200 research outputs found
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
Lagrangian dynamical geography of the Gulf of Mexico
We construct a Markov-chain representation of the surface-ocean Lagrangian
dynamics in a region occupied by the Gulf of Mexico (GoM) and adjacent portions
of the Caribbean Sea and North Atlantic using satellite-tracked drifter
trajectory data, the largest collection so far considered. From the analysis of
the eigenvectors of the transition matrix associated with the chain, we
identify almost-invariant attracting sets and their basins of attraction. With
this information we decompose the GoM's geography into weakly dynamically
interacting provinces, which constrain the connectivity between distant
locations within the GoM. Offshore oil exploration, oil spill contingency
planning, and fish larval connectivity assessment are among the many activities
that can benefit from the dynamical information carried in the geography
constructed here.Comment: Submitted to Scientific Report
Ray stability in weakly range-dependent sound channels
Ray stability is investigated in environments consisting of a
range-independent background sound-speed profile on which a range-dependent
perturbation, such as that produced by internal waves in deep ocean
environments, is superimposed. Numerical results show that ray stability is
strongly influenced by the background sound speed profile. Ray instability is
shown to increase with increasing magnitude of alpha := I omega^{prime} /
omega, where 2 pi / omega(I) is the range of a ray double loop and I is the ray
action variable. The mechanism, shear-induced instability enhancement, by which
alpha controls ray instability is described.Comment: To appear in JAS
Enduring Lagrangian coherence of a Loop Current ring assessed using independent observations
Ocean flows are routinely inferred from low-resolution satellite altimetry
measurements of sea surface height assuming a geostrophic balance. Recent
nonlinear dynamical systems techniques have revealed that surface currents
derived from altimetry can support mesoscale eddies with material boundaries
that do not filament for many months, thereby representing effective transport
mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale
eddy boundaries detected from altimetry is constrained by the impossibility of
current altimeters to resolve ageostrophic submesoscale motions. These may act
to prevent Lagrangian coherence from manifesting in the rigorous form described
by the nonlinear dynamical systems theories. Here we use a combination of
satellite ocean color and surface drifter trajectory data, rarely available
simultaneously over an extended period of time, to provide observational
evidence for the enduring Lagrangian coherence of a Loop Current ring detected
from altimetry. We also seek indications of this behavior in the flow produced
by a data-assimilative system which demonstrated ability to reproduce observed
relative dispersion statistics down into the marginally submesoscale range.
However, the simulated flow, total surface and subsurface or subsampled
emulating altimetry, is not found to support the long-lasting Lagrangian
coherence that characterizes the observed ring. This highlights the importance
of the Lagrangian metrics produced by the nonlinear dynamical systems tools
employed here in assessing model performance.Comment: In press in nature.com/Scientific Report
Transition paths of marine debris and the stability of the garbage patches
We used transition path theory (TPT) to infer "reactive" pathways of floating
marine debris trajectories. The TPT analysis was applied on a pollution-aware
time-homogeneous Markov chain model constructed from trajectories produced by
satellite-tracked undrogued buoys from the NOAA Global Drifter Program. The
latter involved coping with the openness of the system in physical space, which
further required an adaptation of the standard TPT setting. Directly connecting
pollution sources along coastlines with garbage patches of varied strengths,
the unveiled reactive pollution routes represent alternative targets for ocean
cleanup efforts. Among our specific findings we highlight: constraining a
highly probable pollution source for the Great Pacific Garbage Patch;
characterizing the weakness of the Indian Ocean gyre as a trap for plastic
waste; and unveiling a tendency of the subtropical gyres to export garbage
toward the coastlines rather than to other gyres in the event of anomalously
intense winds.Comment: Submitted to Chao
Building a Maxey--Riley framework for surface ocean inertial particle dynamics
A framework for the study of surface ocean inertial particle motion is built
