Coherent deglacial changes in western Atlantic Ocean circulation

Ng, Hong Chin; Robinson, Laura F.; McManus, Jerry F; Mohamed, K.; Jacobel, Allison W; Ivanovic, Ruza; Gregoire, Lauren; Chen, Tianyu

doi:10.1038/s41467-018-05312-3

Cited by 135 publications

(171 citation statements)

References 80 publications

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“…However, high‐resolution proxies collectively suggest Heinrich events are unlikely the trigger, and the AMOC shutoff process is far more complicated than previously thought. Instead, in support of our framework, the AMOC transition from warm on to off is likely a two‐step process (Ng et al, ): in response to gradual cooling associated with atmospheric CO 2 decrease (Barker et al, ), an early slowdown of circulation may have occurred. It was accompanied by an early external freshwater input (Chen et al, ; Ménot et al, ) mostly likely from Eurasian ice sheets (Grousset et al, ; Ng et al, ; Peck et al, ).…”

Section: Implications Of the Amoc Stability Paradigm For The Past CLIsupporting

confidence: 68%

“…Instead, in support of our framework, the AMOC transition from warm on to off is likely a two‐step process (Ng et al, ): in response to gradual cooling associated with atmospheric CO 2 decrease (Barker et al, ), an early slowdown of circulation may have occurred. It was accompanied by an early external freshwater input (Chen et al, ; Ménot et al, ) mostly likely from Eurasian ice sheets (Grousset et al, ; Ng et al, ; Peck et al, ). As a second step, an off state persisted for quite a few hundreds to a thousand years, with substantial ice rafting events from the LIS (Heinrich events) as a consequence (Barker et al, ; Hodell et al, ; Marcott et al, ).…”

Section: Implications Of the Amoc Stability Paradigm For The Past CLIsupporting

confidence: 68%

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Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

et al. 2019

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The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC—or a permanent change in its forcing—could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present‐day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.

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Section: Implications Of the Amoc Stability Paradigm For The Past CLIsupporting

confidence: 68%

Section: Implications Of the Amoc Stability Paradigm For The Past CLIsupporting

confidence: 68%

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

et al. 2019

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“…This sets the stage for the beginning of H1 (17.5–14.7 ka BP) during which we add a freshwater forcing of 0.075 Sv (1 Sv = 10 6 m 3 s −1 ) into the North Atlantic, with a

δ^{18} O_{w}

signature of −20‰ (Ferguson & Jasechko, ). This corresponds to the equivalent of approximately 20 m of sea level rise, in agreement with some paleoproxy reconstructions (Clark et al, ; Lambeck et al, ) and results in a weakening of NADW (McManus et al, ; Ng et al, ). To enable the recovery and overshoot of NADW (Liu et al, ) and therefore the transition to BA, a negative freshwater forcing of −0.2 Sv without a δ 18 O signal (i.e., salt) is added to the same region of the North Atlantic between 15 and 14.65 ka BP.…”

Section: Methodssupporting

confidence: 86%

Assessing the Spatial Origin of Meltwater Pulse 1A Using Oxygen‐Isotope Fingerprinting

Yeung

Menviel

Meißner

et al. 2019

Paleoceanog and Paleoclimatol

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One of the major phases of sea level rise during the last deglaciation (∼19-11 thousand years before present [ka BP]) is Meltwater Pulse-1A (MWP-1A; ∼14.5 ka BP), when sea levels rose by 8.6 to 18 m in less than 400 years. Whether the meltwater originated from the partial disintegration of northern hemispheric ice sheets, from Antarctica, or both, remains controversial. Here we perform a series of idealized transient simulations of the last deglaciation, focusing on MWP-1A, with a three-dimensional oxygen-isotope enabled Earth System Climate Model. Three meltwater scenarios are considered during MWP-1A: a sole northern hemispheric source discharging into the North Atlantic, a sole Antarctic source, and a combined northern hemispheric-Antarctic source. A comparison of simulated changes in the oxygen-isotope composition ( 18 O) of seawater and calcite with published marine sediment records points to a significant contribution from Antarctica. The best model-data fit is obtained with a contribution from both hemispheres. While the simulated changes over the 350 years of MWP-1A are overestimated in our simulations, the millennial-scale changes (∼14.6-13 ka BP) are underestimated, potentially alluding to a longer and sustained meltwater input over the whole period. Meltwater was not applied in the Arctic, the Gulf of Mexico, or the North Pacific in our simulations, and therefore, scenarios with meltwater originating from these regions cannot be excluded.

