[Show abstract][Hide abstract] ABSTRACT: Global ship-based programs, with highly accurate, full water column physical and biogeochemical observations repeated decadally since the 1970s, provide a crucial resource for documenting ocean change. The ocean, a central component of Earth's climate system, is taking up most of Earth's excess anthropogenic heat, with about 19% of this excess in the abyssal ocean beneath 2,000 m, dominated by Southern Ocean warming. The ocean also has taken up about 27% of anthropogenic carbon, resulting in acidification of the upper ocean. Increased stratification has resulted in a decline in oxygen and increase in nutrients in the Northern Hemisphere thermocline and an expansion of tropical oxygen minimum zones. Southern Hemisphere thermocline oxygen increased in the 2000s owing to stronger wind forcing and ventilation. The most recent decade of global hydrography has mapped dissolved organic carbon, a large, bioactive reservoir, for the first time and quantified its contribution to export production (∼20%) and deep-ocean oxygen utilization. Ship-based measurements also show that vertical diffusivity increases from a minimum in the thermocline to a maximum within the bottom 1,500 m, shifting our physical paradigm of the ocean's overturning circulation. Expected final online publication date for the Annual Review of Marine Science Volume 8 is January 03, 2016. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
[Show abstract][Hide abstract] ABSTRACT: Oceanic internal waves are closely linked to turbulence. Here, a relationship between vertical wavenumber (kz) spectra of finescale vertical kinetic energy (VKE) and turbulent dissipation ε is presented using more than 250 joint profiles from five diverse dynamic regimes, spanning latitudes between the equator and 60°. In the majority of the spectra VKE varies as . Scaling VKE with collapses the off-equatorial spectra to within , but underestimates the equatorial spectrum. The simple empirical relationship between VKE and ε fit the data better than a common shear-and-strain finescale parameterization, which significantly underestimates ε in the two data sets that are least consistent with the Garrett-Munk (GM) model. The new relationship between finescale VKE and dissipation rate can be interpreted as an alternative, single-parameter scaling for turbulent dissipation in terms of finescale internal-wave vertical velocity that requires no reference to the GM model spectrum.
[Show abstract][Hide abstract] ABSTRACT: The impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows.
Geophysical Research Letters 05/2015; 42(9):n/a-n/a. DOI:10.1002/2015GL063216 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A publicly available implementation of the shear method for processing horizontal velocities acquired with Lowered Acoustic Doppler Current Profiler (LADCP) systems has been applied to measurements obtained with TRDI Explorer ADCPs mounted on Webb Slocum gliders, in order to determine whether LADCP methods are suitable for processing glider-ADCP data. Quality of the resulting profiles was assessed primarily by comparison with simultaneous velocity profiles collected near the gliders using a vessel-mounted (vmADCP) system. Due to the way the Explorer ADCPs are installed on the gliders, the ADCP data from the upward portions of the dives are degraded. As a consequence, the standard LADCP method of using surface GPS fixes and time-series measurements of horizontal flow past the instrument cannot be applied to reference the baroclinic velocity profiles calculated with the shear method. For dives within range of the seabed, bottom-tracking (BT) can be used to reference the velocities instead, which implies velocity errors increasing with increasing distance from the seabed. During a short test cruise, a set of comparatively shallow glider dive profiles were collected during two primary groups of missions off the California coast. During the so-called A-mission dives, gliderattitude measurements, updated every 4 s, were used to transform the ADCP beam to earth coordinates, resulting in rms velocitycomponent differences between glider-and vmADCP-derived velocities of approximately 6cm/s. During the B-mission dives, attitude data from a external sensor were used instead, resulting in reduced rms velocity-component differences of 3-4 cm·s-1, similar to what is achievable for typical deep-water LADCP profiles. These tests indicate that useful absolute velocity profiles can be obtained with Explorer ADCPs mounted on Slocum gliders, as long as i) a suitable external attitude measurement system is used to transform the ADCP measurements into earth coordinates, and ii) the dives are deep enough to measure the glider motion over ground using bottom tracking.Without BT data, the ADCP-glider system can still provide useful measurements of vertical shear of horizontal velocity.
