714 research outputs found
Measurements of near-surface turbulence and mixing from autonomous ocean gliders
Author Posting. © The Oceanography Society, 2017. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 30, no. 2 (2017): 116â125, doi:10.5670/oceanog.2017.231.As autonomous sampling technologies have matured, ocean sensing concepts with long histories have migrated from their traditional ship-based roots to new platforms. Here, we discuss the case of ocean microstructure sensing, which provides the basis for direct measurement of small-scale turbulence processes that lead to mixing and buoyancy flux. Due to their hydrodynamic design, gliders are an optimal platform for microstructure sensing. A buoyancy-driven glider can profile through the ocean with minimal vibrational noise, a common limitation of turbulence measurements from other platforms. Moreover, gliders collect uncontaminated data during both descents and ascents, permitting collection of near-surface measurements unattainable from ship-based sensing. Persistence and the capability to sample in sea states not feasible for deck-based operations make glider-based microstructure sampling a profoundly valuable innovation. Data from two recent studies illustrate the novel aspects of glider-based turbulence sensing. Surface stable layers, characteristic of conditions with incoming solar radiation and weak winds, exemplify a phenomenon not easily sampled with ship-based methods. In the North Atlantic, dissipation rate measurements in these layers revealed unexpected turbulent mixing during times of peak warming, when enhanced stratification in a thin layer led to an internal wave mode that received energy from the deeper internal wave field of the thermocline. Hundreds of profiles were obtained in the Bay of Bengal through a barrier layer that separates a strongly turbulent surface layer from a surprisingly quiescent interior just 20 m below. These studies demonstrate the utility of buoyancy-driven gliders for collecting oceanic turbulence measurements.We thank the US Office of Naval Research (ONR)
for supporting the development of autonomous
glider systems and the integration effort to incorporate
microstructure sensing. The National Science
Foundation supported the SPURS microstructure
glider effort. ONR supported for the glider program
in the Bay of Bengal
Turbulence observations in a buoyant hydrothermal plume on the East Pacific Rise
Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 180â181, doi:10.5670/oceanog.2012.15.Hot vent fluid enters the ocean at high-temperature hydrothermal vents, also known as black smokers. Because of the large temperature difference between the vent fluid and oceanic near-bottom waters, the hydrothermal effluent initially rises as a buoyant plume through the water column. During its rise, the plume engulfs and mixes with background ocean water. This process, called entrainment, gradually reduces the density of the rising plume until it reaches its level of neutral buoyancy, where the plume density equals that of the background water, and it begins to spread along a surface of constant density.The data presented here were collected
in the context of National Science
Foundation grants OCE-0425361 and
OCE-0728766
Contamination of Finescale Strain Estimates of Turbulent Kinetic Energy Dissipation by Frontal Physics
Finescale strain parameterization (FSP) of turbulent kinetic energy dissipation rate has become a widely used method for observing ocean mixing, solving a coverage problem where direct turbulence measurements are absent but CTD profiles are available. This method can offer significant value, but there are limitations in its broad application to the global ocean. FSP often fails to produce reliable results in frontal zones where temperatureâsalinity (T/S) intrusive features contaminate the CTD strain spectrum, as well as where the aspect ratio of the internal wave spectrum is known to vary greatly with depth, as frequently occurs in the Southern Ocean. In this study we use direct turbulence measurements from Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) and glider microstructure measurements from Autonomous Sampling of Southern Ocean Mixing (AUSSOM) to show that FSP can have large biases (compared to direct turbulence measurement) below the mixed layer when physics associated with T/S fronts are meaningfully present. We propose that the FSP methodology be modified to 1) include a density ratio (RÏ)-based data exclusion rule to avoid contamination by double diffusive instabilities in frontal zones such as the Antarctic Circumpolar Current, the Gulf Stream, and the Kuroshio, and 2) conduct (or leverage available) microstructure measurements of the depth-varying shear-to-strain ratio RÏ(z) prior to performing FSP in each dynamically unique region of the global ocean
An introduction to the special issue on internal waves
Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, No. 2 (2012):15-19, doi:10.5670/oceanog.2012.37.This special issue of Oceanography presents a survey of recent work on internal waves in the ocean. The undersea analogue to the surface waves we see breaking on beaches, internal waves play an important role in transferring heat, energy, and momentum in the ocean. When they break, the turbulence they produce is a vital aspect of the ocean's meridional overturning circulation. Numerical circulation models must parameterize internal waves and their breaking because computers will likely never be powerful enough to simultaneously resolve climate and internal wave scales. The demonstrated sensitivity of these models to the magnitude and distribution of internal wave-driven mixing is the primary motivation for the study of oceanic internal waves. Because internal waves can travel far from their source regions to where they break, progress requires understanding not only their generation but also their propagation through the eddying ocean and the processes that eventually lead to their breaking. Additionally, in certain regions such as near coasts and near strong generation regions, internal waves can develop into sharp fronts wherein the thermocline dramatically shoals hundreds of meters in only a few minutes. These "nonlinear" internal waves can have horizontal currents of several knots (1 knot is roughly 2 meters per second), and are strong enough to significantly affect surface navigation of vessels. Vertical current anomalies often reach one knot as well, posing issues for subsurface navigation and engineering structures associated with offshore energy development. Finally, the upwelling and turbulent mixing supported by internal waves can be vital for transporting nutrient-rich fluid into coastal ecosystems such as coral reefs
Diapycnal advection by double diffusion and turbulence in the ocean
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999Observations of diapycnal mixing rates are examined and related to diapycnal
advection for both double-diffusive and turbulent regimes.
