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Mirabel, A. P., e N. V. Vakulenko. "On advective model of the ventilated thermocline". Океанология 59, n.º 1 (18 de abril de 2019): 5–11. http://dx.doi.org/10.31857/s0030-15745915-11.
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A model of an advective thermocline is proposed for the case of continuously stratified Sverdrup circulation with a ventilated layer caused by the divergence of flows in the Ekman layer: an immiscible layer with homogenized vorticity and a layer of abyssal liquid, which applies to anticyclonic gyre waters. The results of calculations for the Atlantic Ocean (region 15-52°N, 00-63°E) made with this model are presented. With an abyssal density of 28.0, the values of the surface density and density of the unventilated layer grow to the north from 24.2 to 27.0 and from 27.8 to 27.9, respectively, with an almost zonal distribution, i.e. ventilation zones have latitudinal circles. From calculations of the depths of wind circulation, it follows that the ventilating layer is as deep as 900 m in the north-western region and raises to 250 m in the southern and eastern parts of the basin. The same tendency is traced for the depth of the gyre, but here there is an increase in depth from 500 to 1500 m. The active dynamics in the ventilating layer and the shadow area on the eastern border are noted. The structure of the thermocline is demonstrated with a typical zonal section, characterizing a much larger isopycnic increment for ventilated layers than in non-ventilated layers.
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Lago, Véronique, e Matthew H. England. "Projected Slowdown of Antarctic Bottom Water Formation in Response to Amplified Meltwater Contributions". Journal of Climate 32, n.º 19 (27 de agosto de 2019): 6319–35. http://dx.doi.org/10.1175/jcli-d-18-0622.1.
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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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Orsi, Alejandro H., Stanley S. Jacobs, Arnold L. Gordon e Martin Visbeck. "Cooling and ventilating the Abyssal Ocean". Geophysical Research Letters 28, n.º 15 (1 de agosto de 2001): 2923–26. http://dx.doi.org/10.1029/2001gl012830.
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Johnson, Gregory C., Sarah G. Purkey e John L. Bullister. "Warming and Freshening in the Abyssal Southeastern Indian Ocean*". Journal of Climate 21, n.º 20 (15 de outubro de 2008): 5351–63. http://dx.doi.org/10.1175/2008jcli2384.1.
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Abstract Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.
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Okazaki, Y., T. Sagawa, H. Asahi, K. Horikawa e J. Onodera. "Ventilation changes in the western North Pacific since the last glacial period". Climate of the Past 8, n.º 1 (3 de janeiro de 2012): 17–24. http://dx.doi.org/10.5194/cp-8-17-2012.
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Abstract. We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pacific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sediment with a high sedimentation rate. The 14C data on fragile and robust planktic foraminiferal shells were concordant with each other, ensuring high quality of the reconstructed ventilation record. The radiocarbon activity changes were consistent with the atmospheric record, suggesting that no massive mixing of old carbon from the abyssal reservoir occurred throughout the glacial to deglacial periods.
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Okazaki, Y., T. Sagawa, H. Asahi, K. Horikawa e J. Onodera. "Ventilation changes in the western North Pacific since the last glacial period". Climate of the Past Discussions 7, n.º 4 (18 de agosto de 2011): 2719–39. http://dx.doi.org/10.5194/cpd-7-2719-2011.
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Abstract. We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pacific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sediment with a high sedimentation rate. The 14C data on fragile and robust planktic foraminiferal shells were concordant with each other, ensuring high quality of the reconstructed ventilation record. The radiocarbon activity changes were consistent with the atmospheric record, suggesting that no massive mixing of old carbon from the abyssal reservoir occurred throughout the glacial to deglacial periods.
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Van Roekel, Luke P., Taka Ito, Patrick T. Haertel e David A. Randall. "Lagrangian Analysis of the Meridional Overturning Circulation in an Idealized Ocean Basin". Journal of Physical Oceanography 39, n.º 9 (1 de setembro de 2009): 2175–93. http://dx.doi.org/10.1175/2009jpo4110.1.
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Abstract The Lagrangian ocean model is used as a tool to simulate the response of the basin-scale overturning circulation to spatially variable diapycnal mixing in an idealized ocean basin. The model explicitly calculates the positions, velocities, and tracer properties of water parcels. Owing to its Lagrangian formulation, numerical diffusion is completely eliminated and water parcel pathways and water mass ages can be quantified within the framework of the discrete, advective transit time distribution. To illustrate the ventilation pathways, simulated trajectories were tracked backward in time from the interior ocean to the surface mixed layer where the water parcel was last in contact with the atmosphere. This new diagnostic has been applied to examine the response of the meridional overturning circulation to highly localized diapycnal mixing through sensitivity experiments. In particular, the focus is on three simulations: the first holds vertical diffusivity uniform; in the second, the vertical diffusivity is confined within an equatorial box; and the third simulation has a diffusivity pattern based on idealized hurricane-induced mixing. Domain-integrated deep ventilation rates and heat transport are similar between the first two cases. However, locally enhanced mixing yields about 30% younger water mass age in the tropical thermocline due to intense localized upwelling. In the third simulation, a slower ventilation rate of deep waters is found to be due to the lack of abyssal mixing. These results are interpreted using the classical theories of abyssal circulation, highlighting the strong sensitivity of the ventilation pathways to the spatial distribution of diapycnal mixing.
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Vreugdenhil, Catherine A., Andrew McC Hogg, Ross W. Griffiths e Graham O. Hughes. "Adjustment of the Meridional Overturning Circulation and Its Dependence on Depth of Mixing". Journal of Physical Oceanography 46, n.º 3 (março de 2016): 731–47. http://dx.doi.org/10.1175/jpo-d-15-0050.1.
