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1
Primeau, François W., et Mark Holzer. « The Ocean’s Memory of the Atmosphere : Residence-Time and Ventilation-Rate Distributions of Water Masses ». Journal of Physical Oceanography 36, no 7 (1 juillet 2006) : 1439–56. http://dx.doi.org/10.1175/jpo2919.1.
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Abstract A conceptually new approach to diagnosing tracer-independent ventilation rates is developed. Tracer Green functions are exploited to partition ventilation rates according to the ventilated fluid’s residence time in the ocean interior and according to where this fluid enters and exits the interior. In the presence of mixing by mesoscale eddies, which are reasonably represented by diffusion, ventilation rates for overlapping entry and exit regions cannot meaningfully be characterized by a single rate. It is a physical consequence of diffusive transport that fluid elements that spend an infinitesimally short time in the interior cause singularly large ventilation rates for overlapping entry and exit regions. Therefore, ventilation must generally be characterized by a ventilation-rate distribution, ϕ, partitioned according to the time that the ventilated fluid spends in the interior between successive surface contacts. An offline forward and adjoint time-averaged OGCM is used to illustrate the rich detail that ϕ and the closely related probability density function of residence times ℛ provide on the way the ocean communicates with the surface. These diagnostics quantify the relative importance of various surface regions for ventilating the interior ocean by either exposing old water masses to the atmosphere or by forming newly ventilated ones. The model results suggest that the Southern Ocean plays a dominant role in ventilating the ocean, both as a region where new waters are ventilated into the interior and where old waters are first reexposed to the atmosphere.
2
Bopp, L., L. Resplandy, A. Untersee, P. Le Mezo et M. Kageyama. « Ocean (de)oxygenation from the Last Glacial Maximum to the twenty-first century : insights from Earth System models ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 375, no 2102 (7 août 2017) : 20160323. http://dx.doi.org/10.1098/rsta.2016.0323.
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All Earth System models project a consistent decrease in the oxygen content of oceans for the coming decades because of ocean warming, reduced ventilation and increased stratification. But large uncertainties for these future projections of ocean deoxygenation remain for the subsurface tropical oceans where the major oxygen minimum zones are located. Here, we combine global warming projections, model-based estimates of natural short-term variability, as well as data and model estimates of the Last Glacial Maximum (LGM) ocean oxygenation to gain some insights into the major mechanisms of oxygenation changes across these different time scales. We show that the primary uncertainty on future ocean deoxygenation in the subsurface tropical oceans is in fact controlled by a robust compensation between decreasing oxygen saturation (O 2sat ) due to warming and decreasing apparent oxygen utilization (AOU) due to increased ventilation of the corresponding water masses. Modelled short-term natural variability in subsurface oxygen levels also reveals a compensation between O 2sat and AOU, controlled by the latter. Finally, using a model simulation of the LGM, reproducing data-based reconstructions of past ocean (de)oxygenation, we show that the deoxygenation trend of the subsurface ocean during deglaciation was controlled by a combination of warming-induced decreasing O 2sat and increasing AOU driven by a reduced ventilation of tropical subsurface waters. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
3
Naveira Garabato, Alberto C., Graeme A. MacGilchrist, Peter J. Brown, D. Gwyn Evans, Andrew J. S. Meijers et Jan D. Zika. « High-latitude ocean ventilation and its role in Earth's climate transitions ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 375, no 2102 (7 août 2017) : 20160324. http://dx.doi.org/10.1098/rsta.2016.0324.
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The processes regulating ocean ventilation at high latitudes are re-examined based on a range of observations spanning all scales of ocean circulation, from the centimetre scales of turbulence to the basin scales of gyres. It is argued that high-latitude ocean ventilation is controlled by mechanisms that differ in fundamental ways from those that set the overturning circulation. This is contrary to the assumption of broad equivalence between the two that is commonly adopted in interpreting the role of the high-latitude oceans in Earth's climate transitions. Illustrations of how recognizing this distinction may change our view of the ocean's role in the climate system are offered. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
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Katavouta, Anna, et Richard G. Williams. « Ocean carbon cycle feedbacks in CMIP6 models : contributions from different basins ». Biogeosciences 18, no 10 (27 mai 2021) : 3189–218. http://dx.doi.org/10.5194/bg-18-3189-2021.
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Abstract. The ocean response to carbon emissions involves the combined effect of an increase in atmospheric CO2, acting to enhance the ocean carbon storage, and climate change, acting to decrease the ocean carbon storage. This ocean response can be characterised in terms of a carbon–concentration feedback and a carbon–climate feedback. The contribution from different ocean basins to these feedbacks on centennial timescales is explored using diagnostics of ocean carbonate chemistry, physical ventilation and biological processes in 11 CMIP6 Earth system models. To gain mechanistic insight, the dependence of these feedbacks on the Atlantic Meridional Overturning Circulation (AMOC) is also investigated in an idealised climate model and the CMIP6 models. For the carbon–concentration feedback, the Atlantic, Pacific and Southern oceans provide comparable contributions when estimated in terms of the volume-integrated carbon storage. This large contribution from the Atlantic Ocean relative to its size is due to strong local physical ventilation and an influx of carbon transported from the Southern Ocean. The Southern Ocean has large anthropogenic carbon uptake from the atmosphere, but its contribution to the carbon storage is relatively small due to large carbon transport to the other basins. For the carbon–climate feedback estimated in terms of carbon storage, the Atlantic and Arctic oceans provide the largest contributions relative to their size. In the Atlantic, this large contribution is primarily due to climate change acting to reduce the physical ventilation. In the Arctic, this large contribution is associated with a large warming per unit volume. The Southern Ocean provides a relatively small contribution to the carbon–climate feedback, due to competition between the climate effects of a decrease in solubility and physical ventilation and an increase in accumulation of regenerated carbon. The more poorly ventilated Indo-Pacific Ocean provides a small contribution to the carbon cycle feedbacks relative to its size. In the Atlantic Ocean, the carbon cycle feedbacks strongly depend on the AMOC strength and its weakening with warming. In the Arctic, there is a moderate correlation between the AMOC weakening and the carbon–climate feedback that is related to changes in carbonate chemistry. In the Pacific, Indian and Southern oceans, there is no clear correlation between the AMOC and the carbon cycle feedbacks, suggesting that other processes control the ocean ventilation and carbon storage there.
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Sallée, Jean-Baptiste, Kevin Speer, Steve Rintoul et S. Wijffels. « Southern Ocean Thermocline Ventilation ». Journal of Physical Oceanography 40, no 3 (1 mars 2010) : 509–29. http://dx.doi.org/10.1175/2009jpo4291.1.
