Abyssal ocean overturning shaped by seafloor distribution

Lavergne, Casimir de; Madec, Gurvan; Roquet, Fabien; Holmes, R. M.; McDougall, Trevor J.

doi:10.1038/nature24472

Cited by 93 publications

(91 citation statements)

References 98 publications

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“…The increase in mixing with depth in the stratified interior is at least partially offset by the decrease in mixing area with depth (de Lavergne et al, ). In any case, this new conceptual picture is most likely to be relevant below 2,000 m where small‐scale mixing driven by internal wave breaking provides the dominant source of energy to lift water masses, and it is as yet unclear how important it is going to be for the AMOC.…”

Section: Concluding Discussionmentioning

confidence: 99%

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

et al. 2019

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Revolutionary observational arrays, together with a new generation of ocean and climate models, have provided new and intriguing insights into the Atlantic Meridional Overturning Circulation (AMOC) over the last two decades. Theoretical models have also changed our view of the AMOC, providing a dynamical framework for understanding the new observations and the results of complex models. In this paper we review recent advances in conceptual understanding of the processes maintaining the AMOC. We discuss recent theoretical models that address issues such as the interplay between surface buoyancy and wind forcing, the extent to which the AMOC is adiabatic, the importance of mesoscale eddies, the interaction between the middepth North Atlantic Deep Water cell and the abyssal Antarctic Bottom Water cell, the role of basin geometry and bathymetry, and the importance of a three‐dimensional multiple‐basin perspective. We review new paradigms for deep water formation in the high‐latitude North Atlantic and the impact of diapycnal mixing on vertical motion in the ocean interior. And we discuss advances in our understanding of the AMOC's stability and its scaling with large‐scale meridional density gradients. Along with reviewing theories for the mean AMOC, we consider models of AMOC variability and discuss what we have learned from theory about the detection and meridional propagation of AMOC anomalies. Simple theoretical models remain a vital and powerful tool for articulating our understanding of the AMOC and identifying the processes that are most critical to represent accurately in the next generation of numerical ocean and climate models.

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Section: Concluding Discussionmentioning

confidence: 99%

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

et al. 2019

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“…It may be valuable to undertake further analysis, such as for comparison with specific proxy sites, comparison with other models, exploration of the atmospheric dynamics, or consideration of the changes in ocean circulation according to existing theoretical frameworks (e.g. Gnanadesikan 1999; Wolfe and Cessi 2011;Nikurashin and Vallis 2012;de Lavergne et al 2017). To this end we will make all simulation output freely available.…”

Section: Discussionmentioning

confidence: 99%

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Galbraith

Lavergne

2018

Clim Dyn

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Over the past few million years, the Earth descended from the relatively warm and stable climate of the Pliocene into the increasingly dramatic ice age cycles of the Pleistocene. The influences of orbital forcing and atmospheric CO 2 on landbased ice sheets have long been considered as the key drivers of the ice ages, but less attention has been paid to their direct influences on the circulation of the deep ocean. Here we provide a broad view on the influences of CO 2 , orbital forcing and ice sheet size according to a comprehensive Earth system model, by integrating the model to equilibrium under 40 different combinations of the three external forcings. We find that the volume contribution of Antarctic (AABW) vs. North Atlantic (NADW) waters to the deep ocean varies widely among the simulations, and can be predicted from the difference between the surface densities at AABW and NADW deep water formation sites. Minima of both the AABW-NADW density difference and the AABW volume occur near interglacial CO 2 (270-400 ppm). At low CO 2 , abundant formation and northward export of sea ice in the Southern Ocean contributes to very salty and dense Antarctic waters that dominate the global deep ocean. Furthermore, when the Earth is cold, low obliquity (i.e. a reduced tilt of Earth's rotational axis) enhances the Antarctic water volume by expanding sea ice further. At high CO 2 , AABW dominance is favoured due to relatively warm subpolar North Atlantic waters, with more dependence on precession. Meanwhile, a large Laurentide ice sheet steers atmospheric circulation as to strengthen the Atlantic Meridional Overturning Circulation, but cools the Southern Ocean remotely, enhancing Antarctic sea ice export and leading to very salty and expanded AABW. Together, these results suggest that a 'sweet spot' of low CO 2 , low obliquity and relatively small ice sheets would have poised the AMOC for interruption, promoting Dansgaard-Oeschger-type abrupt change. The deep ocean temperature and salinity simulated under the most representative 'glacial' state agree very well with reconstructions from the Last Glacial Maximum (LGM), which lends confidence in the ability of the model to estimate large-scale changes in water-mass geometry. The model also simulates a circulation-driven increase of preformed radiocarbon reservoir age, which could explain most of the reconstructed LGM-preindustrial ocean radiocarbon change. However, the radiocarbon content of the simulated glacial ocean is still higher than reconstructed for the LGM, and the model does not reproduce reconstructed LGM deep ocean oxygen depletions. These ventilation-related disagreements probably reflect unresolved physical aspects of ventilation and ecosystem processes, but also raise the possibility that the LGM ocean circulation was not in equilibrium. Finally, the simulations display an increased sensitivity of both surface air temperature and AABW volume to orbital forcing under low CO 2 . We suggest that this enhanced orbital sensitivity contributed to the developm...

