A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

Abstract. Changes in the latitudinal position and strength of the Southern Hemisphere westerlies (SHW) are thought to be tightly coupled to important climate processes, such as cross-equatorial heat fluxes, Atlantic Meridional Overturning Circulation (AMOC), the bipolar seesaw, Southern Ocean ventilation and atmospheric CO2 levels. However, many uncertainties regarding magnitude, direction, and causes and effects of past SHW shifts still exist due to lack of suitable sites and scarcity of information on SHW dynamics, especially from the last glacial. Here we present a detailed hydroclimate multiproxy record from a 36.4–18.6 kyr old lake sediment sequence on Nightingale Island (NI). It is strategically located at 37∘ S in the central South Atlantic (SA) within the SHW belt and situated just north of the marine Subtropical Front (SF). This has enabled us to assess hydroclimate changes and their link to the regional climate development as well as to large-scale climate events in polar ice cores. The NI record exhibits a continuous impact of the SHW, recording shifts in both position and strength, and between 36 and 31 ka the westerlies show high latitudinal and strength-wise variability possibly linked to the bipolar seesaw. This was followed by 4 kyr of slightly falling temperatures, decreasing humidity and fairly southerly westerlies. After 27 ka temperatures decreased 3–4 ∘C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly-positioned SHW seem to be related to periods of increased CO2 outgassing from the ocean, while changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, our results show that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial. Finally we can conclude that multiproxy lake records from oceanic islands have the potential to record atmospheric variability coupled to large-scale climate shifts over vast oceanic areas.

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A south Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial

10.5194/cp-2019-65 ◽

2019 ◽

Author(s):

Svante Björck ◽

Jesper Sjolte ◽

Karl Ljung ◽

Florian Adolphi ◽

Roger Flower ◽

...

Keyword(s):

Atmospheric Circulation ◽

Southern Hemisphere ◽

Large Scale ◽

Ice Cores ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Driving Mechanism ◽

Last Glacial ◽

Climate Shifts ◽

The Last Glacial

Abstract. The period 36–18 ka was a dynamic phase of the last glacial, with large climate shifts in both hemispheres. Through the bipolar seesaw, the Antarctic Isotope Maxima and Greenland DO events were part of a global concert of large scale climate changes. The interaction between atmospheric processes and Atlantic meridional overturning circulation (AMOC) is crucial for such shifts, controlling upwelling- and carbon cycle dynamics, and generating climate tipping points. Here we report the first temperature and humidity record for the glacial period from the central South Atlantic (SA). The presented data resolves ambiguities about atmospheric circulation shifts during bipolar climate events recorded in polar ice cores. A unique lake sediment sequence from Nightingale Island at 37° S in the SA, covering 36.4–18.6 ka, exhibits continuous impact of the Southern Hemisphere Westerlies (SHW), recording shifts in their position and strength. The SHW displayed high latitudinal and strength-wise variability 36–31 ka locked to the bipolar seesaw, followed by 4 ka of slightly falling temperatures, decreasing humidity and fairly southern westerlies. After 27.5 ka temperatures decreased 3–4 °C, marking the largest hydroclimate change with drier conditions and a variable SHW position. We note that periods with more intense and southerly positioned SHW are correlated with periods of increased CO2 outgassing from the ocean. Changes in the cross-equatorial gradient during large northern temperature changes appear as the driving mechanism for the SHW shifts. Together with coeval shifts of the South Pacific westerlies, it shows that most of the Southern Hemisphere experienced simultaneous atmospheric circulation changes during the latter part of the last glacial.

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Modes of response of the subsurface western South Atlantic to the last glacial Dansgaard-Oeschger cycles

10.5194/egusphere-egu2020-10124 ◽

2020 ◽

Author(s):

Thiago Santos ◽

João Ballalai ◽

Daniel Franco ◽

Rômulo Oliveira ◽

Douglas Lessa ◽

...