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Abstract An approximate mass (volume) budget in the surface layer of the Southern Ocean is used to investigate the intensity and regional variability of the ventilation process, discussed here in terms of subduction and upwelling. Ventilation resulting from Ekman pumping is estimated from satellite winds, the geostrophic mean component is assessed from a climatology strengthened with Argo data, and the eddy-induced advection is included via the parameterization of Gent and McWilliams, together with eddy mixing estimates. All three components contribute significantly to ventilation. Finally, the seasonal cycle of the upper ocean is resolved using Argo data. The circumpolar-averaged circulation shows an upwelling in the Antarctic Intermediate Water (AAIW) density classes, which is carried north into a zone of dense Subantarctic Mode Water (SAMW) subduction. Although no consistent net production is found in the light SAMW density classes, a large subduction of Subtropical Mode Water (STMW) is observed. The STMW area is fed by convergence of a southward and a northward residual meridional circulation. The eddy-induced contribution is important for the water mass transport in the vicinity of the Antartic Circumpolar Current. It balances the horizontal northward Ekman transport as well as the vertical Ekman pumping. While the circumpolar-averaged upper cell structure is consistent with the average surface fluxes, it hides strong longitudinal regional variations and does not represent any local regime. Subduction shows strong regional variability with bathymetrically constrained hotspots of large subduction. These hotspots are consistent with the interior potential vorticity structure and circulation in the thermocline. Pools of SAMW and AAIW of different densities are found along the circumpolar belt in association with the regional pattern of subduction and interior circulation.
6
Thiele, G., et J. L. Sarmiento. « Tracer dating and ocean ventilation ». Journal of Geophysical Research 95, no C6 (1990) : 9377. http://dx.doi.org/10.1029/jc095ic06p09377.
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Shepherd, John G., Peter G. Brewer, Andreas Oschlies et Andrew J. Watson. « Ocean ventilation and deoxygenation in a warming world : introduction and overview ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 375, no 2102 (7 août 2017) : 20170240. http://dx.doi.org/10.1098/rsta.2017.0240.
Texte intégralRésumé :
Changes of ocean ventilation rates and deoxygenation are two of the less obvious but important indirect impacts expected as a result of climate change on the oceans. They are expected to occur because of (i) the effects of increased stratification on ocean circulation and hence its ventilation, due to reduced upwelling, deep-water formation and turbulent mixing, (ii) reduced oxygenation through decreased oxygen solubility at higher surface temperature, and (iii) the effects of warming on biological production, respiration and remineralization. The potential socio-economic consequences of reduced oxygen levels on fisheries and ecosystems may be far-reaching and significant. At a Royal Society Discussion Meeting convened to discuss these matters, 12 oral presentations and 23 posters were presented, covering a wide range of the physical, chemical and biological aspects of the issue. Overall, it appears that there are still considerable discrepancies between the observations and model simulations of the relevant processes. Our current understanding of both the causes and consequences of reduced oxygen in the ocean, and our ability to represent them in models are therefore inadequate, and the reasons for this remain unclear. It is too early to say whether or not the socio-economic consequences are likely to be serious. However, the consequences are ecologically, biogeochemically and climatically potentially very significant, and further research on these indirect impacts of climate change via reduced ventilation and oxygenation of the oceans should be accorded a high priority. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
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Mecking, Sabine, et Kyla Drushka. « Linking northeastern North Pacific oxygen changes to upstream surface outcrop variations ». Biogeosciences 21, no 5 (7 mars 2024) : 1117–33. http://dx.doi.org/10.5194/bg-21-1117-2024.
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Abstract. Understanding the response of the ocean to global warming, including the renewal of ocean waters from the surface (ventilation), is important for future climate predictions. Oxygen distributions in the ocean thermocline have proven an effective way to infer changes in ventilation because physical processes (ventilation and circulation) that supply oxygen are thought to be primarily responsible for changes in interior oxygen concentrations. Here, the focus is on the North Pacific thermocline, where some of the world's oceans' largest oxygen variations have been observed. These variations, described as bi-decadal cycles on top of a small declining trend, are strongest on subsurface isopycnals that outcrop into the mixed layer of the northwestern North Pacific in late winter. In this study, surface density time series are reconstructed in this area using observational data only and focusing on the time period from 1982, the first full year of the satellite sea surface temperature record, to 2020. It is found that changes in the annual maximum outcrop area of the densest isopycnals outcropping in the northwestern North Pacific are correlated with interannual oxygen variability observed at Ocean Station P (OSP) downstream at about a 10-year lag. The hypothesis is that ocean ventilation and uptake of oxygen is greatly reduced when the outcrop areas are small and that this signal travels within the North Pacific Current to OSP, with 10 years being at the higher end of transit times reported in other studies. It is also found that sea surface salinity (SSS) dominates over sea surface temperature (SST) in driving interannual fluctuations in annual maximum surface density in the northwestern North Pacific, highlighting the role that salinity may play in altering ocean ventilation. In contrast, SSS and SST contribute about equally to the long-term declining surface density trends that are superimposed on the interannual cycles.
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Jones, C. S., et Ryan P. Abernathey. « Isopycnal Mixing Controls Deep Ocean Ventilation ». Geophysical Research Letters 46, no 22 (16 novembre 2019) : 13144–51. http://dx.doi.org/10.1029/2019gl085208.
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Schiffbauer, James D., et Natalia Bykova. « Paleontology : Ediacaran ecology drove ocean ventilation ». Current Biology 34, no 15 (août 2024) : R734—R736. http://dx.doi.org/10.1016/j.cub.2024.06.043.
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Li, Lingwei, Zhengyu Liu, Jinbo Du, Lingfeng Wan et Jiuyou Lu. « Mechanisms of global ocean ventilation age change during the last deglaciation ». Climate of the Past 20, no 5 (15 mai 2024) : 1161–75. http://dx.doi.org/10.5194/cp-20-1161-2024.
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Abstract. Marine radiocarbon (14C) is widely used to trace deep-ocean circulation, providing insight into the atmosphere–ocean exchange of CO2 during the last deglaciation. Evidence shows a significantly depleted Δ14C in the glacial deep ocean, suggesting an increased ventilation age at the Last Glacial Maximum (LGM). In this study, using two transient simulations with tracers of 14C and ideal age (IAGE), we found that the oldest ventilation age is not observed at the LGM. In contrast, the models show a modestly younger ventilation age during the LGM compared to the present day. The global mean ventilation ages averaged below 1 km are approximately 800 (630) years and 930 (2000) years at the LGM and in the present day, respectively, in two simulations. This younger glacial ventilation age is mainly caused by the stronger glacial Antarctic Bottom Water (AABW) transport associated with sea ice expansion. Notably, the ocean ventilation age is significantly older predominantly in the deep Pacific during deglaciation compared to the age at the LGM, with global mean ventilation ages peaking at 1900 and 2200 years around 14–12 ka in two simulations, primarily due to the weakening of AABW transport.
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Tschumi, T., F. Joos, M. Gehlen et C. Heinze. « Deep ocean ventilation, carbon isotopes, marine sedimentation and the deglacial CO<sub>2</sub> ; rise ». Climate of the Past 7, no 3 (22 juillet 2011) : 771–800. http://dx.doi.org/10.5194/cp-7-771-2011.