The role of double-diffusive mixing at the site of the North Atlantic Tracer Release
Experiment is considered. The strength of salt-finger mixing is analyzed in terms
of the stability parameters for shear and double-diffusive convection, and a nondimensional ratio of the thermal and energy dissipation rates. While the model for
turbulence describes most dissipation occurring in high shear, dissipation in low
shear is better described by the salt-finger model, and a method for estimating
the salt-finger enhancement of the diapycnal haline diffusivity over the thermal
diffusivity is proposed. Best agreement between tracer-inferred mixing rates and
microstructure based estimates is achieved when the salt-finger enhancement of
haline flux is taken into account.
The role of turbulence occurring above rough bathymetry in the abyssal Brazil
Basin is also considered. The mixing levels along sloping bathymetry exceed the
levels observed on ridge crests and canyon floors. Additionally, mixing levels
modulate in phase with the spring-neap tidal cycle. A model of the dissipation
rate is derived and used to specify the turbulent mixing rate and constrain the
diapycnal advection in an inverse model for the steady circulation. The inverse
model solution reveals the presence of a secondary circulation with zonal character.
These results suggest that mixing in abyssal canyons plays an important role in
the mass budget of Antarctic Bottom Water.This work was supported by contracts N00014-92-1323 and N00014-97-10087
of the Office of Naval Research and grant OCE94-15589 of the National Science
Foundation
Les aménagements gagnants d'une CLAAC : ce qu'en disent les étudiants
Comprend des références bibliographiques
Les conditions d'efficacité des classes d'apprentissage actif
PA-2013-012La prĂ©sente recherche a Ă©tĂ© subventionnĂ©e par le ministĂšre de lâĂducation et de lâEnseignement supĂ©rieur dans le cadre du Programme dâaide Ă la recherche sur lâenseignement et lâapprentissage (PAREA).Comprend des rĂ©fĂ©rences bibliographiques
Connections between ocean bottom topography and Earthâs climate
Author Posting. © Oceanography Society, 2004. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 17, 1 (2004): 65-74.The seafloor is one of the critical controls on the
oceanâs general circulation. Its influence comes through
a variety of mechanisms including the contribution of
mixing in the oceanâs interior through the generation of
internal waves created by currents flowing over rough
topography. The influence of topographic roughness on
the oceanâs general circulation occurs through a series
of connected processes. First, internal waves are generated
by currents and tides flowing over topographic
features in the presence of stratification. Some portion
of these waves is sufficiently nonlinear that they immediately
break creating locally enhanced vertical mixing.
The majority of the internal waves radiate away from
the source regions, and likely contribute to the background
mixing observed in the ocean interior. The
enhancement of vertical mixing over regions of rough
topography has important implications for the abyssal
stratification and circulation. These in turn have implications
for the storage and transport of energy in the climate
system, and ultimately the response of the climate
system to natural and anthropogenic forcing. Finally,
mixing of the stratified ocean leads to changes in sea
level; these changes need to be considered when predicting
future sea level.SRJ
was supported by the National Science Foundation
under grant OCE-0241061 and an Office of Naval
Research Young Investigator Award, LCS was supported
by the Office of Naval Research under grant
N00014-03-1-0307, and STG was supported by the
National Science Foundation under grant OCE-
9985203/OCE-0049066 and by the National
Aeronautics and Space Administration under JPL contract
1224031
Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge
Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04â0.10-Sv (1 Sv ⥠106 m3 sâ1) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy Δ increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides, Δ varies from O(10â9) to O(10â10) W kgâ1 below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of Δ. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves
Efficient sampling of spreading processes on complex networks using a composition and rejection algorithm
Efficient stochastic simulation algorithms are of paramount importance to the
study of spreading phenomena on complex networks. Using insights and analytical
results from network science, we discuss how the structure of contacts affects
the efficiency of current algorithms. We show that algorithms believed to
require or even operations per
update---where is the number of nodes---display instead a polynomial
scaling for networks that are either dense or sparse and heterogeneous. This
significantly affects the required computation time for simulations on large
networks. To circumvent the issue, we propose a node-based method combined with
a composition and rejection algorithm, a sampling scheme that has an
average-case complexity of per update for general
networks. This systematic approach is first set-up for Markovian dynamics, but
can also be adapted to a number of non-Markovian processes and can enhance
considerably the study of a wide range of dynamics on networks.Comment: 12 pages, 7 figure
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