Keyword(s):

Ice Cores ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Last Glacial ◽

Mis 3 ◽

Subsurface Layers ◽

Basal Melting ◽

Mis 5 ◽

The Last Glacial ◽

Millennial Scale

The last glacial was an interval characterized by a sequence of abrupt millennial-scale events well documented mainly from the Greenland and Antarctica ice-cores. Although the triggers are not fully understood, most of the works agree that they occurred in consonance with oscillations in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Paleoceanographic reconstructions have shown that cold millennial-scale stadials were accompanied by high temperatures in the subsurface to intermediate waters of the Atlantic Ocean that may have acted in both the basal melting of ice-sheets and in the rapid atmospheric warming during the onset of warm interstadials. Assuming that recent transient models indicated an accentuated response of the subsurface western South Atlantic to the millennial-scale disturbances, here we present a paleoceanographic reconstruction in this area based on the deep-dwelling planktic foraminifer Globorotalia inflata. Our high-resolution oxygen isotope (d18O) presents a sequence of millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles, mainly during Marine Isotope Stage (MIS) 5. On the other hand, during MIS 3, this millennial-scale feature is absent or weakly represented. Cross-spectral analyzes indicate a meaningful north-to-south forcing over the western South Atlantic subsurface during early-glacial. Mg/Ca-derived temperature and ice-volume free seawater d18O (d18OIVF-SW) executed for the MIS 5 interval demonstrated that the subsurface western South Atlantic was warmer and saltier (colder and fresher) during early glacial stadial (interstadials). We hypothesized that a wide reorganization of the northward heat transport throughout the last glacial occurred, in which regions so far south as 24 &#186;S worked as prominent heat reservoirs in periods of weakened AMOC during MIS 5 but not necessarily during MIS 3. Our data suggest that future impacts over the AMOC along the Brazilian margin will likely be recognized in the subsurface layers of the western South Atlantic.

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δ13C decreases in the upper western South Atlantic during Heinrich Stadials 3 and 2

Climate of the Past ◽

10.5194/cp-13-345-2017 ◽

2017 ◽

Vol 13 (4) ◽

pp. 345-358 ◽

Cited By ~ 4

Author(s):

Marília C. Campos ◽

Cristiano M. Chiessi ◽

Ines Voigt ◽

Alberto R. Piola ◽

Henning Kuhnert ◽

...

Keyword(s):

Climate Change ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Glacial Period ◽

Last Deglaciation ◽

Last Glacial Period ◽

Last Glacial ◽

Change Events ◽

The Last Glacial ◽

Millennial Scale

Abstract. Abrupt millennial-scale climate change events of the last deglaciation (i.e. Heinrich Stadial 1 and the Younger Dryas) were accompanied by marked increases in atmospheric CO2 (CO2atm) and decreases in its stable carbon isotopic ratios (δ13C), i.e. δ13CO2atm, presumably due to outgassing from the ocean. However, information on the preceding Heinrich Stadials during the last glacial period is scarce. Here we present δ13C records from two species of planktonic foraminifera from the western South Atlantic that reveal major decreases (up to 1 ‰) during Heinrich Stadials 3 and 2. These δ13C decreases are most likely related to millennial-scale periods of weakening of the Atlantic meridional overturning circulation and the consequent increase (decrease) in CO2atm (δ13CO2atm). We hypothesise two mechanisms that could account for the decreases observed in our records, namely strengthening of Southern Ocean deep-water ventilation and weakening of the biological pump. Additionally, we suggest that air–sea gas exchange could have contributed to the observed δ13C decreases. Together with other lines of evidence, our data are consistent with the hypothesis that the CO2 added to the atmosphere during abrupt millennial-scale climate change events of the last glacial period also originated in the ocean and reached the atmosphere by outgassing. The temporal evolution of δ13C during Heinrich Stadials 3 and 2 in our records is characterized by two relative minima separated by a relative maximum. This w structure is also found in North Atlantic and South American records, further suggesting that such a structure is a pervasive feature of Heinrich Stadial 2 and, possibly, also Heinrich Stadial 3.