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Abstract. The link between the atmospheric CO2 level and the ventilation state of the deep ocean is an important building block of the key hypotheses put forth to explain glacial-interglacial CO2 fluctuations. In this study, we systematically examine the sensitivity of atmospheric CO2 and its carbon isotope composition to changes in deep ocean ventilation, the ocean carbon pumps, and sediment formation in a global 3-D ocean-sediment carbon cycle model. Our results provide support for the hypothesis that a break up of Southern Ocean stratification and invigorated deep ocean ventilation were the dominant drivers for the early deglacial CO2 rise of ~35 ppm between the Last Glacial Maximum and 14.6 ka BP. Another rise of 10 ppm until the end of the Holocene is attributed to carbonate compensation responding to the early deglacial change in ocean circulation. Our reasoning is based on a multi-proxy analysis which indicates that an acceleration of deep ocean ventilation during early deglaciation is not only consistent with recorded atmospheric CO2 but also with the reconstructed opal sedimentation peak in the Southern Ocean at around 16 ka BP, the record of atmospheric δ13CCO2, and the reconstructed changes in the Pacific CaCO3 saturation horizon.
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Tschumi, T., F. Joos, M. Gehlen et C. Heinze. « Deep ocean ventilation, carbon isotopes, marine sedimentation and the deglacial CO<sub>2</sub> ; rise ». Climate of the Past Discussions 6, no 5 (27 septembre 2010) : 1895–958. http://dx.doi.org/10.5194/cpd-6-1895-2010.
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Abstract. The link between the atmospheric CO2 level and the ventilation state of the deep ocean is an important building block of the key hypotheses put forth to explain glacial-interglacial CO2 fluctuations. In this study, we systematically examine the sensitivity of atmospheric CO2 and its carbon isotope composition to changes in deep ocean ventilation, the ocean carbon pumps, and sediment formation in a global three-dimensional ocean-sediment carbon cycle model. Our results provide support for the hypothesis that a break up of Southern Ocean stratification and invigorated deep ocean ventilation were the dominant drivers for the early deglacial CO2 rise of ~35 ppm between the Last Glacial Maximum and 14.6 ka BP. Another rise of 10 ppm until the end of the Holocene is attributed to carbonate compensation responding to the early deglacial change in ocean circulation. Our reasoning is based on a multi-proxy analysis which indicates that an acceleration of deep ocean ventilation during the early deglaciation is not only consistent with recorded atmospheric CO2 but also with the reconstructed opal sedimentation peak in the Southern Ocean at around 16 ka BP, the record of atmospheric δ13CCO2, and the reconstructed changes in the Pacific CaCO3 saturation horizon.
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Klocker, Andreas. « Opening the window to the Southern Ocean : The role of jet dynamics ». Science Advances 4, no 10 (octobre 2018) : eaao4719. http://dx.doi.org/10.1126/sciadv.aao4719.
Texte intégralRésumé :
The surface waters of the Southern Ocean act as a control valve through which climatically important tracers such as heat, freshwater, and CO2 are transferred between the atmosphere and the ocean. The process that transports these tracers through the surface mixed layer into the ocean interior is known as ocean ventilation. Changes in ocean ventilation are thought to be important for both rapid transitions of the ocean’s global overturning circulation during the last deglaciation and the uptake and storage of excess heat and CO2 as a consequence of anthropogenic climate change. I show how the interaction between Southern Ocean jets, topographic features, and ocean stratification can lead to rapid changes in Southern Ocean ventilation as a function of wind stress. For increasing winds, this interaction leads from a state in which tracers are confined to the surface mixed layer to a state in which tracers fill the ocean interior. For sufficiently high winds, the jet dynamics abruptly change, allowing the tracer to ventilate a water mass known as Antarctic Intermediate Water in the mid-depth Southern Ocean. Abrupt changes in Antarctic Intermediate Water ventilation have played a major role in rapid climate transitions in Earth’s past, and combined with the results presented here, this would suggest that jet dynamics could play a prominent role in contributing to, or even triggering, rapid transitions of the global climate system.
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MacGilchrist, Graeme A., Helen L. Johnson, David P. Marshall, Camille Lique, Matthew Thomas, Laura C. Jackson et Richard A. Wood. « Locations and Mechanisms of Ocean Ventilation in the High-Latitude North Atlantic in an Eddy-Permitting Ocean Model ». Journal of Climate 33, no 23 (1 décembre 2020) : 10113–31. http://dx.doi.org/10.1175/jcli-d-20-0191.1.
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AbstractA substantial fraction of the deep ocean is ventilated in the high-latitude North Atlantic. Consequently, the region plays a crucial role in transient climate change through the uptake of carbon dioxide and heat. However, owing to the Lagrangian nature of the process, many aspects of deep Atlantic Ocean ventilation and its representation in climate simulations remain obscure. We investigate the nature of ventilation in the high-latitude North Atlantic in an eddy-permitting numerical ocean circulation model using a comprehensive set of Lagrangian trajectory experiments. Backward-in-time trajectories from a model-defined North Atlantic Deep Water (NADW) reveal the locations of subduction from the surface mixed layer at high spatial resolution. The major fraction of NADW ventilation results from subduction in the Labrador Sea, predominantly within the boundary current (~60% of ventilated NADW volume) and a smaller fraction arising from open ocean deep convection (~25%). Subsurface transformations—due in part to the model’s parameterization of bottom-intensified mixing—facilitate NADW ventilation, such that water subducted in the boundary current ventilates all of NADW, not just the lighter density classes. There is a notable absence of ventilation arising from subduction in the Greenland–Iceland–Norwegian Seas, due to the re-entrainment of those waters as they move southward. Taken together, our results emphasize an important distinction between ventilation and dense water formation in terms of the location where each takes place, and their concurrent sensitivities. These features of NADW ventilation are explored to understand how the representation of high-latitude processes impacts properties of the deep ocean in a state-of-the-science numerical simulation.
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Peng, T.-H. « Changes in Ocean Ventilation Rates Over the Last 7000 Years Based on 14C Variations in the Atmosphere and Oceans ». Radiocarbon 31, no 03 (1989) : 481–92. http://dx.doi.org/10.1017/s0033822200012078.
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Changes in the ocean ventilation rate may be one of the causes for a net decrease of 100‰ Δ 14C in atmospheric CO2 over the last 8000 years. Ocean ventilation rates of the past can be derived from the 14C record preserved in planktonic and benthic foraminifera in deep-sea sediments. Results of 14C dating using accelerator mass spectrometry on deep sea sediments from the South China Sea show that the age differences between planktonic (G sacculifer) and benthic foraminifera increase from 1350 yr ca 7000 yr ago to 1590 yr at present. An 11-box geochemical model of global ocean circulation was used for this study. Both tree-ring-determined atmospheric 14C values and foraminifera 14C age differences are used as constraints to place limits on patterns of changes in ocean ventilation rates and in atmospheric 14C production rates. Results indicate: 1) 14C production rates in the atmosphere may have decreased by as much as 30% between 7000 and 3000 yr ago, and may have increased again by ca 15% in the past 2000 yr, and 2) the global ocean ventilation rate may not have been at steady state over the last 7000 yr, but may have slowed by as much as 35%.
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Sen Gupta, Alexander, et Matthew H. England. « Evaluation of Interior Circulation in a High-Resolution Global Ocean Model. Part II : Southern Hemisphere Intermediate, Mode, and Thermocline Waters ». Journal of Physical Oceanography 37, no 11 (1 novembre 2007) : 2612–36. http://dx.doi.org/10.1175/2007jpo3644.1.
