scholarly journals Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

2013 ◽  
Vol 9 (4) ◽  
pp. 4771-4806 ◽  
Author(s):  
L. Menviel ◽  
A. Timmermann ◽  
T. Friedrich ◽  
M. H. England
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
External Boundary ◽  
3 Dimensional ◽  
Ice Volume ◽  
Freshwater Fluxes ◽  
Meridional Overturning ◽  
The Continuum

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger (DO) and Heinrich events (HE) is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-dimensional intermediate complexity Earth system model covering the period 50 ka BP to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing Northern Hemisphere ice-volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our analysis is consistent with the idea that variations in ice sheet calving and related changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of DO and HE variability seen in paleorecords across the globe.

scholarly journals Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

2014 ◽  
Vol 10 (1) ◽  
pp. 63-77 ◽  
Author(s):  
L. Menviel ◽  
A. Timmermann ◽  
T. Friedrich ◽  
M. H. England
Keyword(s):  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
External Boundary ◽  
Ice Volume ◽  
Freshwater Fluxes ◽  
Meridional Overturning ◽  
Northern Hemispheric ◽  
The Continuum ◽  
Millennial Scale

Abstract. Millennial-scale variability associated with Dansgaard–Oeschger events is arguably one of the most puzzling climate phenomena ever discovered in paleoclimate archives. Here, we set out to elucidate the underlying dynamics by conducting a transient global hindcast simulation with a 3-D intermediate complexity earth system model covering the period 50 to 30 ka BP. The model is forced by time-varying external boundary conditions (greenhouse gases, orbital forcing, and ice-sheet orography and albedo) and anomalous North Atlantic freshwater fluxes, which mimic the effects of changing northern hemispheric ice volume on millennial timescales. Together these forcings generate a realistic global climate trajectory, as demonstrated by an extensive model/paleo data comparison. Our results are consistent with the idea that variations in ice-sheet calving and subsequent changes of the Atlantic Meridional Overturning Circulation were the main drivers for the continuum of glacial millennial-scale variability seen in paleorecords across the globe.

scholarly journals Impact of mid-glacial ice sheets on deep ocean circulation and global climate: Role of surface cooling on the AMOC

2020 ◽  
Author(s):  
Sam Sherriff-Tadano ◽  
Ayako Abe-Ouchi ◽  
Akira Oka
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Global Climate ◽  
Surface Wind ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Strengthening Effect ◽  
Surface Cooling ◽  
Glacial Ice

Abstract. This study explores the effect of southward expansion of mid-glacial ice sheets on the global climate and the Atlantic meridional overturning circulation (AMOC), as well as the processes by which the ice sheets modify the AMOC. For this purpose, simulations of Marine Isotope Stage (MIS) 3 and 5a are performed with an atmosphere-ocean general circulation model. In the MIS3 and MIS5a simulations, the global average temperature decreases by 5.0 °C and 2.2 °C, respectively, compared with the preindustrial climate simulation. The AMOC weakens by 3 % in MIS3, whereas it is enhanced by 16 % in MIS5a, both of which are consistent with a reconstruction. Sensitivity experiments extracting the effect of the expansion of glacial ice sheets from MIS5a to MIS3 show a global cooling of 1.1 °C, contributing to about 40 % of the total surface cooling from MIS5a to MIS3. These experiments also demonstrate that the ice sheet expansion leads to a surface cooling of 2 °C over the Southern Ocean as a result of colder North Atlantic deep water. We find that the southward expansion of the mid-glacial ice sheet exerts a small impact on the AMOC. Partially coupled experiments reveal that the global surface cooling by the glacial ice sheet tends to reduce the AMOC by increasing the sea ice at both poles, and hence compensates for the strengthening effect of the enhanced surface wind over the North Atlantic. Our results show that the total effect of glacial ice sheets on the AMOC is determined by the two competing effects, surface wind and surface cooling. The relative strength of surface wind and surface cooling depends on the ice sheet configuration, and the strength of the surface cooling can be comparable to that of surface wind when changes in the extent of ice sheet are prominent.

Observed Evidence of a Stable Atlantic Meridional Overturning Circulation since the 1990s

2020 ◽  
Author(s):  
Yao Fu ◽  
Feili Li ◽  
Johannes Karstensen ◽  
N. Penny Holliday ◽  
Chunzai Wang
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Distribution Model ◽  
Overturning Circulation ◽  
State Estimate ◽  
The Past ◽  
Stable Partition ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulation (AMOC) is crucially important in the global climate system due to its role in the meridional heat and freshwater distribution. Model simulations and constructed AMOC indices suggest that the AMOC may have been weakening for decades. However, direct AMOC observations, introduced in 2004 in the subtropics (the RAPID program) and in 2014 in the subpolar North Atlantic (the OSNAP program), are not sufficiently long to capture changes dating back to previous periods. Here we use repeated hydrographic sections in the subtropical and subpolar North Atlantic through the early 1990s to the mid-2010s, combined with a box inverse model that is constrained using satellite altimetry, to analyze hydrographic changes and the AMOC. In combination with a state-of-the-art ocean state estimate, GECCO2, we show that despite dramatic hydrographic changes in the subtropical and subpolar North Atlantic over the past two and half decades, the AMOC has not significantly weakened over the same period. Our hydrography-based estimates also illustrate a remarkably stable partition of the subpolar overturning between the Labrador basin and the eastern subpolar basins on decadal timescales since the 1990s.</p>

scholarly journals Impact of ice sheet meltwater fluxes on the climate evolution at the onset of the Last Interglacial