from the Maxey--Riley set. A new set is obtained by vertically averaging each
term of the original set, adapted to account for Earth's rotation effects,
across the extent of a sufficiently small spherical particle that floats at an
assumed unperturbed air--sea interface with unsteady nonuniform winds and ocean
currents above and below, respectively. The inertial particle velocity is shown
to exponentially decay in time to a velocity that lies close to an average of
seawater and air velocities, weighted by a function of the seawater-to-particle
density ratio. Such a weighted average velocity turns out to fortuitously be of
the type commonly discussed in the search-and-rescue literature, which alone
cannot explain the observed role of anticyclonic mesoscale eddies as traps for
marine debris or the formation of great garbage patches in the subtropical
gyres, phenomena dominated by finite-size effects. A heuristic extension of the
theory is proposed to describe the motion of nonspherical particles by means of
a simple shape factor correction, and recommendations are made for
incorporating wave-induced Stokes drift, and allowing for inhomogeneities of
the carrying fluid density. The new Maxey--Riley set outperforms an ocean
adaptation that ignored wind drag effects and the first reported adaption that
attempted to incorporate them.Comment: To appear in Phys. Fluid
Markov-chain-inspired search for MH370
Markov-chain models are constructed for the probabilistic description of the
drift of marine debris from Malaysian Airlines flight MH370. En route from
Kuala Lumpur to Beijing, the MH370 mysteriously disappeared in the southeastern
Indian Ocean on 8 March 2014, somewhere along the arc of the 7th ping ring
around the Inmarsat-3F1 satellite position when the airplane lost contact. The
models are obtained by discretizing the motion of undrogued satellite-tracked
surface drifting buoys from the global historical data bank. A spectral
analysis, Bayesian estimation, and the computation of most probable paths
between the Inmarsat arc and confirmed airplane debris beaching sites are shown
to constrain the crash site, near 25S on the Inmarsat arc.Comment: Submitted to Chao
Sampling--Dependent Transition Paths of Iceland-Scotland Overflow Water
In this note, we apply Transition Path Theory (TPT) from Markov chains to
shed light on the problem of Iceland-Scotland Overflow Water (ISOW) equatorward
export. A recent analysis of observed trajectories of submerged floats demanded
revision of the traditional abyssal circulation theory, which postulates that
ISOW should steadily flow along a deep boundary current (DBC) around the
subpolar North Atlantic prior to exiting it. The TPT analyses carried out here
allow to focus the attention on the portions of flow from the origin of ISOW to
the region where ISOW exits the subpolar North Atlantic and suggest that
insufficient sampling may be biasing the aforementioned demand. The analyses,
appropriately adapted to represent a continuous input of ISOW, are carried out
on three time-homogeneous Markov chains modeling the ISOW flow. One is
constructed using a high number of simulated trajectories homogeneously
covering the flow domain. The other two use much fewer trajectories which
heterogeneously cover the domain. The trajectories in the latter two chains are
observed trajectories or simulated trajectories subsampled at the observed
frequency. While the densely sampled chain supports a well-defined DBC, the
more heterogeneously sampled chains do not, irrespective of whether observed or
simulated trajectories are used. Studying the sampling sensitivity of the
Markov chains, we can give recommendations for enlarging the existing float
dataset to improve the significance of conclusions about time-asymptotic
aspects of the ISOW circulation.Comment: Submitted to JPO. Comments welcomed
Particle acoustic detection in gravitational wave aluminum resonant antennas
The results on cosmic rays detected by the gravitational antenna NAUTILUS
have motivated an experiment (RAP) based on a suspended cylindrical bar, which
is made of the same aluminum alloy as NAUTILUS and is exposed to a high energy
electron beam. Mechanical vibrations originate from the local thermal expansion
caused by warming up due to the energy lost by particles crossing the material.
The aim of the experiment is to measure the amplitude of the fundamental
longitudinal vibration at different temperatures. We report on the results
obtained down to a temperature of about 4 K, which agree at the level of about
10% with the predictions of the model describing the underlying physical
process.Comment: RAP experiment, 16 pages, 7 figure
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