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Abstract A high-resolution, offline ocean general circulation model, incorporating a realistic parameterization of mixed layer convection, is used to diagnose pathways and time scales of Southern Hemisphere intermediate, mode, and lower thermocline water ventilation. The use of such an offline methodology represents the only feasible way of simulating the long time scales required to validate the internal pathways of a high-resolution ocean model. Simulated and observed chlorofluorocarbon-11 (CFC-11) are in reasonably good agreement, demonstrating the model’s skill in representing realistic ventilation. Regional passive dye and age tracer experiments aid in the identification of pathways originating from different Southern Hemisphere locations. Northern Hemisphere penetration of intermediate, mode, and thermocline waters is most extensive and rapid into the North Atlantic Ocean because these waters are involved in closing the Atlantic meridional overturning cell. However, less than 8% of this ventilation is derived from subduction within the South Atlantic in the simulation. Instead, this water enters the Atlantic just to the south of South Africa, having originally subducted primarily in the east Indian Ocean, but also in the west Indian Ocean and the west Pacific region where a pathway advects water westward to the south of Australia. This pathway also plays a large part, together with water overturned in the east Indian Ocean, in ventilating the northern reaches of the Indian basin. Northward propagation in the Pacific Ocean is limited to the low latitudes of the Northern Hemisphere and is almost exclusively accomplished by water subducted in the South Pacific. A small contribution is made from the other basins from water that spreads northward, fed by a circumpolar pathway associated with the Antarctic Circumpolar Current that forms a conduit for intermediate and mode water exchange between all three basins. Intermediate water is injected into and branches off this pathway in all basins, but most vigorously in the southeastern Pacific.
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Ruth, Eivind, et Øyvind N. Smogeli. « Ventilation of Controllable Pitch Thrusters ». Marine Technology and SNAME News 43, no 04 (1 octobre 2006) : 170–79. http://dx.doi.org/10.5957/mt1.2006.43.4.170.
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In oil and gas exploration and exploitation, many ships and rigs conduct station-keeping operations by the use of dynamic positioning (DP) systems or thruster-assisted position mooring (PM) systems. Such systems use thrusters to control the ship or rig position and heading. In severe weather conditions, thrusters may experience large and rapid changes in the propeller loading due to ventilation and in and out of water effects. To reduce the negative effects of this undesired dynamic loading on the mechanical components and the electrical power plant, the thruster controllers can locally compensate for the disturbances. To be able to design and verify such controllers, a model of the losses is needed. In this paper, experiments with ventilating controllable pitch propellers are presented. The results are used to develop a ventilation model for use in simulations. The model is verified by comparing simulations with time series from experiments. Scaling laws for ventilating propellers are also discussed.
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Katavouta, Anna, Richard G. Williams et Philip Goodwin. « The Effect of Ocean Ventilation on the Transient Climate Response to Emissions ». Journal of Climate 32, no 16 (19 juillet 2019) : 5085–105. http://dx.doi.org/10.1175/jcli-d-18-0829.1.
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Abstract The surface warming response to carbon emissions is affected by how the ocean sequesters excess heat and carbon supplied to the climate system. This ocean uptake involves the ventilation mechanism, where heat and carbon are taken up by the mixed layer and transferred to the thermocline and deep ocean. The effect of ocean ventilation on the surface warming response to carbon emissions is explored using simplified conceptual models of the atmosphere and ocean with and without explicit representation of the meridional overturning. Sensitivity experiments are conducted to investigate the effects of (i) mixed layer thickness, (ii) rate of ventilation of the ocean interior, (iii) strength of the meridional overturning, and (iv) extent of subduction in the Southern Ocean. Our diagnostics focus on a climate metric, the transient climate response to carbon emissions (TCRE), defined by the ratio of surface warming to the cumulative carbon emissions, which may be expressed in terms of separate thermal and carbon contributions. The variability in the thermal contribution due to changes in ocean ventilation dominates the variability in the TCRE on time scales from years to centuries, while that of the carbon contribution dominates on time scales from centuries to millennia. These ventilated controls are primarily from changes in the mixed layer thickness on decadal time scales, and in the rate of ventilated transfer from the mixed layer to the thermocline and deep ocean on centennial and millennial time scales, which is itself affected by the strength of the meridional overturning and extent of subduction in the Southern Ocean.
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Watson, Andrew J., Geoffrey K. Vallis et Maxim Nikurashin. « Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2 ». Nature Geoscience 8, no 11 (28 septembre 2015) : 861–64. http://dx.doi.org/10.1038/ngeo2538.
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Reverdin, Gilles, Ray F. Weiss et William J. Jenkins. « Ventilation of the Atlantic Ocean equatorial thermocline ». Journal of Geophysical Research 98, no C9 (1993) : 16289. http://dx.doi.org/10.1029/93jc00976.
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Broecker, W. « Ventilation of the Glacial Deep Pacific Ocean ». Science 306, no 5699 (12 novembre 2004) : 1169–72. http://dx.doi.org/10.1126/science.1102293.
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Blanke, Bruno, Sabrina Speich, Gurvan Madec et Rudy Maugé. « A global diagnostic of interior ocean ventilation ». Geophysical Research Letters 29, no 8 (avril 2002) : 108–1. http://dx.doi.org/10.1029/2001gl013727.
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MacGilchrist, Graeme A., David P. Marshall, Helen L. Johnson, Camille Lique et Matthew Thomas. « Characterizing the chaotic nature of ocean ventilation ». Journal of Geophysical Research : Oceans 122, no 9 (septembre 2017) : 7577–94. http://dx.doi.org/10.1002/2017jc012875.
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Max, L., L. Lembke-Jene, J. R. Riethdorf, R. Tiedemann, D. Nürnberg, H. Kühn et A. Mackensen. « Pulses of enhanced North Pacific Intermediate Water ventilation from the Okhotsk Sea and Bering Sea during the last deglaciation ». Climate of the Past 10, no 2 (21 mars 2014) : 591–605. http://dx.doi.org/10.5194/cp-10-591-2014.
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Abstract. Under modern conditions only North Pacific Intermediate Water is formed in the northwest Pacific Ocean. This situation might have changed in the past. Recent studies with general circulation models indicate a switch to deep-water formation in the northwest Pacific during Heinrich Stadial 1 (17.5–15.0 ka) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the intermediate-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in δ13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the δ13C intermediate-water (700–1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (>2100 m water depth) during the last glacial termination. Thus, intermediate- and deep-water masses of the northwest Pacific have a differing ventilation history during the last deglaciation.
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Max, L., L. Lembke-Jene, J. R. Riethdorf, R. Tiedemann, D. Nürnberg, H. Kühn et A. Mackensen. « Pulses of enhanced North Pacific Intermediate Water ventilation from the Okhotsk Sea and Bering Sea during the last deglaciation ». Climate of the Past Discussions 9, no 6 (7 novembre 2013) : 6221–53. http://dx.doi.org/10.5194/cpd-9-6221-2013.
Texte intégralRésumé :
Abstract. Under modern conditions only North Pacific Intermediate Water is formed in the Northwest Pacific Ocean. This situation might have changed in the past. Recent studies with General Circulation Models indicate a switch to deep-water formation in the Northwest Pacific during Heinrich Stadial 1 (17.5–15.0 kyr) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the Northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the mid-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in δ13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the δ13C intermediate water (700–1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (>2100 m water depth) during the last glacial termination. Thus, intermediate and deep-water masses of the Northwest Pacific have a differing ventilation history during the last deglaciation.