[Show abstract][Hide abstract] ABSTRACT: Population connectivity refers to the exchange of individuals among populations: it affects gene flow, regulates population size and function, and mitigates recovery from natural or anthropogenic disturbances. Many populations in the deep sea are spatially fragmented, and will become more so with increasing resource exploitation. Understanding population connectivity is critical for spatial management. For most benthic species, connectivity is achieved by the planktonic larval stage, and larval dispersal is, in turn, regulated by complex interactions between biological and oceanographic processes. Coupled biophysical models, incorporating ocean circulation and biological traits, such as planktonic larval duration (PLD), have been used to estimate population connectivity and generate spatial management plans in coastal and shallow waters. In the deep sea, knowledge gaps in both the physical and biological components are delaying the effective use of this approach. Here, we review the current efforts in conservation in the deep sea and evaluate (1) the relevance of using larval dispersal in the design of marine protected areas and (2) the application of biophysical models in the study of population connectivity. Within biophysical models, PLD can be used to estimate dispersal distance. We propose that a PLD that guarantees a minimum dispersal distance for a wide range of species should be used in the planning of marine protected areas in the deep sea. Based on a review of data on species found at depths >200 m, a PLD of 35 and 69 days ensures a minimum distance for 50 and 75%, respectively, of eurybathic and deep-sea species. We note that more data are required to enhance accuracy and address the high variability in PLD between and within taxonomic groups, limiting generalizations that are often appealing to decision-makers. Given the imminent expansion of resource exploitation in the deep sea, data relevant to spatial management are needed urgently.
[Show abstract][Hide abstract] ABSTRACT: Submarine hydrothermal vents perturb the deep-ocean microbiome by injecting reduced chemical species into the water column that act as an energy source for chemosynthetic organisms. These systems thus provide excellent natural laboratories for studying the response of microbial communities to shifts in marine geochemistry. The present study explores the processes that regulate coupled microbial-geochemical dynamics in hydrothermal plumes by means of a novel mathematical model, which combines thermodynamics, growth and reaction kinetics, and transport processes derived from a fluid dynamics model. Simulations of a plume located in the ABE vent field of the Lau basin were able to reproduce metagenomic observations well and demonstrated that the magnitude of primary production and rate of autotrophic growth are largely regulated by the energetics of metabolisms and the availability of electron donors, as opposed to kinetic parameters. Ambient seawater was the dominant source of microbes to the plume and sulphur oxidisers constituted almost 90% of the modelled community in the neutrally-buoyant plume. Data from drifters deployed in the region allowed the different time scales of metabolisms to be cast in a spatial context, which demonstrated spatial succession in the microbial community. While growth was shown to occur over distances of tens of kilometers, microbes persisted over hundreds of kilometers. Given that high-temperature hydrothermal systems are found less than 100 km apart on average, plumes may act as important vectors between different vent fields and other environments that are hospitable to similar organisms, such as oil spills and oxygen minimum zones.The ISME Journal advance online publication, 6 February 2015; doi:10.1038/ismej.2015.4.
The ISME Journal 02/2015; 9(8). DOI:10.1038/ismej.2015.4 · 9.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vertical mixing caused by breaking of internal tides plays a major role in maintaining the deep-ocean stratification. This study compares observations of dissipation from microstructure measurements to calculations of the vertical energy flux from barotropic to internal tides, taking into account the temporal variation due to the spring-neap tidal cycle. The dissipation data originate from two surveys in the Brazil Basin Tracer Release Experiment (BBTRE), and one over the LArval Dispersal along the Deep East Pacific Rise (LADDER3), supplemented with a few stations above the North-Atlantic Ridge (GRAVILUCK) and in the western Pacific (IZU). A good correlation is found between logarithmic values of energy flux and local dissipation in BBTRE, suggesting that the theory is able to predict energy fluxes. For the LADDER3, the local dissipation is much smaller than the calculated energy flux, which is very likely due to the different topographic features of BBTRE and LADDER3. The East Pacific Rise consists of a few isolated seamounts, so that most of the internal wave energy can radiate away from the generation site, whereas the Brazil Basin is characterised by extended rough bathymetry, leading to a more local dissipation. The results from all four field surveys support the general conclusion that the fraction of the internal-tide energy flux that is dissipated locally is very different in different regions.Keywords: Internal tides, microstructure measurement, vertical mixing(Published: 3 October 2014)Citation: Tellus A 2014, 66, 23240, http://dx.doi.org/10.3402/tellusa.v66.23240
[Show abstract][Hide abstract] ABSTRACT: The Southern Ocean plays a pivotal role in the global ocean
circulation and climate1–3. There, the deep water masses of
the world ocean upwell to the surface and subsequently sink
to intermediate and abyssal depths, forming two overturning
cells that exchange substantial quantities of heat and carbon
with the atmosphere4,5. The sensitivity of the upper cell to
climatic changes in forcing is relatively well established6.
However, little is known about how the lower cell responds,
and in particular whether small-scale mixing in the abyssal
Southern Ocean, an important controlling process of the lower
cell7,8, is influenced by atmospheric forcing. Here, we present
observational evidence that relates changes in abyssal mixing
to oceanic eddy variability on timescales of months to decades.
