Explore the vast range of titles available.

The density stratification in the ocean is directly related to the diapycnal mixing, which drives the abyssal cell of the Meridional Overturning Circulation (MOC). It is important to understand how stratification has been changing in the world's deep and abyssal oceans under climate change. Using repeat hydrographic data obtained since the 1990s, we find a decreasing stratification associated with changes in the source Antarctica Bottom Water (AABW) properties in its formation basins as well as in basins along its dispersal pathways. Averaged south of 60°S, the squared buoyancy frequency N2 shows a negative trend of −6% per decade in waters deeper than 4,000 m. The observed decadal reduction in stratification is associated with large spatial variability, especially in the Southern Ocean basins with multiple AABW sources. Additionally, there are also significant differences between neighboring basins that are related to the blocking effect of topography. Plain Language Summary Cold and dense Antarctica Bottom Water (AABW) forms near the coastline of Antarctica, sinks to the ocean bottom, moves northward in deep branches of the Meridional Overturning Circulation, becomes lighter through mixing, and eventually upwells to shallower depths. The AABW properties change under the changing climate. Repeat hydrography sections occupied approximately once per decade since the 1990s have revealed continuously warming and freshening of the AABW. In many regions of the ocean, the warming and freshening trends are strongest at the bottom. This causes a decrease in the vertical gradient of density, namely stratification. As the level of mixing is strongly related to stratification, it is important to quantify its trend and understand its spatial structure. We quantify the trend in stratification globally and find a reduction in stratification in the Southern Ocean and along the spreading pathways of the AABW at lower latitudes. Key Points Three decades of repeat hydrographic sections reveal decreasing density stratification in the deep and abyssal layers of the Southern Ocean South of 60°S, N2 has reduced by 6% per decade in the ocean below 4,000 m, with a peak reduction rate of 15% per decade at 4,800 m The decreasing stratification is found along the pathway of Antarctica Bottom Water and its vertical structure depends on the topography of basin boundaries

Mesoscale 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 contrast, during times of weak subinertial flows, kinetic energy in the finescale and near-inertial bands is lower, Ri values are higher, and near-inertial waves propagate predominantly downward through the water column. Diapycnal diffusivities estimated indirectly from a simple Ri-based parameterization are consistent with results from a tracer-release experiment and a microstructure survey bracketing the mooring measurements. These observations are consistent with energy transfer (a “cascade”) from subinertial flows, including mesoscale eddies, to near-inertial oscillations, turbulence, and mixing. This interpretation suggests that, in addition to topographic roughness and tidal forcing, parameterization of deep-ocean mixing should also take subinertial flows into account. The findings presented here are expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean.

Internal wave–wave interaction theories and observations support a parameterization for the turbulent dissipation rate ɛ and eddy diffusivity K that depends on internal wave shear 〈Vz2〉 and strain 〈ξz2〉 variances. Its latest incarnation is applied to about 3500 lowered ADCP/CTD profiles from the Indian, Pacific, North Atlantic, and Southern Oceans. Inferred diffusivities K are functions of latitude and depth, ranging from 0.03 × 10−4 m2 s−1 within 2° of the equator to (0.4–0.5) × 10−4 m2 s−1 at 50°–70°. Diffusivities K also increase with depth in tropical and subtropical waters. Diffusivities below 4500-m depth exhibit a peak of 0.7 × 10−4 m2 s−1 between 20° and 30°, latitudes where semidiurnal parametric subharmonic instability is expected to be active. Turbulence is highly heterogeneous. Though the bulk of the vertically integrated dissipation ∫ɛ is contributed from the main pycnocline, hotspots in ∫ɛ show some correlation with small-scale bottom roughness and near-bottom flow at sites where strong surface tidal dissipation resulting from tide–topography interactions has been implicated. Average vertically integrated dissipation rates are 1.0 mW m−2, lying closer to the 0.8 mW m−2 expected for a canonical (Garrett and Munk) internal wave spectrum than the global-averaged deep-ocean surface tide loss of 3.3 mW m−2.

Atmospheric forcing, which is known to have a strong influence on surface ocean dynamics and production, is typically not considred in studies of the deep sea. Our observations and models demonstrate an unexpected influence of surface-generated mesoscale eddies in the transport of hydrothermal vent efflux and of vent larvae away from the northern East Pacific Rise. Transport by these deep-reaching eddies provides a mechanism for spreading the hydrothermal chemical and heat flux into the deep-ocean interior and for dispersing propagules hundreds of kilometers between isolated and ephemeral communities. Because the eddies interacting with the East Pacific Rise are formed seasonally and are sensitive to phenomena such as El Niño, they have the potential to introduce seasonal to interannual atmospheric variations into the deep sea.