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Wilson, Earle A., Stephen C. Riser, Ethan C. Campbell et Annie P. S. Wong. « Winter Upper-Ocean Stability and Ice–Ocean Feedbacks in the Sea Ice–Covered Southern Ocean ». Journal of Physical Oceanography 49, no 4 (avril 2019) : 1099–117. http://dx.doi.org/10.1175/jpo-d-18-0184.1.
Texte intégralRésumé :
AbstractIn this study, under-ice ocean data from profiling floats, instrumented seals, and shipboard casts are used to assess wintertime upper-ocean stability and heat availability in the sea ice–covered Southern Ocean. This analysis reveals that the southern Weddell Sea, which features a weak upper-ocean stratification and relatively strong thermocline, is preconditioned for exceptionally high rates of winter ventilation. This preconditioning also facilitates a strong negative feedback to winter ice growth. Idealized experiments with a 1D ice–ocean model show that the entrainment of heat into the mixed layer of this region can maintain a near-constant ice thickness over much of winter. However, this quasi-equilibrium is attained when the pycnocline is thin and supports a large temperature gradient. We find that the surface stress imparted by a powerful storm may upset this balance and lead to substantial ice melt. This response can be greatly amplified when coincident with anomalous thermocline shoaling. In more strongly stratified regions, such as near the sea ice edge of the major gyres, winter ice growth is weakly limited by the entrainment of heat into the mixed layer. Thus, the thermodynamic coupling between winter sea ice growth and ocean ventilation has significant regional variability. This regionality will influence the response of the Southern Ocean ice–ocean system to future changes in ocean stratification and surface forcing.
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Waugh, Darryn W., Francois Primeau, Tim DeVries et Mark Holzer. « Recent Changes in the Ventilation of the Southern Oceans ». Science 339, no 6119 (31 janvier 2013) : 568–70. http://dx.doi.org/10.1126/science.1225411.
Texte intégralRésumé :
Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole, and there is intense debate on the impact of this on the ocean's circulation and uptake and redistribution of atmospheric gases. We used measurements of chlorofluorocarbon-12 (CFC-12) made in the southern oceans in the early 1990s and mid- to late 2000s to examine changes in ocean ventilation. Our analysis of the CFC-12 data reveals a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters, suggesting that the formation of the Antarctic ozone hole has caused large-scale coherent changes in the ventilation of the southern oceans.
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Michalsky, Joseph J., Mark Kutchenreiter et Charles N. Long. « Significant Improvements in Pyranometer Nighttime Offsets Using High-Flow DC Ventilation ». Journal of Atmospheric and Oceanic Technology 34, no 6 (juin 2017) : 1323–32. http://dx.doi.org/10.1175/jtech-d-16-0224.1.
Texte intégralRésumé :
AbstractVentilators are used to keep the domes of pyranometers clean and dry, but they affect the nighttime offset as well. This paper examines different ventilation strategies. For the several commercial single-black-detector pyranometers with ventilators examined here, high-flow-rate [50 cubic feet per minute (CFM) and higher] 12-VDC (where VDC refers to voltage direct current) fans lower the offsets, lower the scatter, and improve the predictability of the offsets during the night compared with lower-flow-rate (35 CFM) 120-VAC (where VAC refers to voltage alternating current) fans operated in the same ventilator housings. Black-and-white pyranometers sometimes show improvement with DC ventilation, but in some cases DC ventilation makes the offsets slightly worse. Since the offsets for these black-and-white pyranometers are always small, usually no more than 1 W m−2, whether AC or DC ventilated, changing their ventilation to higher CFM DC ventilation is not imperative. Future work should include all major manufacturers of pyranometers and unventilated and ventilated pyranometers. An important outcome of future research will be to clarify under what circumstances nighttime data can be used to predict daytime offsets.
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Waugh, Darryn W. « Changes in the ventilation of the southern oceans ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 372, no 2019 (13 juillet 2014) : 20130269. http://dx.doi.org/10.1098/rsta.2013.0269.
Texte intégralRésumé :
Changes in the ventilation of the southern oceans over the past few decades are examined using ocean measurements of CFC-12 and model simulations. Analysis of CFC-12 measurements made between the late 1980s and late 2000s reveal large-scale coherent changes in the ventilation, with a decrease in the age of subtropical Subantarctic Mode Waters (SAMW) and an increase in the age of Circumpolar Deep Waters. The decrease in SAMW age is consistent with the observed increase in wind stress curl and strength of the subtropical gyres over the same period. A decrease in the age of SAMW is also found in Community Climate System Model version 4 perturbation experiments where the zonal wind stress is increased. This decrease is due to both more rapid transport along isopycnals and the movement of the isopycnals. These results indicate that the intensification of surface winds in the Southern Hemisphere has caused large-scale coherent changes in the ventilation of the southern oceans.
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Lago, Véronique, et Matthew H. England. « Projected Slowdown of Antarctic Bottom Water Formation in Response to Amplified Meltwater Contributions ». Journal of Climate 32, no 19 (27 août 2019) : 6319–35. http://dx.doi.org/10.1175/jcli-d-18-0622.1.
Texte intégralRésumé :
Abstract The sinking and recirculation of Antarctic Bottom Water (AABW) are a major regulator of the storage of heat, carbon, and nutrients in the ocean. This sinking is sensitive to changes in surface buoyancy, in particular because of freshening since salinity plays a greater role in determining density at cold temperatures. Acceleration in Antarctic ice-shelf and land-ice melt could thus significantly impact the ventilation of the world’s oceans, yet future projections do not usually include this effect in models. Here we use an ocean–sea ice model to investigate the potential long-term impact of Antarctic meltwater on ocean circulation and heat storage. The freshwater forcing is derived from present-day estimates of meltwater input from drifting icebergs and basal melt, combined with RCP2.6, RCP4.5, and RCP8.5 scenarios of projected amplification of Antarctic meltwater. We find that the additional freshwater induces a substantial slowdown in the formation rate of AABW, reducing ventilation of the abyssal ocean. Under both the RCP4.5 and RCP8.5 meltwater scenarios, there is a near-complete shutdown of AABW formation within just 50 years, something that is not captured by climate model projections. The abyssal overturning at ~30°S also weakens, with an ~20-yr delay relative to the onset of AABW slowdown. After 200 years, up to ~50% of the original volume of AABW has disappeared as a result of abyssal warming, induced by vertical mixing in the absence of AABW ventilation. This result suggests that climate change could induce the disappearance of present-day abyssal water masses, with implications for the global distribution of heat, carbon, and nutrients.
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Stöven, T., T. Tanhua et M. Hoppema. « Transient tracer applications in the Southern Ocean ». Ocean Science Discussions 11, no 5 (16 octobre 2014) : 2289–335. http://dx.doi.org/10.5194/osd-11-2289-2014.