Observational estimates of mixing rates, obtained along a
repeat hydrographic transect across Drake Passage, are shown
to be dependent on local oceanic eddy energy, derived from
moored current meter and altimetric measurements. As the
intensity of the regional eddy field is regulated by the Southern
Hemisphere westerly winds9,10, our findings suggest that
Southern Ocean abyssal mixing and overturning are sensitive
to climatic perturbations in wind forcing.
[Show abstract][Hide abstract] ABSTRACT: sectors of the Antarctic ice sheet are vulnerable to increases in melting at the bases of fringing ice shelves, with melt rates depending on ocean temperatures and circulations in the sub-ice cavities. Here we analyze an oceanographic data set obtained in austral summer 2009 in Pine Island Bay, which is bounded in the east by the calving front of the Amundsen Sea's fast-moving Pine Island Ice Shelf. The upper-ocean velocity field in the ice-free bay was dominated by a 700 m deep and 50 km wide gyre circulating 1.5 Sv of water clockwise around the bay. Ice cavity water was observed in a surface-intensified and southward-intensified boundary current along the ice front, and in a small ice cove at the end of the southern shear margin of the ice shelf. Repeat measurements in the cove reveal persistent cavity water export of ≈ 0.25 Sv during 10 days of sampling. Vertical velocities in the cove above the ice draft were dominated by buoyancy-frequency oscillations with amplitudes of several cm/s but without significant net upwelling. In combination with the seawater properties, this observation indicates that much of the upwelling occurs within fractured ice near the cove, potentially contributing to weakening the ice shelf shear margin.
[Show abstract][Hide abstract] ABSTRACT: The fate of a deep boundary current that originates in the Southeast Pacific and flows southward along the continental slope of South America is elucidated. The current transports poorly ventilated water of low salinity (a type of Pacific Deep Water, PDW), into Drake Passage. East of Drake Passage, the boundary current breaks into fresh anticyclonic eddies, nine examples of which were observed in mooring data from December 2009 to March 2012. The observed eddies appear to originate mainly from a topographic separation point close to 60°W, have typical diameters of 20–60 km and accompanying Rossby numbers of 0.1–0.3. These features are likely to be responsible for transporting PDW meridionally across the Antarctic Circumpolar Current, explaining the near homogenization of Circumpolar Deep Water properties downstream of Drake Passage. This mechanism of boundary current breakdown may constitute an important process in the Southern Ocean overturning circulation. Mid‐depth eddies observed east of Drake PassageEddies contain fresh Pacific Deep WaterEddies induce transport of water properties across the ACC
Geophysical Research Letters 01/2014; 41(1). DOI:10.1002/2013GL058617 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or by active swimming. Alternatively, they may be retained near the bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50′N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75 m above the bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near the bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000 m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley—which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.
Deep Sea Research Part II Topical Studies in Oceanography 08/2013; 92:46–57. DOI:10.1016/j.dsr2.2013.03.032 · 2.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  The spatial distribution of turbulent dissipation rates and internal wavefield characteristics is analyzed across two contrasting regimes of the Antarctic Circumpolar Current (ACC), using microstructure and finestructure data collected as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Mid-depth turbulent dissipation rates are found to increase from in the Southeast Pacific to in the Scotia Sea, typically reaching within a kilometer of the seabed. Enhanced levels of turbulent mixing are associated with strong near-bottom flows, rough topography, and regions where the internal wavefield is found to have enhanced energy, a less-inertial frequency content and a dominance of upward propagating energy. These results strongly suggest that bottom-generated internal waves play a major role in determining the spatial distribution of turbulent dissipation in the ACC. The energy flux associated with the bottom internal wave generation process is calculated using wave radiation theory, and found to vary between 0.8 mW m−2 in the Southeast Pacific and 14 mW m−2 in the Scotia Sea. Typically, 10%–30% of this energy is found to dissipate within 1 km of the seabed. Comparison between turbulent dissipation rates inferred from finestructure parameterizations and microstructure-derived estimates suggests a significant departure from wave-wave interaction physics in the near-field of wave generation sites.