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Synoptic climate change as a driver of late Quaternary glaciations in the mid-latitudes of the Southern Hemisphere

Climate of the Past ◽

10.5194/cp-2-11-2006 ◽

2006 ◽

Vol 2 (1) ◽

pp. 11-19 ◽

Cited By ~ 18

Author(s):

H. Rother ◽

J. Shulmeister

Keyword(s):

New Zealand ◽

Southern Hemisphere ◽

Large Scale ◽

Late Quaternary ◽

Synoptic Climatology ◽

Balance Model ◽

Last Glacial ◽

Regional Flow ◽

Quaternary Glaciations ◽

The Last Glacial

Abstract. The relative timing of late Quaternary glacial advances in mid-latitude (40-55° S) mountain belts of the Southern Hemisphere (SH) has become a critical focus in the debate on global climate teleconnections. On the basis of glacial data from New Zealand (NZ) and southern South America it has been argued that interhemispheric synchrony or asynchrony of Quaternary glacial events is due to Northern Hemisphere (NH) forcing of SH climate through either the ocean or atmosphere systems. Here we present a glacial snow-mass balance model that demonstrates that large scale glaciation in the temperate and hyperhumid Southern Alps of New Zealand can be generated with moderate cooling. This is because the rapid conversion of precipitation from rainfall to snowfall drives massive ice accumulation at small thermal changes (1-4°C). Our model is consistent with recent paleo-environmental reconstructions showing that glacial advances in New Zealand during the Last Glacial Maximum (LGM) and the Last Glacial Interglacial Transition (LGIT) occurred under very moderate cooling. We suggest that such moderate cooling could be generated by changes in synoptic climatology, specifically through enhanced regional flow of moist westerly air masses. Our results imply that NH climate forcing may not have been the exclusive driver of Quaternary glaciations in New Zealand and that synoptic style climate variations are a better explanation for at least some late Quaternary glacial events, in particular during the LGIT (e.g. Younger Dryas and/or Antarctic Cold Reversal).

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Ocean Model Diagnosis of Interannual Coevolving SST Variability in the South Indian and South Atlantic Oceans

Journal of Climate ◽

10.1175/jcli3422.1 ◽

2005 ◽

Vol 18 (15) ◽

pp. 2864-2882 ◽

Cited By ~ 72

Author(s):

J. C. Hermes ◽

C. J. C. Reason

Keyword(s):

Atmospheric Circulation ◽

Southern Hemisphere ◽

Large Scale ◽

Ocean Model ◽

South Atlantic ◽

Heat Fluxes ◽

Global Ocean ◽

The South ◽

Sst Variability ◽

South Indian

Abstract A global ocean model (ORCA2) forced with 50 yr of NCEP–NCAR reanalysis winds and heat fluxes has been used to investigate the evolution and forcing of interannual dipolelike sea surface temperature (SST) variability in the South Indian and South Atlantic Oceans. Although such patterns may also exist at times in only one of these basins and not the other, only events where there are coherent signals in both basins during the austral summer have been chosen for study in this paper. A positive (negative) event occurs when there is a significant warm (cool) SST anomaly evident in the southwest of both the South Indian and South Atlantic Oceans and a cool (warm) anomaly in the eastern subtropics. The large-scale forcing of these events appears to consist of a coherent modulation of the wavenumber-3 or -4 pattern in the Southern Hemisphere atmospheric circulation such that the semipermanent subtropical anticyclone in each basin is shifted from its summer mean position and its strength is modulated. A relationship to the Antarctic Oscillation is also apparent, and seems to strengthen after the mid-1970s. The modulated subtropical anticyclones lead to changes in the tropical easterlies and midlatitude westerlies in the South Atlantic and South Indian Oceans that result in anomalies in latent heat fluxes, upwelling, and Ekman heat transports, all of which contribute to the SST variability. In addition, there are significant modulations to the strong Rossby wave signals in the South Indian Ocean. The results of this study confirm the ability of the ORCA2 model to represent these dipole patterns and indicate connections between large-scale modulations of the Southern Hemisphere midlatitude atmospheric circulation and coevolving SST variability in the South Atlantic and South Indian Oceans.

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Modelling of mineral dust for interglacial and glacial climate conditions with a focus on Antarctica

Climate of the Past ◽

10.5194/cp-11-765-2015 ◽

2015 ◽

Vol 11 (5) ◽

pp. 765-779 ◽

Cited By ~ 17

Author(s):

N. Sudarchikova ◽

U. Mikolajewicz ◽

C. Timmreck ◽

D. O'Donnell ◽

G. Schurgers ◽

...