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AbstractThe relative roles of advective processes and mixing on the temporal adjustment of the meridional overturning circulation are examined, in particular the effects of mixing in either the abyssal or upper ocean. Laboratory experiments with convectively driven overturning and imposed stirring rates show that the circulation adjusts toward an equilibrium state on time scales governed by mixing in the upper boundary layer region but independent of the mixing rate in the bulk of the interior. The equilibrium state of the stratification is dependent only on the rate of mixing in the boundary layer. An idealized high-resolution ocean model shows adjustment (of a two-cell circulation) dominated primarily by the advective ventilation time scale, consistent with a view of the circulation determined by water mass transformation occurring primarily near the surface. Both the experiments and the model results indicate that adjustments of the circulation are controlled by surface buoyancy uptake (or rejection) and that the nonequilibrium circulation is dominated by advective processes, especially if the average abyssal ocean diffusivity is less than 3 × 10−5 m2 s−1.
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Jenkins, William J., Kathryn L. Elder, Ann P. McNichol e Karl von Reden. "The Passage of the Bomb Radiocarbon Pulse into the Pacific Ocean". Radiocarbon 52, n.º 3 (2010): 1182–90. http://dx.doi.org/10.1017/s0033822200046257.
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We report and compare radiocarbon observations made on 2 meridional oceanographic sections along 150°W in the South Pacific in 1991 and 2005. The distributions reflect the progressive penetration of nuclear weapons-produced 14C into the oceanic thermocline. The changes over the 14 yr between occupations are demonstrably large relative to any possible drift in our analytical standardization. The computed difference field based on the gridded data in the upper 1600 m of the section exhibits a significant decrease over time (approaching 40 to 50‰ in Δ14C) in the upper 200–300 m, consistent with the decadal post-bomb decline in atmospheric 14C levels. A strong positive anomaly (increase with time), centered on the low salinity core of the Antarctic Intermediate Water (AAIW), approaches 50–60‰ in Δ14C, a clear signature of the downstream evolution of the 14C transient in this water mass. We use this observation to estimate the transit time of AAIW from its “source region” in the southeast South Pacific and to compute the effective reservoir age of this water mass. The 2 sections show small but significant changes in the abyssal 14C distributions. Between 1991 and 2005, Δ14C has increased by 9‰ below 2000 m north of 55°S. This change is accompanied overall by a modest increase in salinity and dissolved oxygen, as well as a slight decrease in dissolved silica. Such changes are indicative of greater ventilation. Calculation of “phosphate star” also indicates that this may be due to a shift from the Southern Ocean toward North Atlantic Deep Water as the ventilation source of the abyssal South Pacific.
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Doney, Scott C., e William J. Jenkins. "Ventilation of the Deep Western Boundary Current and Abyssal Western North Atlantic: Estimates from Tritium and3He Distributions". Journal of Physical Oceanography 24, n.º 3 (março de 1994): 638–59. http://dx.doi.org/10.1175/1520-0485(1994)024<0638:votdwb>2.0.co;2.
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Galbraith, Eric D., e SL Jaccard. "An increase in the ventilation of the abyssal North Pacific Ocean at the end of the last ice age". PAGES news 16, n.º 1 (janeiro de 2008): 13–14. http://dx.doi.org/10.22498/pages.16.1.13.
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Babagoli Matikolaei, Javad, Abbasali Aliakbari Bidokhti e Maryam Shiea. "Some aspects of the deep abyssal overflow between the middle and southern basins of the Caspian Sea". Ocean Science 15, n.º 2 (29 de abril de 2019): 459–76. http://dx.doi.org/10.5194/os-15-459-2019.
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Abstract. The present study investigates the deep gravity current between the middle and southern Caspian Sea basins caused by the density difference of deep waters. Oceanographic data, a numerical model and a dynamic model are used to consider the structure of this Caspian Sea abyssal overflow. The CTD data are obtained from UNESCO, and the three-dimensional COHERENS ocean model results are used to study the abyssal currents in the southern basin of the Caspian Sea. The deep overflow is driven by the density difference, which is mainly owing to the temperature difference, between the middle and southern basins, especially in winter. Due to cold weather in the northern basin, water sinks at high latitudes and after filling the middle basin it overflows into the southern basin. As the current passes through the Absheron Strait (or sill), we use the analytic model of Falcini and Salusti (2015) for the overflow gravity current to estimate the changes in the vorticity and potential vorticity of the flow over the Absheron sill; the effects of entrainment and friction are also considered. Due to the importance of the overflow with respect to deep water ventilation, a simple dynamical model of the boundary currents based on the shape of the Absheron Strait is used to estimate typical mass transport and flushing time; the flushing time is found to be about 15 to 20 years for the southern basin of the Caspian Sea. This timescale is important for the region's ecosystem and with respect to the impacts of pollution due to oil exploration. In addition, by reviewing the drilled oil and gas wells in the Caspian Sea, the results show that the deep overflow moves over some of these wells. Thus, the deep flow could be an important factor influencing oil pollution in the deeper region of the southern Caspian Sea.
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JACOBS, STANLEY S. "Bottom water production and its links with the thermohaline circulation". Antarctic Science 16, n.º 4 (30 de novembro de 2004): 427–37. http://dx.doi.org/10.1017/s095410200400224x.