Species inhabiting deep-sea hydrothermal vents are strongly influenced by the geological setting, as it provides the chemical-rich fluids supporting the food web, creates the patchwork of seafloor habitat, and generates catastrophic disturbances that can eradicate entire communities. The patches of vent habitat host a network of communities (a metacommunity) connected by dispersal of planktonic larvae. The dynamics of the metacommunity are influenced not only by birth rates, death rates and interactions of populations at the local site, but also by regional influences on dispersal from different sites. The connections to other communities provide a mechanism for dynamics at a local site to affect features of the regional biota. In this paper, we explore the challenges and potential benefits of applying metacommunity theory to vent communities, with a particular focus on effects of disturbance. We synthesize field observations to inform models and identify data gaps that need to be addressed to answer key questions including: 1) what is the influence of the magnitude and rate of disturbance on ecological attributes such as time to extinction or resilience in a metacommunity; 2) what interactions between local and regional processes control species diversity, and 3) which communities are 'hot spots' of key ecological significance. We conclude by assessing our ability to evaluate resilience of vent metacommunities to human disturbance (e.g., deep-sea mining). Although the resilience of a few highly disturbed vent systems in the eastern Pacific has been quantified, these values cannot be generalized to remote locales in the western Pacific or mid Atlantic where disturbance rates are different and information on local controls is missing.

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.

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.

Over mid-ocean ridges, the interaction between the currents and the topography gives rise to complex flows, which drive the transport properties of biogeochemical constituents, and especially those associated with hydrothermal vents, thus impacting associated ecosystems. This paper describes the circulation in the rift valley along the Azores sector of the North Mid-Atlantic Ridge, using a combination of \\textit{in-situ} data from several surveys and realistic high-resolution modelling. It confirms the presence of a mean deep current with an up-valley branch intensified along the right inner flank of the valley (looking downstream), and a weaker down-valley branch flowing at shallower depth along the opposite flank. The hydrographic properties of the rift-valley water, and in particular the along-valley density gradient that results from a combination of the topographic isolation, the deep flow and the related mixing, are quantified. We also show that the deep currents exhibit significant variability and can be locally intense, with typical values greater than \\SI{10}{cm/s}. Finally, insights on the dynamical forcings of the deep currents and their variability are provided using numerical simulations, showing that tidal forcing of the mean circulation is important and that the overlying mesoscale turbulence triggers most of the variability.

Direct measurements of oceanic turbulent parameters were taken upstream of and across Drake Passage, in the region of the Subantarctic and Polar Fronts. Values of turbulent kinetic energy dissipation rate ε estimated by microstructure are up to two orders of magnitude lower than previously published estimates in the upper 1000 m. Turbulence levels in Drake Passage are systematically higher than values upstream, regardless of season. The dissipation of thermal variance χ is enhanced at middepth throughout the surveys, with the highest values found in northern Drake Passage, where water mass variability is the most pronounced. Using the density ratio, evidence for double-diffusive instability is presented. Subject to double-diffusive physics, the estimates of diffusivity using the Osborn–Cox method are larger than ensemble statistics based on ε and the buoyancy frequency.

Ocean ridge mixing The mixing of warm upper-ocean water with the colder water beneath is an essential component of global ocean circulation because it increases the buoyancy of deep water, but it is not clear where this mixing takes place. Louis St Laurent and Andreas Thurnherr have measured the mixing that occurs as water flows through a narrow passage on the crest of the Mid-Atlantic Ridge. The rates they observe are unusually high: if similar mixing rates occur in the other narrow passages on the Mid-Atlantic Ridge in the North Atlantic basin, these sites could account for as much buoyancy flux at the depth of the passages as the rest of the basin combined. So mixing in narrow passages on mid-ocean ridges — sites that have previously been overlooked — may make a significant contribution to buoyancy flux at the global scale. Observations from the crest of the Mid-Atlantic Ridge suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Measurements show that large diffusivities characterize the mixing downstream of a sill in a well stratified boundary layer, with mixing levels remaining of the order of 10 −4 m 2 s −1 at the base of the main thermocline. Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing 1 , 2 . This mixing is thought to result primarily from instability of the oceanic internal wave field 3 , but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence 4 and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m 2  s - 1 characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10 - 4  m 2  s - 1 at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10 - 5  m 2  s - 1 that characterize much of the global thermocline and the abyssal ocean 5 . Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone 6 , 7 , a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.