Texte intégralRésumé :
Abstract. Transient tracers can be used to constrain the Inverse-Gaussian transit time distribution (IG-TTD) and thus provide information about ocean ventilation. Individual transient tracers have different time and application ranges which are defined by their atmospheric history (chronological transient tracers) or their decay rate (radioactive transient tracers). The classification ranges from tracers for highly ventilated water masses, e.g. sulfur hexafluoride (SF6), the decay of Tritium (δ3H) and to some extent also dichlorodifluoromethane (CFC-12) to tracers for less ventilated deep ocean basins, e.g. CFC-12, Argon-39 (39Ar) and radiocarbon (14C). The IG-TTD can be empirically constrained by using transient tracer couples with sufficiently different input functions. Each tracer couple has specific characteristics which influence the application limit of the IG-TTD. Here we provide an overview of commonly used transient tracer couples and their validity areas within the IG-TTD by using the concept of tracer age differences (TAD). New measured CFC-12 and SF6 data from a section along 10° E in the Southern Ocean in 2012 are presented. These are combined with a similar data set of 1998 along 6° E in the Southern Ocean as well as with 39Ar data from the early 1980s in the western Atlantic Ocean and the Weddell Sea for investigating the application limit of the IG-TTD and to analyze changes in ventilation in the Southern Ocean. We found that the IG-TTD can be constrained south to 46° S which corresponds to the Subantarctic Front (SAF) denoting the application limit. The constrained IG-TTD north of the SAF shows a slight increase in mean ages between 1998 and 2012 in the upper 1200 m between 42–46° S. The absence of SF6 inhibits ventilation analyses below this depth. The time lag analysis between the 1998 and 2012 data shows an increase in ventilation down to 1000 m and a steady ventilation between 2000 m-bottom south of the SAF between 51–55° S.
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Gnanadesikan, A., et J. L. Russell. « How does ocean ventilation change under global warming ? » Ocean Science 3, no 1 (6 février 2007) : 43–53. http://dx.doi.org/10.5194/os-3-43-2007.
Texte intégralRésumé :
Abstract. Since the upper ocean takes up much of the heat added to the earth system by anthropogenic global warming, one would expect that global warming would lead to an increase in stratification and a decrease in the ventilation of the ocean interior. However, multiple simulations in global coupled climate models using an ideal age tracer which is set to zero in the mixed layer and ages at 1 yr/yr outside this layer show that the intermediate depths in the low latitudes, Northwest Atlantic, and parts of the Arctic Ocean become younger under global warming. This paper reconciles these apparently contradictory trends, showing that the decreases result from changes in the relative contributions of old deep waters and younger surface waters. Implications for the tropical oxygen minimum zones, which play a critical role in global biogeochemical cycling are considered in detail.
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Stocker, Thomas F., et Daniel G. Wright. « The Effect of a Succession of Ocean Ventilation Changes on 14C ». Radiocarbon 40, no 1 (1997) : 359–66. http://dx.doi.org/10.1017/s0033822200018233.
Texte intégralRésumé :
Using the model of Stacker and Wright (1996), we investigate the effect of a succession of ocean ventilation changes on the atmospheric concentration of radiocarbon, δ14Catm, the surface reservoir ages, the top-to-bottom age differences, and the calendar-14C age relationships in different regions of the ocean. The model includes a representation of the cycling of 14C through the atmosphere, the ocean and the land biosphere. Ocean ventilation changes are triggered by increasing rates of freshwater discharge into the North Atlantic, which are determined according to a simple feedback mechanism between the melting rates and the climatic state of the North Atlantic region. The results demonstrate that ventilation changes can cause δ14Catm fluctuations of 25%, surface reservoir age fluctuations of 100 yr in the Pacific (200 yr in the Atlantic) and top-to-bottom age variations of 500 yr in the Pacific (1000 yr in the Atlantic). We also show that 14C age estimates based on marine organisms that live in the near-surface region of the ocean and take up the signal of surface 14C can result in apparent age reversals if the assumption of a constant reservoir age is made.
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Duplessy, Jean-Claude, Maurice Arnold, Edouard Bard, Anne Juillet-Leclerc, Nejib Kallel et Laurent Labeyrie. « AMS 14C Study of Transient Events and of the Ventilation Rate of the Pacific Intermediate Water During the Last Deglaciation ». Radiocarbon 31, no 03 (1989) : 493–502. http://dx.doi.org/10.1017/s003382220001208x.
Texte intégralRésumé :
14C analysis of monospecific samples of planktonic and benthic foraminifera were performed in deep-sea sediment cores from the Atlantic and Pacific Oceans by Accelerator Mass Spectrometry (AMS). These measurements demonstrate that the Younger Dryas cold event, first described in the north Atlantic, is also present at the same time in the north Pacific Ocean. The comparison of the 14C ages of planktonic and benthic foraminifera from the same sediment level in two Pacific cores shows that the ventilation time of the Pacific Ocean was greater than today during the last ice age, but significantly less than today during the deglaciation.
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Gnanadesikan, A., J. L. Russell et F. Zeng. « How does ocean ventilation change under global warming ? » Ocean Science Discussions 3, no 4 (11 juillet 2006) : 805–26. http://dx.doi.org/10.5194/osd-3-805-2006.
Texte intégralRésumé :
Abstract. Since the upper ocean takes up much of the heat added to the earth system by anthropogenic global warming, one would expect that global warming would lead to an increase in stratification and a decrease in the ventilation of the ocean interior. However, multiple simulations in global coupled climate models using an ideal age tracer which is set to zero in the mixed layer and ages at 1 yr/yr outside this layer show that the intermediate depths in the low latitudes become younger under global warming. This paper reconciles these apparently contradictory trends, showing that a decrease in upwelling of old water from below is responsible for the change. Implications for global biological cycling are considered.
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Fine, Rana A., Kevin A. Maillet, Kevin F. Sullivan et Debra Willey. « Circulation and ventilation flux of the Pacific Ocean ». Journal of Geophysical Research : Oceans 106, no C10 (15 octobre 2001) : 22159–78. http://dx.doi.org/10.1029/1999jc000184.
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Fine, Rana A., William M. Smethie, John L. Bullister, Monika Rhein, Dong-Ha Min, Mark J. Warner, Alain Poisson et Ray F. Weiss. « Decadal ventilation and mixing of Indian Ocean waters ». Deep Sea Research Part I : Oceanographic Research Papers 55, no 1 (janvier 2008) : 20–37. http://dx.doi.org/10.1016/j.dsr.2007.10.002.
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Broecker, Wallace S., Elizabeth Clark, Irena Hajdas et Georges Bonani. « Glacial ventilation rates for the deep Pacific Ocean ». Paleoceanography 19, no 2 (2 avril 2004) : n/a. http://dx.doi.org/10.1029/2003pa000974.
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Watson, Andrew J., Timothy M. Lenton et Benjamin J. W. Mills. « Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia ». Philosophical Transactions of the Royal Society A : Mathematical, Physical and Engineering Sciences 375, no 2102 (7 août 2017) : 20160318. http://dx.doi.org/10.1098/rsta.2016.0318.