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For more than a century it has been known that the abyssal basins of the world ocean are primarily occupied by relatively cold and fresh waters that originate in the Southern Ocean. Their distinguishing characteristics are acquired by exposure of surface and shelf waters to ‘ventilation’ by the polar atmosphere and to the melting and freezing of ice over and near the Antarctic continental shelf. Subsequent mixing with deep water over the continental slope results in ‘Bottom Water’ that forms the southern sinking limb of the global ‘Thermohaline Circulation.’ Over recent decades, oceanographers have wrestled with a variety of bottom water and thermohaline circulation problems, ranging from basic definitions to forcing and formation sites, source components and properties, generation processes and rates, mixing and sinking, pathways and transports. A brief review of these efforts indicates both advances and anomalies in our understanding of Antarctic Bottom Water production and circulation. Examples from ongoing work illustrate increasing interest in the temporal variability of bottom water in relation to climate change.
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Meredith, Michael P., Alberto C. Naveira Garabato, Arnold L. Gordon e Gregory C. Johnson. "Evolution of the Deep and Bottom Waters of the Scotia Sea, Southern Ocean, during 1995–2005*". Journal of Climate 21, n.º 13 (1 de julho de 2008): 3327–43. http://dx.doi.org/10.1175/2007jcli2238.1.
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Abstract The Southern Ocean hosts the formation of the densest layers of the oceanic overturning circulation and provides a climatically sensitive element of deep ocean ventilation. An oceanographic section across the eastern Scotia Sea occupied in 1995, 1999, and 2005 reveals significant variability in the deep and bottom waters of Southern Ocean origin. Warming (∼0.1°C) of the warm midlayer waters in the Scotia Sea between 1995 and 1999 reversed through to 2005, reflecting changes seen earlier upstream in the Weddell Sea. The volume of deep waters with potential temperature less than 0°C decreased during 1995–2005, though such a reduction was only clear between 1995 and 1999 at the southern end of the section. The abyssal waters of the eastern Scotia Sea changed circulation between 1995 and 1999, with the dominant point of their entry to the basin shifting from the south to the northeast; by 2005, the former route had regained dominance. These changes are best explained by interannual variations in the deep waters exiting the Weddell Sea, superimposed on a longer-term (decadal) warming trend. The interannual variations are related to changes in the strength of the Weddell Gyre, reflecting large-scale atmospheric variability that may include the El Niño–Southern Oscillation phenomenon. The Scotia Sea is the most direct pathway for dense waters of the overturning circulation emanating from the Weddell Sea to fill much of the World Ocean abyss. The regional changes reported here have the potential to affect the climatically significant ventilation of the global ocean abyss.
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Sarnthein, Michael, Kevin Küssner, Pieter M. Grootes, Blanca Ausin, Timothy Eglinton, Juan Muglia, Raimund Muscheler e Gordon Schlolaut. "Plateaus and jumps in the atmospheric radiocarbon record – potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis". Climate of the Past 16, n.º 6 (23 de dezembro de 2020): 2547–71. http://dx.doi.org/10.5194/cp-16-2547-2020.
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Abstract. Changes in the geometry of ocean meridional overturning circulation (MOC) are crucial in controlling past changes of climate and the carbon inventory of the atmosphere. However, the accurate timing and global correlation of short-term glacial-to-deglacial changes of MOC in different ocean basins still present a major challenge. The fine structure of jumps and plateaus in atmospheric and planktic radiocarbon (14C) concentration reflects changes in atmospheric 14C production, ocean–atmosphere 14C exchange, and ocean mixing. Plateau boundaries in the atmospheric 14C record of Lake Suigetsu, now tied to Hulu Cave U∕Th model ages instead of optical varve counts, provide a stratigraphic “rung ladder” of up to 30 age tie points from 29 to 10 cal ka for accurate dating of planktic oceanic 14C records. The age differences between contemporary planktic and atmospheric 14C plateaus record the global distribution of 14C reservoir ages for surface waters of the Last Glacial Maximum (LGM) and deglacial Heinrich Stadial 1 (HS-1), as documented in 19 and 20 planktic 14C records, respectively. Elevated and variable reservoir ages mark both upwelling regions and high-latitude sites covered by sea ice and/or meltwater. 14C ventilation ages of LGM deep waters reveal opposed geometries of Atlantic and Pacific MOC. Like today, Atlantic deep-water formation went along with an estuarine inflow of old abyssal waters from the Southern Ocean up to the northern North Pacific and an outflow of upper deep waters. During early HS-1, 14C ventilation ages suggest a reversed MOC and ∼1500-year flushing of the deep North Pacific up to the South China Sea, when estuarine circulation geometry marked the North Atlantic, gradually starting near 19 ka. High 14C ventilation ages of LGM deep waters reflect a major drawdown of carbon from the atmosphere. The subsequent major deglacial age drop reflects changes in MOC accompanied by massive carbon releases to the atmosphere as recorded in Antarctic ice cores. These new features of MOC and the carbon cycle provide detailed evidence in space and time to test and refine ocean models that, in part because of insufficient spatial model resolution and reference data, still poorly reproduce our data sets.
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Sarnthein, M., B. Schneider e P. M. Grootes. "Peak glacial <sup>14</sup>C ventilation ages suggest major draw-down of carbon into the abyssal ocean". Climate of the Past 9, n.º 6 (15 de novembro de 2013): 2595–614. http://dx.doi.org/10.5194/cp-9-2595-2013.
