18,516 research outputs found
Quantifying Eulerian Eddy Leakiness in an Idealized Model
An idealized eddy‐resolving ocean basin, closely resembling the North Pacific Ocean, is simulated using MITgcm. We identify rotationally coherent Lagrangian vortices (RCLVs) and sea surface height (SSH) eddies based on the Lagrangian and Eulerian framework, respectively. General statistical results show that RCLVs have a much smaller coherent core than SSH eddies with the ratio of radius is about 0.5. RCLVs are often enclosed by SSH anomaly contours, but SSH eddy identification method fails to detect more than half of RCLVs. Based on their locations, two types of eddies are classified into three categories: overlapping RCLVs and SSH eddies, nonoverlapping SSH eddies, and nonoverlapping RCLVs. Using Lagrangian particles, we examine the processes of leakage and intrusion around SSH eddies. For overlapping SSH eddies, over the lifetime, the material coherent core only accounts for about 25% and about 50% of initial water leak from eddy interior. The remaining 25% of water can still remain inside the boundary, but only in the form of filaments outside the coherent core. For nonoverlapping SSH eddies, more water leakage (about 60%) occurs at a faster rate. Guided by the number and radius of SSH eddies, fixed circles and moving circles are randomly selected to diagnose the material flux around these circles. We find that the leakage and intrusion trends of moving circles are quite similar to that of nonoverlapping SSH eddies, suggesting that the material coherence properties of nonoverlapping SSH eddies are not significantly different from random pieces of ocean with the same size
Ocean Eddy Identification and Tracking using Neural Networks
Global climate change plays an essential role in our daily life. Mesoscale
ocean eddies have a significant impact on global warming, since they affect the
ocean dynamics, the energy as well as the mass transports of ocean circulation.
From satellite altimetry we can derive high-resolution, global maps containing
ocean signals with dominating coherent eddy structures. The aim of this study
is the development and evaluation of a deep-learning based approach for the
analysis of eddies. In detail, we develop an eddy identification and tracking
framework with two different approaches that are mainly based on feature
learning with convolutional neural networks. Furthermore, state-of-the-art
image processing tools and object tracking methods are used to support the eddy
tracking. In contrast to previous methods, our framework is able to learn a
representation of the data in which eddies can be detected and tracked in more
objective and robust way. We show the detection and tracking results on sea
level anomalies (SLA) data from the area of Australia and the East Australia
current, and compare our two eddy detection and tracking approaches to identify
the most robust and objective method.Comment: accepted for International Geoscience and Remote Sensing Symposium
201
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
Jets and Topography: Jet Transitions and the Impact on Transport in the Antarctic Circumpolar Current
The Southern Ocean’s Antarctic Circumpolar Current (ACC) naturally lends itself to interpretations using a zonally averaged framework. Yet, navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models. In this study, both observational and numerical results indicate that zonal asymmetries, in the form of topography, impact global flow structure and transport properties.
The conclusions are based on a suite of more than 1.5 million virtual drifter trajectories advected using a satellite altimetry–derived surface velocity field spanning 17 years. The focus is on sites of “cross front” transport as defined by movement across selected sea surface height contours that correspond to jets along most of the ACC. Cross-front exchange is localized in the lee of bathymetric features with more than 75% of crossing events occurring in regions corresponding to only 20% of the ACC’s zonal extent.
These observations motivate a series of numerical experiments using a two-layer quasigeostrophic model with simple, zonally asymmetric topography, which often produces transitions in the front structure along the channel. Significantly, regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic. Jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets. Eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering. The combination of high eddy kinetic energy and recirculation features enhances particle exchange. These results stress the complications in developing consistent circumpolar definitions of the ACC fronts
Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin
Transport and mixing properties of surface currents can be detected from
altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with
Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have
the advantage of exploiting both spatial and temporal variability of the
velocity field and are in principle able to unveil subgrid filaments generated
by chaotic stirring. However, one may wonder whether this theoretical advantage
is of practical interest in real-data, mesoscale and submesoscale analysis,
because of the uncertainties and resolution of altimetric products, and the
non-passive nature of biogeochemical tracers. Here we compare the ability of
standard Eulerian diagnostics and the finite-size Lyapunov exponent in
detecting instantaneaous and climatological transport and mixing properties. By
comparing with sea-surface temperature patterns, we find that the two
diagnostics provide similar results for slowly evolving eddies like the first
Alboran gyre. However, the Lyapunov exponent is also able to predict the
(sub-)mesoscale filamentary process occuring along the Algerian current and
above the Balearic Abyssal Plain. Such filaments are also observed, with some
mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov
exponents do not show any compact relation with other Eulerian diagnostics,
unveiling a different structure even at the basin scale. We conclude that
filamentation dynamics can be detected by reprocessing available altimetric
data with Lagrangian tools, giving insight into (sub-)mesoscale stirring
processes relevant to tracer observations and complementing traditional
Eulerian diagnostics
Global tropospheric chemistry: Chemical fluexes in the global atmosphere
In October 1987, NSF, NASA, and NOAA jointly sponsored a workshop at Columbia University to assess the experimental tools and analysis procedures in use and under development to measure and understand gas and particle fluxes across this critical air-surface boundary. Results are presented for that workshop. It is published to summarize the present understanding of the various measurement techniques that are available, identify promising new technological developments for improved measurements, and stimulate thinking about this important measurement challenge
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