Texte intégralRésumé :
The major biogeochemical cycles that keep the present-day Earth habitable are linked by a network of feedbacks, which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes that control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 10 5 –10 6 years, these ocean anoxic events (OAEs) being particularly associated with warm ‘greenhouse’ climates. A powerful mechanism responsible for past OAEs was an increase in phosphorus supply to the oceans, leading to higher ocean productivity and oxygen demand in subsurface water. This can be amplified by positive feedbacks on the nutrient content of the ocean, with low oxygen promoting further release of phosphorus from ocean sediments, leading to a potentially self-sustaining condition of deoxygenation. We use a simple model for phosphorus in the ocean to explore this feedback, and to evaluate the potential for humans to bring on global-scale anoxia by enhancing P supply to the oceans. While this is not an immediate global change concern, it is a future possibility on millennial and longer time scales, when considering both phosphate rock mining and increased chemical weathering due to climate change. Ocean deoxygenation, once begun, may be self-sustaining and eventually could result in long-lasting and unpleasant consequences for the Earth's biosphere. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.
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Liu, Ling Ling, et Rui Xin Huang. « The Global Subduction/Obduction Rates : Their Interannual and Decadal Variability ». Journal of Climate 25, no 4 (8 février 2012) : 1096–115. http://dx.doi.org/10.1175/2011jcli4228.1.
Texte intégralRésumé :
Abstract Ventilation, including subduction and obduction, for the global oceans was examined using Simple Ocean Data Assimilation (SODA) outputs. The global subduction rate averaged over the period from 1959 to 2006 is estimated at 505.8 Sv (1 Sv ≡ 106 m3 s−1), while the corresponding global obduction rate is estimated at 482.1 Sv. The annual subduction/obduction rates vary greatly on the interannual and decadal time scales. The global subduction rate is estimated to have increased 7.6% over the past 50 years, while the obduction rate is estimated to have increased 9.8%. Such trends may be insignificant because errors associated with the data generated by ocean data assimilation could be as large as 10%. However, a major physical mechanism that induced these trends is primarily linked to changes in the Southern Ocean. While the Southern Ocean plays a key role in global subduction and obduction rates and their variability, both the Southern Ocean and equatorial regions are critically important sites of water mass formation/erosion.
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Bachman, Scott D., et Andreas Klocker. « Interaction of Jets and Submesoscale Dynamics Leads to Rapid Ocean Ventilation ». Journal of Physical Oceanography 50, no 10 (1 octobre 2020) : 2873–83. http://dx.doi.org/10.1175/jpo-d-20-0117.1.
Texte intégralRésumé :
ABSTRACTOcean ventilation is the process by which climatically important tracers such as heat and carbon are exchanged between the atmosphere and ocean interior. In this paper a series of numerical simulations are used to study the interaction of submesoscales and a topographically steered jet in driving rapid ventilation. The ventilation is found to increase both as a function of wind stress and model resolution, with a submesoscale-resolving 1/120° model exhibiting the largest ventilation rate. The jet in this simulation is found to be persistently unstable to submesoscale instabilities, which are known to feature intense vertical circulations. The vertical tracer transport is found to scale as a function of the eddy kinetic energy and mean isopycnal slope, whose behaviors change as a function of the wind stress and due to the emergence of a strong potential vorticity gradient due to the lateral shear of the jet. These results highlight the importance of jet–submesoscale interaction as a bridge between the atmosphere and the ocean interior.
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Yin, Qiuzhen. « Insolation-induced mid-Brunhes transition in Southern Ocean ventilation and deep-ocean temperature ». Nature 494, no 7436 (février 2013) : 222–25. http://dx.doi.org/10.1038/nature11790.
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MCPHEE, MILES G. « Is thermobaricity a major factor in Southern Ocean ventilation ? » Antarctic Science 15, no 1 (26 février 2003) : 153–60. http://dx.doi.org/10.1017/s0954102003001159.
Texte intégralRésumé :
The Weddell Polynya, a large expanse of water that originated over Maud Rise (a bathymetric protrusion centred near 64°30′S, 3°E) and remained open during winter in the late 1970s, may have manifested a mode of deep ocean convection where despite large heat loss at the surface, sustained heat transport from below prevents lasting ice formation. In a different dominant mode (the present one), sea ice forms early in the winter and subsequently provides a thermal barrier that quickly quells incipient deep convection, thus preventing wholesale destruction of the ice cover. A possible mechanism for overcoming the thermal barrier is thermobaricity, the pressure dependence of the thermal expansion factor for seawater. An idealized, two-layer version of actual temperature and salinity profiles from the Weddell illustrates that thermobaric mixing can persist for extended periods in an ice-covered ocean, provided realistic melt rates (controlled by salt exchange at the ice/ocean interface) are specified. This furnishes a possible explanation for transient winter polynyas sometime observed in the ice-covered Southern Ocean. Thermobaricity may provide a trigger for widespread convection with possible climate impact.
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Sohail, Taimoor, Bishakhdatta Gayen et Andrew McC. Hogg. « The Dynamics of Mixed Layer Deepening during Open-Ocean Convection ». Journal of Physical Oceanography 50, no 6 (juin 2020) : 1625–41. http://dx.doi.org/10.1175/jpo-d-19-0264.1.
Texte intégralRésumé :
AbstractOpen-ocean convection is a common phenomenon that regulates mixed layer depth and ocean ventilation in the high-latitude oceans. However, many climate model simulations overestimate mixed layer depth during open-ocean convection, resulting in excessive formation of dense water in some regions. The physical processes controlling transient mixed layer depth during open-ocean convection are examined using two different numerical models: a high-resolution, turbulence-resolving nonhydrostatic model and a large-scale hydrostatic ocean model. An isolated destabilizing buoyancy flux is imposed at the surface of both models and a quasi-equilibrium flow is allowed to develop. Mixed layer depth in the turbulence-resolving and large-scale models closely aligns with existing scaling theories. However, the large-scale model has an anomalously deep mixed layer prior to quasi-equilibrium. This transient mixed layer depth bias is a consequence of the lack of resolved turbulent convection in the model, which delays the onset of baroclinic instability. These findings suggest that in order to reduce mixed layer biases in ocean simulations, parameterizations of the connection between baroclinic instability and convection need to be addressed.
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Weinans, Els, Anne Willem Omta, George A. K. van Voorn et Egbert H. van Nes. « A potential feedback loop underlying glacial-interglacial cycles ». Climate Dynamics 57, no 1-2 (18 mars 2021) : 523–35. http://dx.doi.org/10.1007/s00382-021-05724-w.
Texte intégralRésumé :
AbstractThe sawtooth-patterned glacial-interglacial cycles in the Earth’s atmospheric temperature are a well-known, though poorly understood phenomenon. Pinpointing the relevant mechanisms behind these cycles will not only provide insights into past climate dynamics, but also help predict possible future responses of the Earth system to changing CO$$_2$$ 2 levels. Previous work on this phenomenon suggests that the most important underlying mechanisms are interactions between marine biological production, ocean circulation, temperature and dust. So far, interaction directions (i.e., what causes what) have remained elusive. In this paper, we apply Convergent Cross-Mapping (CCM) to analyze paleoclimatic and paleoceanographic records to elucidate which mechanisms proposed in the literature play an important role in glacial-interglacial cycles, and to test the directionality of interactions. We find causal links between ocean ventilation, biological productivity, benthic $$\delta ^{18}$$ δ 18 O and dust, consistent with some but not all of the mechanisms proposed in the literature. Most importantly, we find evidence for a potential feedback loop from ocean ventilation to biological productivity to climate back to ocean ventilation. Here, we propose the hypothesis that this feedback loop of connected mechanisms could be the main driver for the glacial-interglacial cycles.