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Abstract. Ice core records demonstrate a glacial–interglacial atmospheric CO2 increase of ~ 100 ppm, while 14C calibration efforts document a strong decrease in atmospheric 14C concentration during this period. A calculated transfer of ~ 530 Gt of 14C-depleted carbon is required to produce the deglacial coeval rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide Δ14C data set showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the maximum 14C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14C yr, in the extreme reaching 5100 14C yr. Below 2000 m depth the 14C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We propose as a working hypothesis that the modern regression of DIC vs. Δ14C also applies for LGM times, which implies that a mean LGM aging of ~ 600 14C yr corresponded to a global rise of ~ 85–115 μmol DIC kg−1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters may indeed have made it possible to absorb an additional ~ 730–980 Gt DIC, one third of which possibly originated from intermediate waters. We also infer that LGM deep-water O2 dropped to suboxic values of < 10 μmol kg−1 in the Atlantic sector of the Southern Ocean, possibly also in the subpolar North Pacific. The deglacial transfer of the extra-aged, deep-ocean carbon to the atmosphere via the dynamic ocean–atmosphere carbon exchange would be sufficient to account for two trends observed, (1) for the increase in atmospheric CO2 and (2) for the 190‰ drop in atmospheric Δ14C during the so-called HS-1 "Mystery Interval", when atmospheric 14C production rates were largely constant.
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Sarnthein, M., B. Schneider e P. M. Grootes. "Peak glacial <sup>14</sup>C ventilation ages suggest major draw-down of carbon into the abyssal ocean". Climate of the Past Discussions 9, n.º 1 (13 de fevereiro de 2013): 925–65. http://dx.doi.org/10.5194/cpd-9-925-2013.
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Abstract. Ice core records demonstrate a glacial-interglacial atmospheric CO2 increase of ~ 100 ppm. A transfer of ~ 530 Gt C is required to produce the deglacial rise of carbon in the atmosphere and terrestrial biosphere. This amount is usually ascribed to oceanic carbon release, although the actual mechanisms remained elusive, since an adequately old and carbon-enriched deep-ocean reservoir seemed unlikely. Here we present a new, though still fragmentary, ocean-wide 14C dataset showing that during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS-1) the 14C age difference between ocean deep waters and the atmosphere exceeded the modern values by up to 1500 14C yr, in the extreme reaching 5100 yr. Below 2000 m depth the 14C ventilation age of modern ocean waters is directly linked to the concentration of dissolved inorganic carbon (DIC). We assume that the range of regression slopes of DIC vs. Δ14C remained constant for LGM times, which implies that an average LGM aging by ~ 600 14C yr corresponded to a global rise by ~ 85–115 μmol DIC kg−1 in the deep ocean. Thus, the prolonged residence time of ocean deep waters indeed made it possible to absorb an additional ~ 730–980 Gt DIC, ~ 1/3 of which transferred from intermediate waters. We infer that LGM deep-water O2 dropped to suboxic values of < 10 μmol kg−1 in the Atlantic sector of the Southern ocean, possibly also in the subpolar North Pacific. The transfer of aged deep-ocean carbon to the atmosphere and the ocean-atmosphere exchange are sufficient to account for the 190-‰ drop in atmospheric 14C during the so-called HS-1 "Mystery Interval".
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Yi, Liang, Haifeng Wang, Xiguang Deng, Haifan Yuan, Dong Xu e Huiqiang Yao. "Geochronology and Geochemical Properties of Mid-Pleistocene Sediments on the Caiwei Guyot in the Northwest Pacific Imply a Surface-to-Deep Linkage". Journal of Marine Science and Engineering 9, n.º 3 (27 de fevereiro de 2021): 253. http://dx.doi.org/10.3390/jmse9030253.
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Seamounts are ubiquitous topographic units in the global ocean, and their effects on local circulation have attracted great research attention in physical oceanography; however, fewer relevant efforts were made on geological timescales in previous studies. The Caiwei (Pako) Guyot in the Magellan Seamounts of the western Pacific is a typical seamount and oceanographic characteristics have been well documented. In this study, we investigate a sediment core by geochronological and geochemical studies to reveal a topography-induce surface-to-bottom linkage. The principal results are as follows: (1) Two magnetozones are recognized in core MABC–11, which can be correlated to the Brunhes and Matuyama chrons; (2) Elements Ca, Si, Cl, K, Mn, Ti, and Fe are seven elements with high intensities by geochemical scanning; (3) Ca intensity can be tuned to global ice volume to refine the age model on glacial-interglacial timescales; (4) The averaged sediment accumulation rate is ~0.73 mm/kyr, agreeing with the estimate of the excess 230Th data in the upper part. Based on these results, a proxy of element Mn is derived, whose variability can be correlated with changes in global ice volume and deep-water masses on glacial-interglacial timescales. This record is also characterized by an evident 23-kyr cycle, highlighting a direct influence of solar insolation on deep-sea sedimentary processes. Overall, sedimentary archives of the Caiwei Guyot not only record an intensified abyssal ventilation during interglaciations in the western Pacific, but also provide a unique window for investigating the topography-induced linkage between the upper and bottom ocean on orbital timescales.
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Dunne, John P., Jasmin G. John, Alistair J. Adcroft, Stephen M. Griffies, Robert W. Hallberg, Elena Shevliakova, Ronald J. Stouffer et al. "GFDL’s ESM2 Global Coupled Climate–Carbon Earth System Models. Part I: Physical Formulation and Baseline Simulation Characteristics". Journal of Climate 25, n.º 19 (5 de abril de 2012): 6646–65. http://dx.doi.org/10.1175/jcli-d-11-00560.1.
