Glacial Atlantic overturning increased by wind stress in climate models

Muglia, Juan; Schmittner, Andreas

doi:10.1002/2015gl064583

Cited by 119 publications

(188 citation statements)

References 35 publications

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“…The PMIP3 ice sheets induced a strengthening and shoaling of the NADW cell in CCSM4 (compare experiments LGM and LGMCO 2 in Brady et al, 2013). In a recent study, Muglia and Schmittner (2015) found that applying glacial wind stress anomalies from the PMIP3 ensemble in the UVic model led to an increased northward salt transport, enhanced overturning and a deeper NADW cell, which is consistent with the ice-sheet effect in MPI-ESM. The sign of the GHG effect appears more robust.…”

Section: Overturningsupporting

confidence: 62%

“…This inter-model spread was reduced in PMIP3, with most models simulating an increase in NADW strength and a deepening of the NADW cell (Muglia and Schmittner, 2015). The large inter-model spread in PMIP2 and the fact that most models fail to simulate the shoaling of the NADW cell in PMIP3 suggests that there is still a lack of understanding of the mechanisms that determine the glacial AMOC state.…”

Section: Introductionmentioning

confidence: 99%

“…This response is quite common for models that have participated in PMIP3. All models simulate a stronger AMOC and all models but one simulate either a deepening or no change of the NADW cell depth (Muglia and Schmittner, 2015).…”

Section: Total Totalmentioning

confidence: 99%

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The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model

2016

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Abstract. Simulations with the Max Planck Institute EarthSystem Model (MPI-ESM) are used to study the sensitivity of the AMOC and the deep-ocean water masses during the Last Glacial Maximum to different sets of forcings. Analysing the individual contributions of the glacial forcings reveals that the ice sheets cause an increase in the overturning strength and a deepening of the North Atlantic Deep Water (NADW) cell, while the low greenhouse gas (GHG) concentrations cause a decrease in overturning strength and a shoaling of the NADW cell. The effect of the orbital configuration is negligible. The effects of the ice sheets and the GHG reduction balance each other in the deep ocean so that no shoaling of the NADW cell is simulated in the full glacial state. Experiments in which different GHG concentrations with linearly decreasing radiative forcing are applied to a setup with glacial ice sheets and orbital configuration show that GHG concentrations below the glacial level are necessary to cause a shoaling of the NADW cell with respect to the pre-industrial state in MPI-ESM. For a pCO 2 of 149 ppm, the simulated overturning state and the deep-ocean water masses are in best agreement with the glacial state inferred from proxy data. Sensitivity studies confirm that brine release and shelf convection in the Southern Ocean are key processes for the shoaling of the NADW cell. Shoaling occurs only when Southern Ocean shelf water contributes significantly to the formation of Antarctic Bottom Water.

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Section: Overturningsupporting

confidence: 62%

Section: Introductionmentioning

confidence: 99%

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The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model

2016

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“…Changes in freshwater fluxes around Antarctica have also been argued to explain the high deep ocean stratification in a decoupled Community Earth System Model (CESM)1.1.2 ocean simulation under LGM boundary conditions (10). Many other coupled climate models, however, show different and widely diverging changes in the deep ocean circulation and stratification between preindustrial and LGM simulations (25,26). Possible reasons for this disagreement include differences between transient and fully equilibrated solutions (27,28), insufficient increase in Antarctic sea-ice cover and formation (29), and/or compensating effects due to other differences in the boundary conditions, such as changes in the ice sheets (26,30).…”

Section: Discussionmentioning

confidence: 99%

“…Many other coupled climate models, however, show different and widely diverging changes in the deep ocean circulation and stratification between preindustrial and LGM simulations (25,26). Possible reasons for this disagreement include differences between transient and fully equilibrated solutions (27,28), insufficient increase in Antarctic sea-ice cover and formation (29), and/or compensating effects due to other differences in the boundary conditions, such as changes in the ice sheets (26,30). A more detailed analysis of comprehensive LGM climate simulations is needed to better understand differences between them, but is beyond the scope of this study.…”

Section: Discussionmentioning

confidence: 99%

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Jansen

2016

Proc. Natl. Acad. Sci. U.S.A.

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Earth's climate has undergone dramatic shifts between glacial and interglacial time periods, with high-latitude temperature changes on the order of 5-10 • C. These climatic shifts have been associated with major rearrangements in the deep ocean circulation and stratification, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and the ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of glacial climates. This study shows that various inferred changes in the deep ocean circulation and stratification between glacial and interglacial climates can be interpreted as a direct consequence of atmospheric temperature differences. Colder atmospheric temperatures lead to increased sea ice cover and formation rate around Antarctica. The associated enhanced brine rejection leads to a strongly increased deep ocean stratification, consistent with high abyssal salinities inferred for the last glacial maximum. The increased stratification goes together with a weakening and shoaling of the interhemispheric overturning circulation, again consistent with proxy evidence for the last glacial. The shallower interhemispheric overturning circulation makes room for slowly moving water of Antarctic origin, which explains the observed middepth radiocarbon age maximum and may play an important role in ocean carbon storage.LGM | AMOC | stratification | cooling | sea-ice T he deep ocean today is ventilated mainly by two water masses. North Atlantic deep water (NADW) is formed in the North Atlantic before flowing southward at a depth of about 2-3 km and eventually rising back up to the surface in the Southern Ocean. Antarctic bottom water (AABW) is formed around Antarctica and spreads northward into the abyssal basins. The AABW that makes it into the Atlantic then slowly upwells into the lower NADW before returning southward and resurfacing again around Antarctica (1, 2). The glacial equivalent of NADW was likely confined to shallower depths, leaving more of the deep Atlantic filled with water masses originating primarily from around Antarctica (3-5). Moreover, the two water masses appear more distinct, with less mixing between them (6).Multiple studies have pointed toward the potential importance of sea ice and surface buoyancy fluxes around Antarctica in controlling changes in the deep ocean stratification and circulation between the present and Last Glacial Maximum (LGM) (7-11). Specifically, we recently showed that enhanced buoyancy loss around Antarctica is expected to lead to an increase in the abyssal stratification, an upward shift of NADW, and a clearer separation between NADW and southward-flowing AABW (11)-all in agreement with inferences made for differences in circulation and stratification between the present and LGM (3,6,12,13). This gives rise to the hypothesis that strong cooling around Antarctica led to an increased net freezing...

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Dynamical reconstruction of the global ocean state during the Last Glacial Maximum

2017

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The global ocean state for the modern age and for the Last Glacial Maximum (LGM) was dynamically reconstructed with a sophisticated data assimilation technique. A substantial amount of data including global seawater temperature, salinity (only for the modern estimate), and the isotopic composition of oxygen and carbon (only in the Atlantic for the LGM) were integrated into an ocean general circulation model with the help of the adjoint method, thereby the model was optimized to reconstruct plausible continuous fields of tracers, overturning circulation and water mass distribution. The adjoint‐based LGM state estimation of this study represents the state of the art in terms of the length of forward model runs, the number of observations assimilated, and the model domain. Compared to the modern state, the reconstructed continuous sea‐surface temperature field for the LGM shows a global‐mean cooling of 2.2 K, and the reconstructed LGM ocean has a more vigorous Atlantic meridional overturning circulation, shallower North Atlantic Deep Water (NADW) equivalent, stronger stratification, and more saline deep water.

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Glacial Atlantic overturning increased by wind stress in climate models

Cited by 119 publications

References 35 publications

The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model

The effect of greenhouse gas concentrations and ice sheets on the glacial AMOC in a coupled climate model

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Dynamical reconstruction of the global ocean state during the Last Glacial Maximum

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