[Show abstract][Hide abstract] ABSTRACT: Direct measurements of turbulence levels in the Drake Passage region of the Southern Ocean show a marked enhancement over the Phoenix Ridge. At this site, the Antarctic Circumpolar Current (ACC) isconstricted in its flow between the southern tip of South America and the northern tip of the Antarctic Peninsula. Observed turbulent kinetic energy dissipation rates are enhanced in the regions corresponding to the ACC frontal zones where strong flow reaches the bottom. In these areas, turbulent dissipation levels reach 10-8 W kg-1 at abyssal and middepths. The mixing enhancementin the frontal regions is sufficient to elevate the diapycnal turbulent diffusivity acting in the deep water above the axis of the ridge to 1×10-4 m2 s-1. This level is an order of magnitude larger than the mixing levels observed upstream in the ACC above smootherbathymetry. Outside of the frontal regions, dissipation rates are O(10-10) W kg-1, comparable to the background levels of turbulence found throughout most mid- and low-latitude regions of the global ocean.
Journal of Physical Oceanography 08/2012; in press(12):120803124702009. DOI:10.1175/JPO-D-12-027.1 · 2.86 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Deep-sea hydrothermal plume particles are known to sequester seawater trace elements and influence ocean-scale biogeochemical budgets. The relative importance of biotic versus abiotic oxidation–reduction and other particle-forming reaction, however, and the mechanisms of seawater trace element sequestration remain unknown. Suspended particulate material was collected from a non-buoyant hydrothermal plume by in situ filtration at 9°50′N East Pacific Rise during a 3-day, 24 sample, time-series. Twenty-three samples were digested for total elemental analysis. One representative sample was selected for particle-by-particle geochemical analyses including elemental composition by X-ray fluorescence, speciation of Fe, S, and C by 1s X-ray absorption near edge structure spectroscopy, and X-ray diffraction. Consistent with past studies, positive linear correlations were observed for P, V, As, and Cr with Fe in the bulk chemistry. Arsenic was associated with both Fe oxyhydroxides and sulfides but not uniformly distributed among either mineral type. Particle aggregation was common. Aggregates were composed of minerals embedded in an organic matrix; the minerals ranged from <20 nm to >10 μm in diameter. The speciation of major mineral forming elements (Fe, Mn, S) was complex. Over 20 different minerals were observed, nine of which were either unpredicted by thermodynamic modeling or had no close match in the thermodynamic database. Sulfur-bearing phases consisted of polysulfides (S6, S8), and metal sulfides (Fe, Cu, Zn, Mn). Four dominant species, Fe oxyhydroxide, Fe monosulfide, pyrrhotite, and pyrite, accounted for >80% of the Fe present. Particulate Mn was prevalent in both oxidized and reduced minerals. The organic matrix was: (1) always associated with minerals, (2) composed of biomolecules, and (3) rich in S. Possible sources of this S-rich organic matter include entrained near vent biomass and in situ production by S-oxidizing microorganisms. These results indicate that particle aggregation with organic material is prevalent in dispersing hydrothermal plume fluxes, as well as in sinking particulate matter at this site. Particle aggregation and organic coatings can influence the reactivity, transport, and residence time of hydrothermal particles in the water column. Thus a biogeochemical approach is critical to understanding the net effect of hydrothermal fluxes on ocean and sedimentary trace element budgets.
[Show abstract][Hide abstract] ABSTRACT: AbstractMesoscale eddies are ubiquitous in the World Ocean and dominate the energy content on subinertial time scales. Recent theoretical and numerical studies suggest a connection between mesoscale eddies and diapycnal mixing in the deep ocean, especially near rough topography in regions of strong geostrophic flow. However, unambiguous observational evidence for such a connection has not yet been found, and it is still unclear what physical processes are responsible for transferring energy from mesoscale to small-scale processes. Here, the authors present observations demonstrating that finescale variability near the crest of the East Pacific Rise is strongly modulated by low-frequency geostrophic flows, including those due to mesoscale eddies. During times of strong subinertial flows, the authors observed elevated kinetic energy on vertical scales <50 m and in the near-inertial band, predominantly upward-propagating near-inertial waves, and increased incidence of layers with Richardson number . In contr...
[Show abstract][Hide abstract] ABSTRACT: In a paper published in 2002 in this journal, K. Polzin et al. derive corrections for spectra of vertical shear calculated from lowered acoustic Doppler current profiler (LADCP) velocity data. To illustrate and validate the corrections, they use velocities derived with a specific implementation of the shear method for LADCP processing that is no longer supported or widely used. In several recent publications, spectral corrections specific to this old processing method have been applied without modification to LADCP data processed with the more modern and much more widely used velocity-inversion method, which is associated with significantly less damping at high vertical wavenumbers than the older method. The purpose of this work is to derive and validate spectral corrections appropriate for different LADCP processing methods.
Journal of Atmospheric and Oceanic Technology 04/2012; 29(4):597-600. DOI:10.1175/JTECH-D-11-00158.1 · 1.73 Impact Factor