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Abstract The physical climate formulation and simulation characteristics of two new global coupled carbon–climate Earth System Models, ESM2M and ESM2G, are described. These models demonstrate similar climate fidelity as the Geophysical Fluid Dynamics Laboratory’s previous Climate Model version 2.1 (CM2.1) while incorporating explicit and consistent carbon dynamics. The two models differ exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4p1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. Differences in the ocean mean state include the thermocline depth being relatively deep in ESM2M and relatively shallow in ESM2G compared to observations. The crucial role of ocean dynamics on climate variability is highlighted in El Niño–Southern Oscillation being overly strong in ESM2M and overly weak in ESM2G relative to observations. Thus, while ESM2G might better represent climate changes relating to total heat content variability given its lack of long-term drift, gyre circulation, and ventilation in the North Pacific, tropical Atlantic, and Indian Oceans, and depth structure in the overturning and abyssal flows, ESM2M might better represent climate changes relating to surface circulation given its superior surface temperature, salinity, and height patterns, tropical Pacific circulation and variability, and Southern Ocean dynamics. The overall assessment is that neither model is fundamentally superior to the other, and that both models achieve sufficient fidelity to allow meaningful climate and earth system modeling applications. This affords the ability to assess the role of ocean configuration on earth system interactions in the context of two state-of-the-art coupled carbon–climate models.
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Berlinsky, N. "Hydrological factors of formation of oxygen regime of inland seas". Ukrainian hydrometeorological journal, n.º 21 (20 de março de 2018): 90–97. http://dx.doi.org/10.31481/uhmj.21.2018.09.
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Introduction. The regularities of the formation and dynamics of the oxygen regime in inland seas - the Black Sea and Caspian, under the influence of natural and anthropogenic factors are estimated.
Purpose. One of the features of the Black Sea is the absence of dissolved oxygen in the water below 200 meters depth. A comparison is made between the hydrological conditions of the Black Sea and the Caspian, where the depths are also quite large, but the intensity of vertical exchange is different. In addition, it is necessary to distinguish the reasons for the formation of oxygen deficient in the seas. They can be natural and human-made origin as well. The paper presents an analysis of the cause-and-effect patterns of the development of pelagial hypoxia as the examples of the Caspian and the Black Sea.
Methods. The work is the review of the problem literature that allows an assessment of the cur-rent state of gas exchange of pelagic and abyssal marine basins.
Results. Three main formation factors of the features of the hydrological structure and processes responsible for the intensity of gas exchange in the pelagic zone are identified. So, in the Black Sea, saline waters come with the Lower Bosporus Current and flow down the slope, filling the deep layers of the Black Sea basin, forming a stable vertical stratification density, that limiting vertical gas exchange. The second factor is contributing to oxygen saturation of the lower layers during the process of winter vertical circulation that is mainly expressed in the northwestern part.
The third factor is the regime shift of 1976-1978 in the Black Sea as a decrease of the winter temperature and salinity in the surface layer that led to increasing of static stability. According to actively developed convection processes, covering the entire body of the Caspian Sea, in contrast to the Black Sea, there is an intensive exchange between the upper and the deep layers and the intensity of convection depends on the temperature regime of the year. In the Caspian Sea, the regime shift of 1976-1978 led to a twofold increase in the static stability of water below 100 m, the almost complete cessation of the ventilation of the deep waters of the (the process the slope cascading) and the extreme decrease in the concentrations of dissolved oxygen.
Conclusion. In the Black Sea, the formation of a stable of the density vertical stratification is due to the intrusion of saline dense waters of the Lower Bosporus Current, and the winter vertical circulation is expressed only in the northwestern part of the sea, which generally limits vertical gas exchange with the deep water. In the Caspian Sea, convective mixing plays a main role in the for-mation of the hydrological structure of water and the ventilation of the bottom layers. In cold winters in the northern Caspian, strong cooling, as well as salinity during ice formation, creates the conditions for the formation of waters with a density that allows them to flow down the slopes of the bottom along isopycnic surfaces and aerating the deep layers of the sea.
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Yi, Liang, Martín Medina-Elizalde, Liangcheng Tan, David B. Kemp, Yanzhen Li, Gunther Kletetschka, Qiang Xie et al. "Plio-Pleistocene deep-sea ventilation in the eastern Pacific and potential linkages with Northern Hemisphere glaciation". Science Advances 9, n.º 8 (24 de fevereiro de 2023). http://dx.doi.org/10.1126/sciadv.add1467.
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Antarctic bottom water (AABW) production is a key factor governing global ocean circulation, and the present disintegration of the Antarctic Ice Sheet slows it. However, its long-term variability has not been well documented. On the basis of high-resolution chemical scanning of a well-dated marine ferromanganese nodule from the eastern Pacific, we derive a record of abyssal ventilation spanning the past 4.7 million years and evaluate its linkage to AABW formation over this period. We find that abyssal ventilation was relatively weak in the early Pliocene and persistently intensified from 3.4 million years ago onward. Seven episodes of markedly reduced ocean ventilation indicative of AABW formation collapse are identified since the late Pliocene, which were accompanied by key stages of Northern Hemisphere glaciation. We suggest that the interpolar climate synchronization within these inferred seven collapse events may have intensified global glaciation by inducing poleward moisture transport in the Northern Hemisphere.
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Millet, Bruno, William R. Gray, Casimir de Lavergne e Didier M. Roche. "Oxygen isotope constraints on the ventilation of the modern and glacial Pacific". Climate Dynamics, 31 de agosto de 2023. http://dx.doi.org/10.1007/s00382-023-06910-8.
