considered. Both
general theoretical arguments and analytical estimates of time spreads suggest
that travel time stability is largely controlled by a property of the background sound speed profile. Here, is
the range of a ray double loop and is the ray action variable. Numerical
results for both volume scattering by internal waves in deep ocean environments
and rough surface scattering in upward refracting environments are shown to
confirm the expectation that travel time stability is largely controlled by
.Comment: Submitted to J. Acoust. Soc. Am., 30 June 200
Can the structure of motor variability predict learning rate?
Recent studies show that motor variability is actively regulated as an exploration tool to promote learning
in reward-based tasks. However, its role in learning processes during error-based tasks, when a reduction
of the motor variability is required to achieve good performance, is still unclear. In this study, we
hypothesized that error-based learning not only depends on exploration but also on the individuals’
ability to measure and predict the motor error. Previous studies identified a less auto-correlated motor
variability as a higher ability to perform motion adjustments. Two experiments investigated the relationship
between motor learning and variability, analyzing the long-range autocorrelation of the center
of pressure fluctuations through the score of a Detrended Fluctuation Analysis in balance tasks. In
Experiment 1, we assessed the relationship between variability and learning rate using a standing balance
task. Based on the results of this experiment, and to maximize learning, we performed a second
experiment with a more difficult sitting balance task and increased practice. The learning rate of the 2
groups with similar balance performances but different scores was compared. Individuals with a lower
score showed a higher learning rate. Because the scores reveal how the motor output changes over
time, instead of the magnitude of those changes, the higher learning rate is mainly linked to the higher
error sensitivity rather than the exploration strategies. The results of this study highlight the relevance of
the structure of output motor variability as a predictor of learning rate in error-based tasks
O programa nacional de fortalecimento da agricultura familiar no Brasil: uma análise sobre a distribuição regional e setorial dos recursos.
No Brasil, as polĂticas pĂşblicas para o espaço rural sempre tenderam a priorizar a agricultura patronal, em detrimento dos agricultores familiares. Todavia, os estudos realizados pelos ĂłrgĂŁos FAO - INCRA deram subsĂdio para a criação do Programa Nacional de Fortalecimento da Agricultura Familiar (PRONAF), resultando em um novo direcionamento dos investimentos pĂşblicos, os quais passaram a contemplar o segmento dos agricultores familiares. Entende-se o PRONAF como uma polĂtica nĂŁo-compensatĂłria, que, apesar de seus problemas, tem contribuĂdo de fato para mudanças e melhorias no espaço agrário brasileiro. Desde sua criação no final da dĂ©cada de 1990, o PRONAF passou por várias mudanças em sua estrutura administrativa e operacional, a fim de alcançar seus objetivos e adequar-se face a complexa realidade social agrária brasileira. Sendo assim, o presente estudo visa discutir as ações do Estado por meio desse Programa, a partir de suas linhas de atuação, bem como analisar a distribuição de suas concessões de crĂ©dito regional e setorialmente. Assim, os procedimentos metodolĂłgicos utilizados para a realização deste trabalho compreendem pesquisa bibliográfica e documental, alĂ©m de pesquisa em fontes secundárias, no intuito de obter dados e informações relevantes para a análise das relações sociais estabelecidas em meio a esse processo de concretização e espacialização desse Programa. Dentre as implicações do PRONAF pode-se notar em âmbito nacional, uma diminuição da disparidade regional brasileira, bem como a preocupação que o Programa tem demonstrado com os aspectos socioculturais locais e regionais, como forma de garantir que seus investimentos perpassem a dimensĂŁo econĂ´mica, mas valorize outras dimensões, a exemplo dos elementos culturais
Descriptive profile of hip range of motion in elite tennis players
Objective: To describe the range of motion (ROM) profile (flexion, extension, abduction, internal and
external rotation) of the hip in elite tennis players; and (b) to analyse if there are sex-related differences
in the hip ROM.
Design: Cohort study.
Setting: Controlled laboratory environment.
Participants: 81 male and 28 female tennis players completed this study.
Main outcome measures: Descriptive measures of passive hip flexion, extension and abduction, and internal
and external active and passive hip rotation ROM were taken. Magnitude-based inferences on differences between sex (males vs. females) and hip (dominant vs. non-dominant) were made by standardising differences.
Results: No clinically meaningful bilateral and sex-related differences in any of the hip ROM measures. In addition, it was found that both males and females had restricted mobility measures on hip flexion (25 ] and passive [35 ])
Conclusions: Asymmetric hip joint ROM measures found during clinical examination and screening may indicate abnormalities and the need of rehabilitation (e.g., flexibility training). In addition, clinicians should include specific exercises (e.g., stretching) in their conditioning, prevention and rehabilitation programmes aiming to avoid restricted mobility of hip flexion (males ÂĽ 74 ; females ÂĽ 78 ), extension (males ÂĽ 1.5; females ÂĽ 0.4), abduction (males ÂĽ 35 ; females ÂĽ 34 ) and internal rotation (males ÂĽ 30 ; females ÂĽ 35) that might be generated as a consequence of playing tennis
ContribuciĂłn a la corologĂa de las macroalgas marinas bentĂłnicas del litoral malagueño. I.
Continuando con un proyecto sobre la corologĂa de las macroalgas marinas bentĂłnicas del litoral malagueño (Conde, 1981; Conde, 1984; Conde & Soto, 1986) reseñamos en esta nota la presencia en la zona de 5 nuevas especies, y se confirman otras 5, citadas anteriormente en la literatura o de arribazĂłn. La ordenaciĂłn de las especies se ha realizado segĂşn la propuesta de Gallardo Ă©t al. (1985)
Aspectos ultraestructurales de algunas coralináceas (Rhodophyta) del Mediterráneo andaluz (S. de España).
En el presente artĂculo se estudian e interpretan al microscopio electrĂłnico de barrido (M.E.B.) algunos caracteres cualitativos de siete especies de algas coralináceas del litoral andaluz, pertenecientes a los gĂ©neros Corallina,Jania y Amphiroa.Using the scanning electron microscopy some qualitatives characteristics of seven seaweeds species of the mediterranean coasts of AndalucĂa (S. of Spain), of the genus Corallina, Junio y Amphiroa have been studied
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