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  • 1
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    Ice sheet decline and rising atmospheric CO2 control AMOC sensitivity to deglacial meltwater discharge (2022)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Highlights: • Climate model sensitivity experiments are performed using state-of-the-art ice sheet and freshwater reconstructions • Declining Northern Hemisphere ice sheets increase the sensitivity of the AMOC to North Atlantic meltwater discharge • Deglacial rise in atmospheric CO2 concentration decreases the sensitivity of the AMOC to North Atlantic meltwater discharge • Both effects provide a complementary perspective to existing explanations for abrupt AMOC transitions Abstract: The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Coupled climate-ice sheet modelling of MIS-13 reveals a sensitive Cordilleran Ice Sheet (2021)
    Elsevier
    Details
    Publication Date: 2024-02-07
    Description: Previous modelling efforts have investigated climate responses to different Milankovitch forcing during Marine Isotope Stage (MIS) 13. During this time the climate has been highly variable at atmospheric CO2 concentrations of ~240 ppm. As yet, ice sheet-climate feedbacks were missing in previous studies. Therefore we use the state-of-the-art coupled climate-ice sheet model, AWI-ESM-1.2, to investigate the MIS-13 climate and corresponding Northern Hemisphere ice sheet (NHIS) evolution by performing simulations under three different astronomical configurations representing 495, 506 and 517 kyr BP. The simulated excess ice compared to present-day is mainly over the Cordillera, Arctic islands and Tibet. The global mean surface air temperature for the MIS-13 experiments have the same magnitude. At 506 kyr BP with boreal summer at perihelion, the Northern Hemisphere continents are warmer during summer than the other experiments, which could potentially inhibit the development of the ice sheets. The Cordilleran Ice Sheet is found to be especially sensitive to orbital (precession) forcing, at an intermediate CO2 level. This is probably due to its high elevation where the freezing point could be easily maintained. The other ice sheets over northeast America and Eurasia, however, are absent in our simulations. We propose that the alpine-based Cordilleran Ice Sheet is more sensitive and easier to build up than other NHISs in response to the astronomical controlled summer insolation. Dynamic surges are simulated for the Cordilleran Ice Sheet under fixed low orbital forcing. These surges due to internal ice sheet-climate feedbacks could potentially be the mechanism for the millennial scale H-like events.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Deglacial diatom production in the tropical North Atlantic driven by enhanced silicic acid supply (2016)
    Elsevier
    Details
    Publication Date: 2023-08-01
    Description: Major shifts in ocean circulation are thought to be responsible for abrupt changes in temperature and atmospheric CO2 during the last deglaciation, linked to variability in meridional heat transport and deep ocean carbon storage. There is also widespread evidence for shifts in biological production during these times of deglacial CO2 rise, including enhanced diatom production in regions such as the tropical Atlantic. However, it remains unclear as to whether this diatom production was driven by enhanced wind-driven upwelling or density-driven vertical mixing, or by elevated thermocline concentrations of silicic acid supplied to the surface at a constant rate. Here, we demonstrate that silicic acid supply at depth in the NE Atlantic was enhanced during the abrupt climate events of the deglaciation. We use marine sediment archives to show that an increase in diatom production during abrupt climate shifts could only occur in regions of the NE Atlantic where the deep supply of silicic acid could reach the surface. The associated changes are indicative of enhanced regional wind-driven upwelling and/or weakened stratification due to circulation changes during phases of weakened Atlantic meridional overturning. Globally near-synchronous pulses of diatom production and enhanced thermocline concentrations of silicic acid suggest that widespread deglacial surface-driven breakdown of stratification, linked to changes in atmospheric circulation, had major consequences for biological productivity and carbon cycling.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Anti‐Phased Miocene Ice Volume and CO2 Changes by Transient Antarctic Ice Sheet Variability (2020)
    AMER GEOPHYSICAL UNION
    In:  EPIC3Paleoceanography and Paleoclimatology, AMER GEOPHYSICAL UNION, 35(11), pp. e2020PA003971, ISSN: 2572-4525
    Details
    Publication Date: 2021-02-16
    Description: Geological evidence indicates large continental‐scale Antarctic ice volume variations during the early and mid‐Miocene. On million‐year timescales, these variations can largely be explained by equilibrium Antarctic ice sheet (AIS) simulations. In contrast, on shorter orbital timescales, the AIS needs not be in equilibrium with the forcing and ice volume variations may be substantially different. Here, we introduce a conceptual model, based on ice dynamical model results, to investigate the difference between transient variability and equilibrium differences of the Miocene AIS. In our model, an ice sheet will grow (shrink) by a specific rate when it is smaller (larger) than its equilibrium size. We show that phases of concurrent ice volume increase and rising CO2 levels are possible, even though the equilibrium ice volume decreases monotonically with CO2. When the AIS volume is out of equilibrium with the forcing climate, the ice sheet can still be adapting to a relatively large equilibrium size, although CO2 is rising after a phase of decrease. A delayed response of Antarctic ice volume to (covarying) solar insolation and CO2 concentrations can cause discrepancies between Miocene solar insolation and benthic δ18O variability. Increasing forcing frequency leads to a larger disequilibrium and consequently larger CO2‐ice volume phase differences. Furthermore, an amplified forcing amplitude causes larger amplitude ice volume variability, because the growth and decay rates depend on the forcing. It also leads to a reduced average ice volume, resulting from the growth rates generally being smaller than the decay rates.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