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AbstractChanges in Pacific tracer reservoirs and transports are thought to be central to the regulation of atmospheric CO2 on glacial–interglacial timescales. However, there are currently two contrasting views of the circulation of the modern Pacific; the classical view sees southern sourced abyssal waters upwelling to about 1.5 km depth before flowing southward, whereas the bathymetrically constrained view sees the mid-depths (1–2.5 km) largely isolated from the global overturning circulation and predominantly ventilated by diffusion. Furthermore, changes in the circulation of the Pacific under differing climate states remain poorly understood. Through both a modern and a Last Glacial Maximum (LGM) analysis focusing on oxygen isotopes in seawater and benthic foraminifera as conservative tracers, we show that isopycnal diffusion strongly influences the mid-depths of the Pacific. Diapycnal diffusion is most prominent in the subarctic Pacific, where an important return path of abyssal tracers to the surface is identified in the modern state. At the LGM we infer an expansion of North Pacific Intermediate Water, as well as increased layering of the deeper North Pacific which would weaken the return path of abyssal tracers. These proposed changes imply a likely increase in ocean carbon storage within the deep Pacific during the LGM relative to the Holocene.
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Gunn, Kathryn L., Stephen R. Rintoul, Matthew H. England e Melissa M. Bowen. "Recent reduced abyssal overturning and ventilation in the Australian Antarctic Basin". Nature Climate Change, 25 de maio de 2023. http://dx.doi.org/10.1038/s41558-023-01667-8.
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AbstractDense water formed near Antarctica, known as Antarctic bottom water (AABW), drives deep ocean circulation and supplies oxygen to the abyssal ocean. Observations show that AABW has freshened and contracted since the 1960s, yet the drivers of these changes and their impact remain uncertain. Here, using observations from the Australian Antarctic Basin, we show that AABW transport reduced by 4.0 Sv between 1994 and 2009, during a period of strong freshening on the continental shelf. An increase in shelf water salinity between 2009 and 2018, previously linked to transient climate variability, drove a partial recovery (2.2 Sv) of AABW transport. Over the full period (1994 to 2017), the net slowdown of −0.8 ± 0.5 Sv decade−1 thinned well-oxygenated layers, driving deoxygenation of −3 ± 2 μmol kg−1 decade−1. These findings demonstrate that freshening of Antarctic shelf waters weakens the lower limb of the abyssal overturning circulation and reduces deep ocean oxygen content.
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Shimada, Keishi, Yujiro Kitade, Shigeru Aoki, Kohei Mizobata, Lingqiao Cheng, Kunio T. Takahashi, Ryosuke Makabe, Jota Kanda e Tsuneo Odate. "Shoaling of abyssal ventilation in the Eastern Indian Sector of the Southern Ocean". Communications Earth & Environment 3, n.º 1 (19 de maio de 2022). http://dx.doi.org/10.1038/s43247-022-00445-2.
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AbstractAntarctic Bottom Water formation, which ventilates the abyssal layers of the Southern Ocean, is an integral component of the global ocean meridional overturning circulation. Considering evident freshening and density decreases in the source waters, widespread warming in the Southern Ocean suggests a weakening in the Antarctic Bottom Water supply. We demonstrate that the weakening is robust based on water mass warming in the deep and abyssal layers of the Australian-Antarctic Basin, which remained after removing the southward shift effect of the Southern Antarctic Circumpolar Current Front. Moreover, a decrease in apparent oxygen utilisation and reduced warming in the intermediate density layer below Circumpolar Deep Water extended further from the Australian-Antarctic Basin to the South Australian Basin. We suggest that a concurrent weakening in the densest portion and strengthening in the less dense portion shape the multi-basin change in the meridional overturning circulation that originates from the Southern Ocean.
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Hu, Qianwen, Xiaodong Huang, Qinbo Xu, Chun Zhou, Shoude Guan, Xing Xu, Wei Zhao, Qingxuan Yang e Jiwei Tian. "Parametric Subharmonic Instability of Diurnal Internal Tides in the Abyssal South China Sea". Journal of Physical Oceanography, 5 de outubro de 2022. http://dx.doi.org/10.1175/jpo-d-22-0020.1.
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Abstract Internal waves close to the seafloor of abyssal oceans are the key energy suppliers driving near-bottom mixing and the upwelling branches of meridional overturning circulation, but their spatiotemporal variability and intrinsic mechanisms remain largely unclear. In this study, measurements from 10 long-term moorings were used to investigate the internal wave activities in the abyssal South China Sea, which is an important upwelling zone. Strong near-inertial internal waves (NIWs) with current velocity pulses exceeding 5 cm s−1 were observed to dominate the near-bottom internal wave field at approximately 14°N. These abyssal NIWs were phase-coupled with diurnal internal tides (D1), and both displayed common seasonal variations that were larger in winter and summer, providing evidence of diurnal parametric subharmonic instability (PSI) near its critical latitudes (CLs). Emitted from the bottom, near-inertial kinetic energy rapidly decreased by one order of magnitude from depths of ~120 m to ~620 m above the bottom. Near rough topographies, the abyssal PSI was shifted poleward to approximately 14.8°N by negative relative vorticities of passing anticyclonic eddies or topographic Rossby waves. Compared with flat topography, PSI near rough topography was significantly promoted by topographic-localized strong D1 with high-mode structures, creating abyssal NIW bursts. Bottom-reaching shipboard conductivity-temperature-depth profiles revealed that the bottom mixed layers became much thicker when approaching CLs, suggesting that abyssal PSI potentially accelerates the ventilation and upwelling of bottom water. The observational results presented here illustrate notable spatiotemporal variations in abyssal NIWs regulated by PSI and call for consideration of PSI to better understand near-bottom mixing and upwelling.
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Holzer, Mark, Tim DeVries e Casimir de Lavergne. "Diffusion controls the ventilation of a Pacific Shadow Zone above abyssal overturning". Nature Communications 12, n.º 1 (16 de julho de 2021). http://dx.doi.org/10.1038/s41467-021-24648-x.