2015 ◽  
Vol 11 (5) ◽  
pp. 4391-4423 ◽  
Author(s):  
H. Goelzer ◽  
P. Huybrechts ◽  
M.-F. Loutre ◽  
T. Fichefet
Keyword(s):  
Southern Ocean ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Last Interglacial ◽  
Temperature Changes ◽  
Freshwater Fluxes ◽  
Climate Evolution ◽  
Meridional Overturning

Abstract. Large climate perturbations occurred during Termination II when the ice sheets retreated from their glacial configuration. Here we investigate the impact of ice sheet changes and associated freshwater fluxes on the climate evolution at the onset of the Last Interglacial. The period from 135 to 120 kyr BP is simulated with the Earth system model of intermediate complexity LOVECLIM v.1.3 with prescribed evolution of the Antarctic ice sheet, the Greenland ice sheet and the other Northern Hemisphere ice sheets. Variations in meltwater fluxes from the Northern Hemisphere ice sheets lead to North Atlantic temperature changes and modifications of the strength of the Atlantic meridional overturning circulation. By means of the interhemispheric see-saw effect, variations in the Atlantic meridional overturning circulation also give rise to temperature changes in the Southern Hemisphere, which are modulated by the direct impact of Antarctic meltwater fluxes into the Southern Ocean. Freshwater fluxes from the melting Antarctic ice sheet lead to a millennial time scale oceanic cold event in the Southern Ocean with expanded sea ice as evidenced in some ocean sediment cores, which may be used to constrain the timing of ice sheet retreat.

scholarly journals Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming

2020 ◽  
Vol 6 (39) ◽  
pp. eaaz1169 ◽  
Author(s):  
Shaina Sadai ◽  
Alan Condron ◽  
Robert DeConto ◽  
David Pollard
Keyword(s):  
Sea Ice ◽  
Greenhouse Gas Emission ◽  
Global Climate ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Antarctic Ice Sheet ◽  
Coupled Numerical Model ◽  
Subsurface Ocean ◽  
Climate Simulations ◽  
Meridional Overturning

Meltwater and ice discharge from a retreating Antarctic Ice Sheet could have important impacts on future global climate. Here, we report on multi-century (present–2250) climate simulations performed using a coupled numerical model integrated under future greenhouse-gas emission scenarios IPCC RCP4.5 and RCP8.5, with meltwater and ice discharge provided by a dynamic-thermodynamic ice sheet model. Accounting for Antarctic discharge raises subsurface ocean temperatures by >1°C at the ice margin relative to simulations ignoring discharge. In contrast, expanded sea ice and 2° to 10°C cooler surface air and surface ocean temperatures in the Southern Ocean delay the increase of projected global mean anthropogenic warming through 2250. In addition, the projected loss of Arctic winter sea ice and weakening of the Atlantic Meridional Overturning Circulation are delayed by several decades. Our results demonstrate a need to accurately account for meltwater input from ice sheets in order to make confident climate predictions.

scholarly journals Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation

2020 ◽  
Vol 1 (1) ◽  
Author(s):  
Tatsuya Hayashi ◽  
Toshiro Yamanaka ◽  
Yuki Hikasa ◽  
Masahiko Sato ◽  
Yoshihiro Kuwahara ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Deep Water ◽  
Global Climate ◽  
Atlantic Meridional Overturning Circulation ◽  
North Atlantic Deep Water ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Early Stages ◽  
Meridional Overturning

Abstract The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly increased after 2.7 million years ago and was actively maintained even in early stages of individual glacials, in contrast with late stages when it drastically decreased because of iceberg melting. Probably, the active Nordic Seas overturning during the early stages of glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations.

Impact of mid-glacial ice sheets on deep ocean circulation and global climate

2021 ◽  
Author(s):  
Sam Sherriff-Tadano ◽  
Ayako Abe-Ouchi ◽  
Akira Oka
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Surface Wind ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Strengthening Effect ◽  
Surface Cooling ◽  
Glacial Ice ◽  
Cooling Effects