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“…We also note an acute increase (+~40 μmol/kg) of DIC along the western fringe of the Atlantic and Pacific Oceans. A weaker but more spatially extended DIC increase of ~10 μmol/kg in the southern half of the globe is also predicted, which can be attributed partly to the northward spreading of the dense and young AABW (de Lavergne et al, ; Gebbie & Huybers, ). The elongated positive DIC anomaly in the low latitudes of the Pacific Ocean (i.e., between Peru and Melanesia) is driven by an accumulation of metabolic CO 2 , as an increase in both the age of bottom waters (see Figure S2) and the particulate organic carbon sinking flux (see Figure S3) is predicted in this area.…”

Section: Resultsmentioning

confidence: 88%

Reduced CaCO₃Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO₃Dissolution Over the 21st Century

Sulpis

Dufour

Trossman

et al. 2019

Global Biogeochemical Cycles

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Results from a range of Earth System and climate models of various resolution run under high‐CO2 emission scenarios challenge the paradigm that seafloor CaCO3 dissolution will grow in extent and intensify as ocean acidification develops over the next century. Under the “business as usual,” RCP8.5 scenario, CaCO3 dissolution increases in some areas of the deep ocean, such as the eastern central Pacific Ocean, but is projected to decrease in the Northern Pacific and abyssal Atlantic Ocean by the year 2100. The flux of CaCO3 to the seafloor and bottom‐current speeds, both of which are expected to decrease globally through the 21st century, govern changes in benthic CaCO3 dissolution rates over 53% and 31% of the dissolving seafloor, respectively. Below the calcite compensation depth, a reduced CaCO3 flux to the CaCO3‐free seabed modulates the amount of CaCO3 material dissolved at the sediment‐water interface. Slower bottom‐water circulation leads to thicker diffusive boundary layers above the sediment bed and a consequent stronger transport barrier to CaCO3 dissolution. While all investigated models predict a weakening of bottom current speeds over most of the seafloor by the end of the 21st century, strong discrepancies exist in the magnitude of the predicted speeds. Overall, the poor performance of most models in reproducing modern bottom‐water velocities and CaCO3 rain rates coupled with the existence of large disparities in predicted bottom‐water chemistry across models hampers our ability to robustly estimate the magnitude and temporal evolution of anthropogenic CaCO3 dissolution rates and the associated anthropogenic CO2 neutralization.

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Abyssal ocean overturning shaped by seafloor distribution

Cited by 93 publications

References 98 publications

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Reduced CaCO₃Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO₃Dissolution Over the 21st Century

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Abyssal ocean overturning shaped by seafloor distribution

Cited by 93 publications

References 98 publications

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

Reduced CaCO3Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO3Dissolution Over the 21st Century

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Reduced CaCO₃Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO₃Dissolution Over the 21st Century