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AbstractMid-depth North Pacific waters are rich in nutrients and respired carbon accumulated over centuries. The rates and pathways with which these waters exchange with the surface ocean are uncertain, with divergent paradigms of the Pacific overturning: one envisions bottom waters upwelling to 1.5 km depth; the other confines overturning beneath a mid-depth Pacific shadow zone (PSZ) shielded from mean advection. Here global inverse modelling reveals a PSZ where mean ages exceed 1400 years with overturning beneath. The PSZ is supplied primarily by Antarctic and North-Atlantic ventilated waters diffusing from below and from the south. Half of PSZ waters re-surface in the Southern Ocean, a quarter in the subarctic Pacific. The abyssal North Pacific, despite strong overturning, has mean re-surfacing times also exceeding 1400 years because of diffusion into the overlying PSZ. These results imply that diffusive transports – distinct from overturning transports – are a leading control on Pacific nutrient and carbon storage.
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Marchal, Olivier, e Ning Zhao. "On the Estimation of Deep Atlantic Ventilation from Fossil Radiocarbon Records. I. Modern Reference Estimates". Journal of Physical Oceanography, 23 de março de 2021. http://dx.doi.org/10.1175/jpo-d-20-0153.1.
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AbstractRadiocarbon dates of fossil carbonates sampled from sediment cores and the seafloor have been used to infer that deep ocean ventilation during the last ice age was different from today. In this first of paired papers, the time-averaged abyssal circulation in the modern Atlantic is estimated by combining a hydrographic climatology, observational estimates of volume transports, Argo float velocities at 1000 m, radiocarbon data, and geostrophic dynamics. Different estimates of modern circulation, obtained from different prior assumptions about the abyssal flow and different errors in the geostrophic balance, are produced for use in a robust interpretation of fossil records in terms of deviations from the present-day flow, which is undertaken in the second paper.For all estimates, the meridional transport integrated zonally and averaged over a hemisphere, 〈Vk〉, is southward between 1000-4000 m in both hemispheres, northward between 4000-5000 m in the South Atlantic, and insignificant between 4000-5000 m in the North Atlantic. Estimates of 〈Vk〉 obtained from two distinct prior circulations - one based on a level of no motion at 4000 m and one based on Argo oat velocities at 1000 m - become statistically indistinguishable when Δ14C data are considered. The transport time scale, defined as τk = /〈Vk〉, where is the volume of the kth layer, is estimated to about a century between 1000-3000 m in both the South and North Atlantic, 124±9 yr (203±23 yr) between 3000-4000 m in the South (North) Atlantic, and 269±115 yr between 4000-5000 m in the South Atlantic.
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Pavia, Frank J., Shouyi Wang, Jennifer Middleton, Richard W. Murray e Robert F. Anderson. "Trace Metal Evidence for Deglacial Ventilation of the Abyssal Pacific and Southern Oceans". Paleoceanography and Paleoclimatology 36, n.º 9 (30 de agosto de 2021). http://dx.doi.org/10.1029/2021pa004226.
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Marchal, Olivier, e Ning Zhao. "On the Estimation of Deep Atlantic Ventilation from Fossil Radiocarbon Records. Part II. (In)consistency with Modern Estimates". Journal of Physical Oceanography, 7 de junho de 2021. http://dx.doi.org/10.1175/jpo-d-20-0314.1.
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AbstractMeasurements of radiocarbon concentration (Δ14C) in fossil biogenic carbonates have been interpreted as reecting a reduced ventilation of the deep Atlantic during the last ice age. Here we evaluate the (in)consistency of an updated compilation of fossil Δ14C data for the last deglaciation with the abyssal circulation in the modern Atlantic. A 14C transport equation, in which the mean velocity field is a modern field estimate and turbulent flux divergence is treated as a random fluctuation, is fitted to deglacial Δ14C records by using recursive weighted least-squares. This approach allows us to interpret the records in terms of deviations from the modern flow with due regard for uncertainties in the fossil data, the 14C transport equation, and its boundary conditions.We find that the majority of fit residuals could be explained by uncertainties in fossil Δ14C data, for two distinct estimates of the modern flow and of the error variance in the boundary conditions. Thus, most, not all, deglacial data appear consistent with present-day ventilation rates. From 20 to 32% of the residuals exceed in magnitude the published errors in the fossil data by a factor of two. Residuals below 4000 m in the western North Atlantic are all negative, suggesting that deglacial Δ14C values from this region are too low to be explained by modern ventilation. Whilst deep water ventilation appeared different from today at some locations, a larger database and a better understanding of error (co)variances are needed to make reliable paleoceanographic inferences from fossil Δ14C records.
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Raddatz, J., E. Beisel, M. Butzin, A. Schröder-Ritzrau, C. Betzler, R. Friedrich e N. Frank. "Variable ventilation ages in the equatorial Indian Ocean thermocline during the LGM". Scientific Reports 13, n.º 1 (13 de julho de 2023). http://dx.doi.org/10.1038/s41598-023-38388-z.
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AbstractVariations of atmospheric CO2 during the Pleistocene ice-ages have been associated with changes in the drawdown of carbon into the deep-sea. Modelling studies suggest that about one third of the glacial carbon drawdown may not be associated to the deep ocean, but to the thermocline or intermediate ocean. However, the carbon storage capacity of thermocline waters is still poorly constrained. Here we present paired 230Th/U and 14C measurements on scleractinian cold-water corals retrieved from ~ 450 m water depth off the Maldives in the Indian Ocean. Based on these measurements we calculate ∆14C, ∆∆14C and Benthic-Atmosphere (Batm) ages in order to understand the ventilation dynamics of the equatorial Indian Ocean thermocline during the Last Glacial Maximum (LGM). Our results demonstrate a radiocarbon depleted thermocline as low as -250 to -345‰ (∆∆14C), corresponding to ~ 500–2100 years (Batm) old waters at the LGM compared to ~ 380 years today. More broadly, we show that thermocline ventilation ages are one order of magnitude more variable than previously thought. Such a radiocarbon depleted thermocline can at least partly be explained by variable abyssal upwelling of deep-water masses with elevated respired carbon concentrations. Our results therefore have implications for radiocarbon-only based age models and imply that upper thermocline waters as shallow as 400 m depth can also contribute to some of the glacial carbon drawdown.