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  • 5
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    Dependence of abrupt Atlantic meridional ocean circulation changes on climate background states (2013)
    AMER GEOPHYSICAL UNION
    In:  EPIC3Geophysical Research Letters, AMER GEOPHYSICAL UNION, ISSN: 0094-8276
    Details
    Publication Date: 2019-07-17
    Description: Abrupt decadal climate changes during the last glacial-interglacial cycle are less pronounced during maximum glacial conditions and absent during the Holocene. To further understand the underlying dynamics, we conduct hosing experiments for three climate states: Pre-industrial (PI), 32 kilo years before present (ka BP) and Last Glacial Maximum (LGM). Our simulations show that a stronger temperature inversion between the surface and intermediate layer in the South Labrador Sea induces a faster restart of convective processes (32 ka BP 〉 LGM 〉 PI) during the initial resumption of the Atlantic meridional overturning circulation (AMOC). A few decades later, an AMOC overshoot is mainly linked to the advection of warmer and saltier intermediate-layer water from the tropical Atlantic into the South Labrador Sea, which causes a stronger deep-water formation than that before the freshwater perturbation. This mechanism is most pronounced during the 32 ka BP, weaker during the LGM and absent during the PI.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 6
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    Agulhas salt-leakage oscillations during abrupt climate changes of the Late Pleistocene (2013)
    AMER GEOPHYSICAL UNION
    In:  EPIC3Paleoceanography, AMER GEOPHYSICAL UNION, 28, ISSN: 0883-8305
    Details
    Publication Date: 2019-07-17
    Description: An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway (IAO-G), south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt-leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and mid-latitude westerlies that impacted the inter-ocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full-strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 7
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    Abrupt changes in deep Atlantic circulation during the transition to full glacial conditions (2013)
    AMER GEOPHYSICAL UNION
    In:  EPIC3Paleoceanography, AMER GEOPHYSICAL UNION, ISSN: 0883-8305
    Details
    Publication Date: 2019-07-17
    Description: Six Ocean Drilling Program (ODP) sites, in the Northwest Atlantic have been used to investigate kinematic and chemical changes in the ‘Western Boundary Undercurrent’ (WBUC) during the development of full glacial conditions across the Marine Isotope Stage 5a/4 boundary (~70,000 years ago). Sortable silt mean grain size( S͞S )measurements are employed to examine changes in near bottom flow speeds, together with carbon isotopes measured in benthic foraminifera and % planktic foraminiferal fragmentation as proxies for changes in water-mass chemistry. A depth transect of cores, spanning 1.8-4.6 km depth, allows changes in both the strength and depth of the WBUC to be constrained across millennial scale events. S͞S measurements reveal that the flow speed structure of the WBUC during warm intervals (‘interstadials’) was comparable to modern (Holocene) conditions. However, significant differences are observed during cold intervals, with higher relative flow speeds inferred for the shallow component of the WBUC (~ 2 km depth) during all cold ‘stadial’ intervals (including Heinrich Stadial 6), and a substantial weakening of the deep component (~3-4 km) during full glacial conditions. Our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation (involving Glacial North Atlantic Intermediate Water) that was quantitatively distinct from preceding cold stadial events. Furthermore, our chemical proxy data show that the physical response of the WBUC during the last glacial inception was probably coupled to basin-wide changes in the water-mass composition of the deep Northwest Atlantic.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 8
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    Ice sheet decline and rising atmospheric CO2 control AMOC sensitivity to deglacial meltwater discharge (2022)
    Elsevier
    In:  EPIC3Global and Planetary Change, Elsevier, 210, pp. 103755-103755, ISSN: 0921-8181
    Details
    Publication Date: 2023-09-19
    Description: The last deglaciation was characterized by a sequence of abrupt climate events thought to be linked to rapid changes in Atlantic meridional overturning circulation (AMOC). The sequence includes a weakening of the AMOC after the Last Glacial Maximum (LGM) during Heinrich Stadial 1 (HS1), which ends with an abrupt AMOC amplification at the transition to the Bølling/Allerød (B/A). This transition occurs despite persistent deglacial meltwater fluxes that counteract vigorous North Atlantic deep-water formation. Using the Earth system model COSMOS with a range of deglacial boundary conditions and reconstructed deglacial meltwater fluxes, we show that deglacial CO2 rise and ice sheet decline modulate the sensitivity of the AMOC to these fluxes. While declining ice sheets increase the sensitivity, increasing atmospheric CO2 levels tend to counteract this effect. Therefore, the occurrence of a weaker HS1 AMOC and an abrupt AMOC increase in the presence of meltwater, might be explained by these effects, as an alternative to or in combination with changes in the magnitude or routing of meltwater discharge.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
    Format: application/pdf
    Location Call Number Limitation Availability
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    PAPER (OA)
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