<p>This study explores the effect of southward expansion of Northern Hemisphere (American) mid-glacial ice sheets on the global climate and the Atlantic Meridional Overturning Circulation (AMOC), as well as the processes by which the ice sheets modify the AMOC. For this purpose, simulations of Marine Isotope Stage (MIS) 3 (36ka) and 5a (80ka) are performed with an atmosphere-ocean general circulation model. In the MIS3 and MIS5a simulations, the global average temperature decreases by 5.0 °C and 2.2 °C, respectively, compared with the preindustrial climate simulation. The AMOC weakens by 3% in MIS3, whereas it strengthens by 16% in MIS5a, both of which are consistent with an estimate based on <sup>231</sup>Pa/<sup>230</sup>Th. Sensitivity experiments extracting the effect of the southward expansion of glacial ice sheets from MIS5a to MIS3 show a global cooling of 1.1 °C, contributing to about 40% of the total surface cooling from MIS5a to MIS3. These experiments also demonstrate that the ice sheet expansion leads to a surface cooling of 2 °C over the Southern Ocean as a result of colder North Atlantic deep water. We find that the southward expansion of the mid-glacial ice sheet exerts a small impact on the AMOC. Partially coupled experiments reveal that the global surface cooling by the glacial ice sheet tends to reduce the AMOC by increasing the sea ice at both poles, and hence compensates for the strengthening effect of the enhanced surface wind over the North Atlantic. Our results show that the total effect of glacial ice sheets on the AMOC is determined by the two competing effects, surface wind and surface cooling. The relative strength of surface wind and surface cooling effects depends on the ice sheet configuration, and the strength of the surface cooling can be comparable to that of surface wind when changes in the extent of ice sheet are prominent.</p>

scholarly journals The GEOVIDE cruise in May–June 2014 reveals an intense Meridional Overturning Circulation over a cold and fresh subpolar North Atlantic

2017 ◽  
Author(s):  
Patricia Zunino ◽  
Pascale Lherminier ◽  
Herlé Mercier ◽  
Nathalie Daniault ◽  
Maria Isabel García-Ibáñez ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Surface Waters ◽  
Negative Temperature ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
North Atlantic Current ◽  
Overturning Circulation ◽  
Freshwater Fluxes ◽  
Meridional Overturning

Abstract. The GEOVIDE cruise was carried out in the subpolar North Atlantic (SPNA), along the OVIDE section and across the Labrador Sea, in May–June 2014. It was planned to clarify the distribution of the trace elements and their isotopes in the SPNA as part of the GEOTRACES international program. This paper focuses on the state of the circulation and distribution of thermohaline properties during the cruise. In terms of circulation, the comparison with the 2002–2012 mean state shows a more intense Irminger current and also a weaker North Atlantic Current, with a transfer of volume transport from its northern to its central branch. However, those anomalies are compatible with the variability already observed along the OVIDE section in the 2000s. In terms of properties, the surface waters of the eastern SPNA were much colder and fresher than the averages over 2002–2012. Remarkably, in spite of negative temperature anomalies in the surface waters, the heat transport across the OVIDE section, estimated at 0.56 ± 0.06 PW, was the largest measured since 2002. This relatively large value is related to the relatively strong Meridional Overturning Circulation measured across the OVIDE section during GEOVIDE (18.7 ± 3.0 Sv). Analyzing the air-sea heat and freshwater fluxes over the eastern SPNA in relation to the heat and freshwater content changes observed during 2013 and 2014, we concluded that these changes were mainly driven by air–sea heat and freshwater fluxes rather than by ocean circulation.

scholarly journals Impact of mid-glacial ice sheets on deep ocean circulation and global climate

2021 ◽  
Vol 17 (1) ◽  
pp. 95-110
Author(s):  
Sam Sherriff-Tadano ◽  
Ayako Abe-Ouchi ◽  
Akira Oka
Keyword(s):  
North Atlantic ◽  
Global Climate ◽  
Surface Wind ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Strengthening Effect ◽  
Surface Cooling ◽  
Glacial Ice ◽  
Cooling Effects

Abstract. This study explores the effect of southward expansion of Northern Hemisphere (American) mid-glacial ice sheets on the global climate and the Atlantic Meridional Overturning Circulation (AMOC) as well as the processes by which the ice sheets modify the AMOC. For this purpose, simulations of Marine Isotope Stage (MIS) 3 (36 ka) and 5a (80 ka) are performed with an atmosphere–ocean general circulation model. In the MIS3 and MIS5a simulations, the global average temperature decreases by 5.0 and 2.2 ∘C, respectively, compared with the preindustrial climate simulation. The AMOC weakens by 3 % in MIS3, whereas it strengthens by 16 % in MIS5a, both of which are consistent with an estimate based on 231Pa ∕ 230Th. Sensitivity experiments extracting the effect of the southward expansion of glacial ice sheets from MIS5a to MIS3 show a global cooling of 1.1 ∘C, contributing to about 40 % of the total surface cooling from MIS5a to MIS3. These experiments also demonstrate that the ice sheet expansion leads to a surface cooling of 2 ∘C over the Southern Ocean as a result of colder North Atlantic Deep Water. We find that the southward expansion of the mid-glacial ice sheet exerts a small impact on the AMOC. Partially coupled experiments reveal that the global surface cooling by the glacial ice sheet tends to reduce the AMOC by increasing the sea ice at both poles and, hence, compensates for the strengthening effect of the enhanced surface wind over the North Atlantic. Our results show that the total effect of glacial ice sheets on the AMOC is determined by two competing effects: surface wind and surface cooling. The relative strength of surface wind and surface cooling effects depends on the ice sheet configuration, and the strength of the surface cooling can be comparable to that of surface wind when changes in the extent of ice sheet are prominent.

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