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“…Connecting NPIW formation with the sea ice cycle rather than focusing on a seesaw with Atlantic MOC (e.g., Freeman et al, ; Hu, Meehl, Han, Abe‐Ouchi, et al, ; Menviel et al, ; Mikolajewicz et al, ; Okazaki et al, ) allows for enhanced NPIW formation even when Atlantic MOC is relatively strong (Böhm et al, ; Henry et al, ; Jonkers et al, ; Lynch‐Stieglitz et al, ) and releases NPIW formation from the transience of millennial scale events that characterize Atlantic MOC variability (Boyle & Keigwin, ; Henry et al, ; McManus et al, ; Ng et al, ; Praetorius et al, ). Because sea ice expands as temperature cools, NPIW formation would follow global climate (e.g., Lisiecki & Raymo, ) and generate the glacial‐interglacial cycles in physical stratification inferred from the productivity records.…”

Section: Discussionmentioning

confidence: 99%

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

Costa

McManus

Anderson

2018

Paleoceanog and Paleoclimatol

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In the Subarctic Pacific, variability in productivity on glacial‐interglacial timescales is often attributed to changes in stratification and nutrient delivery to the surface, but the mechanisms driving this relationship are poorly constrained. Records extending beyond the last glacial maximum from both the open ocean and the marginal seas are required to investigate the timing and magnitude of different influential processes through the full glacial cycle. In this study we generated 231Pa/230Th over 210,000 years in order to capture two full glacial cycles of paleoproductivity on the Juan de Fuca Ridge in the East Subarctic Pacific. The sedimentary 231Pa/230Th ratios are always equal to or greater than the seawater production ratio (0.093), consistent with enhanced biological scavenging in this region. The temporal pattern of 231Pa/230Th burial is remarkably coherent with changes in climate, with high values (0.20) during peak interglacial periods descending to low values (0.10) during peak glacial conditions, consistent with other long productivity records from this region. We investigate the possible contributions of temperature, sea ice formation, Bering Strait closure, wind strength, upwelling, and subsurface nutrient concentrations as possible mechanisms by which physical and/or chemical stratification emerged during glacial periods. Due to the low sea surface salinity in the North Pacific, cooling actually weakens the density gradient in surface (0–200 m) waters. To create the steep density profiles that characterize physical stratification, additional processes to reduce the salinity of surface waters must occur during glacial periods. We suggest that regional sea ice formation and Bering Strait closure may have contributed to freshening surface waters and enhancing physical stratification during glacial periods. Additionally, simulated weak winds in this region due to the southward shift of the glacial westerlies may have further reduced surface mixing depths in the Subarctic Pacific. Finally, previous model simulations suggest strong glacial wind stress curl in the Subarctic Pacific, but enhanced Ekman divergence of nutrient‐poor subsurface waters would have little impact on stimulating productivity in surface waters of the Subarctic Pacific. We therefore suggest that the combined effects of surface freshening, weak winds, and lower subsurface nutrient concentrations may all have contributed to lower productivity during glacial periods in the Subarctic Pacific.

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Coherent deglacial changes in western Atlantic Ocean circulation

Cited by 135 publications

References 80 publications

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

Assessing the Spatial Origin of Meltwater Pulse 1A Using Oxygen‐Isotope Fingerprinting

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

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