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Tang, Zheng, Tiegang Li, Zhifang Xiong, Haowen Dang e Jingteng Guo. "Covariation of Deep Antarctic Pacific Oxygenation and Atmospheric CO2 during the Last 770 kyr". Lithosphere 2022, Special 9 (22 de fevereiro de 2022). http://dx.doi.org/10.2113/2022/1835176.
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Abstract We present new geochemical evidence of changes in oxygenation of the deep Antarctic Pacific over the last 770 kyr. Our data are derived from redox-sensitive metals and export production proxies extracted from gravity core ANT34/A2-10 at 4217 m water depth. Our results show that oxygen levels in the deep Antarctic Zone (AZ) varied in line with the release of deeply sequestered remineralized carbon to the atmosphere during glacial–interglacial (G–IG) cycles, with lower oxygen concentrations and more carbon storage during glacial periods. Subsequent reductions in the amount of carbon stored at depth were closely associated with improved ventilation during glacial terminations. The systematic and repeated glacial-to-interglacial increases in export production in the AZ region indicate a robust pattern of enhanced Southern Ocean (SO) ventilation during interglacial periods. In addition to the decline in atmospheric CO2 caused by iron fertilization in the Subantarctic AZ (SAZ) during the latter half of the glacial progression, decreases in productivity in the central AZ suggest that the weakening of SO ventilation induced deep AZ carbon sequestration and that this might have made a continuous additional contribution to the CO2 decline from each interglacial peak to glacial maximum. Observed variations in the degree of deep oxygenation and “organic carbon pump” efficiency in the central AZ might be driven primarily by physical “ventilation” processes (i.e., overturning circulation, mixing, and/or air–sea gas exchange). Our records of abyssal oxygenation in the central AZ, which vary in concert with atmospheric CO2 levels over the last several G–IG cycles, provide strong evidence that SO ventilation plays a significant role in controlling variations in both the amount of respired carbon sequestered in the deep ocean and atmospheric CO2 concentrations on G–IG timescales. Specifically, we suggest that the “organic carbon pump” (OCP) in the SAZ and the physical ventilation processes in the AZ (the “carbon venting valve”) acted together synergistically, but dominated at different intervals over G–IG cycles, to repeatedly switch the SO between carbon sink and carbon source, thereby modulating the atmospheric CO2 over the last 770 kyr. These findings provide new insights into the role of the AZ in controlling deep SO carbon sequestration and atmospheric CO2 levels in G–IG cycles.
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Akhoudas, Camille Hayatte, Jean-Baptiste Sallée, F. Alexander Haumann, Michael P. Meredith, Alberto Naveira Garabato, Gilles Reverdin, Loïc Jullion et al. "Ventilation of the abyss in the Atlantic sector of the Southern Ocean". Scientific Reports 11, n.º 1 (24 de março de 2021). http://dx.doi.org/10.1038/s41598-021-86043-2.
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AbstractThe Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m$$^{-3}$$ - 3 $$\gamma _{n}$$ γ n is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.
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Waddell, Lindsey M., Ingrid L. Hendy, Theodore C. Moore e Mitchell W. Lyle. "Ventilation of the abyssal Southern Ocean during the late Neogene: A new perspective from the subantarctic Pacific". Paleoceanography 24, n.º 3 (19 de agosto de 2009). http://dx.doi.org/10.1029/2008pa001661.
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Zhang, Boer, Marianna Linz, Shantong Sun e Andrew F. Thompson. "A framework for constraining ocean mixing rates and overturning circulation from age tracers". Journal of Physical Oceanography, 2 de maio de 2024. http://dx.doi.org/10.1175/jpo-d-23-0162.1.
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Abstract The age of seawater refers to the amount of the time that has elapsed since that water encountered the surface. This age measures the ventilation rate of the ocean, and the spatial distribution of age can be influenced by multiple processes, such as the overturning circulation, ocean mixing, and air-sea exchange. In this work, we aim to gain new quantitative insights about how the ocean’s age tracer distribution reflects the strength of the meridional overturning circulation and diapycnal diffusivity. We propose an integral constraint that relates the age tracer flow across an isopycnal surface to the geometry of the surface. With the integral constraint, a relationship between the globally-averaged effective diapycnal diffusivity and the meridional overturning strength at an arbitrary density level can be inferred from the age tracer concentration near that level. The theory is tested in a set of idealized single-basin simulations. A key insight from this study is that the age difference between regions of upwelling and downwelling, rather than any single absolute age value, is the best indicator of overturning strength. The framework has also been adapted to estimate the strength of abyssal overturning circulation in the modern North Pacific, and we demonstrate that the age field provides an estimate of the circulation strength consistent with previous studies. This framework could potentially constrain ocean circulation and mixing rates from age-like realistic tracers (e.g., radiocarbon) in both past and present climates.
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Sallée, J. B., E. P. Abrahamsen, C. Allaigre, M. Auger, H. Ayres, R. Badhe, J. Boutin et al. "Southern ocean carbon and heat impact on climate". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 381, n.º 2249 (8 de maio de 2023). http://dx.doi.org/10.1098/rsta.2022.0056.
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The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.