Letteratura scientifica selezionata sul tema "Ventilation abyssale"

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.

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.

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.

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.

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.

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.

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.

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.

L’océan Austral est un acteur central de la circulation océanique mondiale et du système climatique terrestre. Malgré l’essor des observations in situ dans cette région reculée du globe depuis les années 1990 (avec notamment le début de « l’ère satellitaire » et des grands programmes internationaux d’observations tel que WOCE, CLIVAR, GO-SHIP, ou ARGO), ce vaste océan reste encore aujourd’hui largement méconnu. Il est pourtant nécessaire de parvenir à mieux observer et comprendre les mécanismes de sa dynamique océanique ainsi que sa variabilité afin de prédire au mieux l’évolution future du système climatique. Notamment, une des particularités qui rend l’océan Austral essentiel dans le système climatique est qu’il est l’un des principaux lieux de ventilation de l’océan profond, qui permet une redistribution et un stockage de chaleur, d’eau douce, de carbone, d’oxygène, et de nutriments, entre autres. Cette ventilation est en partie dirigée par une circulation verticale unique connectant la surface aux abysses océaniques, mise en mouvement par les intenses interactions et échanges de flux d’énergie et de flottabilité entre atmosphère, océan et cryosphère. Je me penche dans cette thèse sur certains aspects de la dynamique australe en m’efforçant d’apporter une vue mécanistique de la circulation grande échelle et des changements en cours. Un fil rouge méthodologique que j’emploie sur l’ensemble de cette thèse est l’utilisation d’observations des isotopes stables de l’eau, traceur passif utilisé couramment dans un grand nombre de disciplines des sciences de la terre, mais jusque récemment assez peu en océanographie physique. La mesure des isotopes de l’eau constitue un outil, qui en tant que traceurs de l’origine de l’eau, permet de mieux caractériser les différentes composantes du cycle hydrologique ainsi que son évolution. En particulier, la composition isotopique de l’eau de mer représente une empreinte importante des masses d’eau, contenant des informations sur les conditions de leur formation et leur évolution. Dans cette thèse, au-delà du travail méthodologique important sur le terrain et en laboratoire pour l’échantillonnage, l’analyse et la calibration des mesures isotopiques, j’utilise les isotopes de l’eau en combinaison avec d’autres traceurs plus conventionnels pour aborder avec un nouveau regard, les questions du rôle des interactions entre océan et calotte polaire à la circulation grande échelle, de la signature des eaux de surface dans les abysses, ou encore de l’impact des changements de régimes atmosphériques ou de fonte de la cryosphère sur l’océan de surface. Au-delà de la seule utilisation des isotopes stables de l’eau, les approches que j’ai mises en place m’ont permis de documenter la quantité de fonte et de regel d’une des plus grandes cavités glaciaires au monde, qui influence les caractéristiques des masses d’eau denses, précurseurs des eaux abyssales se formant en mer de Weddell. Mes résultats mettent également à jour la proportion que représente, in fine, ces eaux denses dans la production des eaux abyssales dans le secteur Atlantique de l’océan Austral. Je détaille les processus qui mènent à la formation des eaux abyssales et avec cette nouvelle force, je montre que des estimations passées de la production d’eaux abyssales en apparente contradiction, s’attaquaient en réalité à différents processus. Finalement, je quantifie les changements des apports en eau douce lors des trois dernières décennies sur les tendances des propriétés de surface dans le secteur Indien de l’océan Austral. Mes résultats démontrent que des changements dans le régime des précipitations expliquent les changements des caractéristiques de l’océan surface affectant la stratification avec des conséquences sur la formation des masses d’eau et la circulation de retournement de l’océan Austral à grande échelle
The Southern Ocean is a key component in global ocean circulation and the Earth's climate system. Despite the increase of in situ observations in this remote region since the 1990s (notably with the « satellite era » and major international observation programs such as WOCE, CLIVAR, GO-SHIP, or ARGO), this immense ocean remains largely unknown. However, it is essential to observe and understand the mechanisms of its dynamics as well as its variability with the aim to predict the future evolution of the climate system. In particular, one important characteristic of the Southern Ocean is that it is one of the main sites of deep ocean ventilation, which allows redistribution and sequestration of heat, freshwater, carbon, oxygen, and nutrients. This ventilation process is mainly associated with a vertical circulation connecting the ocean surface to the abyss, fueled by intense interactions and exchanges of energy and buoyancy fluxes between atmosphere, ocean and cryosphere. In this thesis, I apprehend some aspects of the Southern Ocean dynamics by providing a mechanistic view of large-scale circulation and its ongoing changes. The approach I use throughout this thesis is based on observations of stable water isotopes, a passive tracer commonly used in a large number of earth science disciplines, but until recently only sparsely used in physical oceanography. Stable water isotopes constitute a robust tool which, as a tracer of the origin of water, help to better characterize the different components of the hydrological cycle as well as its evolution. In particular, the isotopic composition of seawater represents an important imprint of water masses, containing information on the conditions of their formation and their evolution. In this thesis, beyond the important methodological work at sea and in the laboratory for the sampling, analysis and calibration of isotopic measurements, I use the stable water isotopes in combination with other more conventional tracers to apprehend, with a new perspective, the questions of the role of interactions between the Southern Ocean and the Antarctic Ice Sheet in large-scale circulation, the signature of surface waters in the abyss, or even the impact of changes in atmospheric or cryosphere regimes on the surface ocean. Beyond the only use of stable water isotopes, original approaches have allowed me to document melting and refreezing of one of the largest ice shelves in the world, which influences the characteristics of the dense waters, precursors of abyssal waters produced in the Weddell Sea. My results also reveal the proportion of these dense waters in bottom water formation in the Atlantic sector of the Southern Ocean. We detail the processes that lead to the formation of bottom waters and with this new insight, we demonstrate that past estimates of bottom water production, in apparent contradiction, were actually focusing on different processes. Finally, I propose to quantify the changes in freshwater inputs over the past three decades that influence the trends in surface properties in the Indian sector of the Southern Ocean. The results demonstrate that changes in the precipitation regime explain changes in the surface ocean characteristics impacting stratification with consequences for large-scale water mass formation and overturning circulation in the Southern Ocean