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Stöven, T., et T. Tanhua. « Ventilation of the Mediterranean Sea constrained by multiple transient tracer measurements ». Ocean Science Discussions 10, no 5 (10 octobre 2013) : 1647–705. http://dx.doi.org/10.5194/osd-10-1647-2013.
Texte intégralRésumé :
Abstract. Ventilation is the prime pathway for ocean surface perturbations, such as temperature anomalies, to be relayed to the ocean interior. It is also the conduit for gas exchange between atmosphere and ocean and thus the mechanism whereby, for instance, the interior ocean is oxygenated and enriched in anthropogenic carbon. The ventilation of the Mediterranean Sea is fast in comparison to the world ocean and has large temporal variability, so that quantification of Mediterranean Sea ventilation rates is challenging and very relevant for Mediterranean oceanography and biogeochemistry. Here we present transient tracer data from a field-campaign in April 2011 that sampled a unique suite of transient tracers (SF6, CFC-12, tritium and 3He) in all major basins of the Mediterranean. We apply the Transit Time Distribution (TTD) model to the data which then constrain the mean age, the ratio of the advective/diffusive transport mechanism, and the presence, or not, of more than one significant (for ventilation) water mass. We find that the eastern part of the Eastern Mediterranean can be reasonable described with a one dimensional Inverse Gaussian (1IG) TTD, and thus constrained with two independent tracers. The ventilation of the Ionian Sea and the Western Mediterranean can only be constrained by a multidimensional TTD. We approximate the ventilation with a two-dimensional Inverse Gaussian (2IG) TTD for these areas and demonstrate one way of constraining a 2IG-TTD from the available transient tracer data. The deep water in the Ionian Sea has higher mean ages than the deep water of the Levantine Basin despite higher transient tracer concentrations. This is partly due to the deep water of Adriatic origin having more diffusive properties in the transport and formation, i.e. a high ratio of diffusion over advection, compared to the deep water of Aegean Sea origin that still dominates the deep Levantine Basin deep water after the Eastern Mediterranean Transient (EMT) in the early 1990s. We also show that the deep Western Mediterranean has approximately 40% contribution of recently ventilated deep water from the Western Mediterranean Transition (WMT) event of the mid-2000s. The deep water has higher transient tracer concentrations than the mid-depth water, but the mean age is similar.
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Wagner, M., et I. L. Hendy. « Trace metal evidence for a poorly ventilated glacial Southern Ocean ». Climate of the Past Discussions 11, no 2 (11 mars 2015) : 637–70. http://dx.doi.org/10.5194/cpd-11-637-2015.
Texte intégralRésumé :
Abstract. Glacial benthic δ13C and Δ14C measurements from the Atlantic Ocean have been interpreted to indicate the existence of a poorly ventilated Southern Ocean with greater CO2 and nutrient contents compared to present. Enhanced storage of CO2 in the deep ocean predicts that oxygen concentrations should have declined at the same time, although no unequivocal evidence for glacial Southern Ocean suboxia has yet been found. Here we take a novel approach by using concentrations of redox-sensitive trace metals to show that Southern Ocean sediments from two cores in the Atlantic sector were suboxic during deglaciation and the last glacial period, implying reduced ventilation and/or elevated export production that significantly altered deep water chemistry. In the Cape Basin, high concentrations of the authigenically deposited trace metal Re coincide with oldest Δ14C values at 3.8 km water depth in the Subantarctic Zone, indicating that poorest Southern Ocean ventilation occurred during the Last Glacial Maximum (~ 23–19 ka). Furthermore, trace metal results suggest that the vertical structure of the glacial Southern Ocean differed from modern deep water mass arrangement such that Lower Circumpolar Deep Water had lower O2 concentrations, and therefore was the likely reservoir of glacial CO2.
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Skinner, Luke, Francois Primeau, Aurich Jeltsch-Thömmes, Fortunat Joos, Peter Köhler et Edouard Bard. « Rejuvenating the ocean : mean ocean radiocarbon, CO2 release, and radiocarbon budget closure across the last deglaciation ». Climate of the Past 19, no 11 (3 novembre 2023) : 2177–202. http://dx.doi.org/10.5194/cp-19-2177-2023.
Texte intégralRésumé :
Abstract. Radiocarbon is a tracer that provides unique insights into the ocean's ability to sequester CO2 from the atmosphere. While spatial patterns of radiocarbon in the ocean interior can indicate the vectors and timescales for carbon transport through the ocean, estimates of the global average ocean–atmosphere radiocarbon age offset (B-Atm) place constraints on the closure of the global carbon cycle. Here, we apply a Bayesian interpolation method to compiled B-Atm data to generate global interpolated fields and mean ocean B-Atm estimates for a suite of time slices across the last deglaciation. The compiled data and interpolations confirm a stepwise and spatially heterogeneous “rejuvenation” of the ocean, suggesting that carbon was released to the atmosphere through two swings of a “ventilation seesaw” operating between the North Atlantic and both the Southern Ocean and the North Pacific. Sensitivity tests using the Bern3D model of intermediate complexity demonstrate that a portion of the reconstructed deglacial B-Atm changes may reflect “phase-attenuation” biases that are unrelated to ocean ventilation and that arise from independent atmospheric radiocarbon dynamics instead. A deglacial minimum in B-Atm offsets during the Bølling–Allerød could partly reflect such a bias. However, the sensitivity tests further demonstrate that when correcting for such biases, ocean “ventilation” could still account for at least one-third of deglacial atmospheric CO2 rise. This contribution to CO2 rise appears to have continued through the Younger Dryas, though much of the impact was likely achieved by the end of the Bølling–Allerød, indicating a key role for marine carbon cycle adjustment early in the deglacial process. Our global average B-Atm estimates place further new constraints on the long-standing mystery of global radiocarbon budget closure across the last deglaciation and suggest that glacial radiocarbon production levels are likely underestimated on average by existing reconstructions.
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Rae, James W. B., et Wally Broecker. « What fraction of the Pacific and Indian oceans' deep water is formed in the Southern Ocean ? » Biogeosciences 15, no 12 (21 juin 2018) : 3779–94. http://dx.doi.org/10.5194/bg-15-3779-2018.
Texte intégralRésumé :
Abstract. In this contribution we explore constraints on the fractions of deep water present in the Indian and Pacific oceans which originated in the northern Atlantic and in the Southern Ocean. Based on PO4* we show that if ventilated Antarctic shelf waters characterize the Southern contribution, then the proportions could be close to 50–50. If instead a Southern Ocean bottom water value is used, the Southern contribution is increased to 75 %. While this larger estimate may best characterize the volume of water entering the Indo-Pacific from the Southern Ocean, it contains a significant portion of entrained northern water. We also note that ventilation may be highly tracer dependent: for instance Southern Ocean waters may contribute only 35 % of the deep radiocarbon budget, even if their volumetric contribution is 75 %. In our estimation, the most promising approaches involve using CFC-11 to constrain the amount of deep water formed in the Southern Ocean. Finally, we highlight the broad utility of PO4* as a tracer of deep water masses, including descending plumes of Antarctic Bottom Water and large-scale patterns of deep ocean mixing, and as a tracer of the efficiency of the biological pump.