Keyword(s):

Southern Hemisphere ◽

Mineral Dust ◽

Atmospheric Transport ◽

Ice Cores ◽

Dust Particles ◽

Dust Deposition ◽

Transport Efficiency ◽

Dust Emissions ◽

Last Glacial ◽

The Last Glacial

Abstract. The mineral dust cycle responds to climate variations and plays an important role in the climate system by affecting the radiative balance of the atmosphere and modifying biogeochemistry. Polar ice cores provide unique information about deposition of aeolian dust particles transported over long distances. These cores are a palaeoclimate proxy archive of climate variability thousands of years ago. The current study is a first attempt to simulate past interglacial dust cycles with a global aerosol–climate model ECHAM5-HAM. The results are used to explain the dust deposition changes in Antarctica in terms of quantitative contribution of different processes, such as emission, atmospheric transport and precipitation, which will help to interpret palaeodata from Antarctic ice cores. The investigated periods include four interglacial time slices: the pre-industrial control (CTRL), mid-Holocene (6000 yr BP; hereafter referred to as "6 kyr"), last glacial inception (115 000 yr BP; hereafter "115 kyr") and Eemian (126 000 yr BP; hereafter "126 kyr"). One glacial time interval, the Last Glacial Maximum (LGM) (21 000 yr BP; hereafter "21 kyr"), was simulated as well to be a reference test for the model. Results suggest an increase in mineral dust deposition globally, and in Antarctica, in the past interglacial periods relative to the pre-industrial CTRL simulation. Approximately two-thirds of the increase in the mid-Holocene and Eemian is attributed to enhanced Southern Hemisphere dust emissions. Slightly strengthened transport efficiency causes the remaining one-third of the increase in dust deposition. The moderate change in dust deposition in Antarctica in the last glacial inception period is caused by the slightly stronger poleward atmospheric transport efficiency compared to the pre-industrial. Maximum dust deposition in Antarctica was simulated for the glacial period. LGM dust deposition in Antarctica is substantially increased due to 2.6 times higher Southern Hemisphere dust emissions, 2 times stronger atmospheric transport towards Antarctica, and 30% weaker precipitation over the Southern Ocean. The model is able to reproduce the order of magnitude of dust deposition globally and in Antarctica for the pre-industrial and LGM climates.

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Was the Atlantic a predominantly Polar Ocean during the last glacial?

10.5194/egusphere-egu2020-21063 ◽

2020 ◽

Author(s):

Marleen Lausecker ◽

Freya Hemsing ◽

Thomas Krengel ◽

Julius Förstel ◽

Andrea Schröder-Ritzrau ◽

...

Keyword(s):

Southern Hemisphere ◽

Last Glacial Maximum ◽

Ocean Circulation ◽

Cold Water ◽

World Ocean ◽

Ice Cores ◽

Glacial Maximum ◽

Global Ocean ◽

Last Glacial ◽

The Last Glacial

The Last Glacial Maximum (LGM) is marked by significant cooling of the global ocean, which was recently estimated to 2.6&#176;C using noble gases trapped in ice cores (1). This cooling is not equally distributed throughout the world oceans, since global ocean circulation models predict regional temperature anomalies during the LGM of up to 7&#176;C (annually and zonally averaged) when compared to modern interior ocean temperature (2). The oceans deep interior thus became haline stratified (3) due to the drop in temperature to near freezing and the global increase in salinity from ice sheet growth. In contrast to a deepening of the modern thermocline as a result of anthropogenic global warming, cooling causes the thermocline to rise in the sub-tropics as more polar waters enter the mid-depth ocean.Here we present glacial thermocline temperature reconstructions since the LGM based on the Li/Mg ratio in aragonite skeletons of precisely dated cold-water corals. Corals have been collected from 300-1000m water depths from sites in the northern and southern Atlantic (62&#176;N to 25&#176;S) and demonstrate synchronous 5 - 7&#176;C glacial cooling, and a dramatic shoaling of the thermocline. Through the deglaciation the warming of the upper thermocline ocean occurs early in the southern hemisphere followed by fluctuating warming and thermocline deepening in the northern Hemisphere, which supports the oceanic climate seesaw proposed by Stocker and Johnson in 2003 (4). We thus propose dramatic changes in export of polar waters towards the Equator and augmented subsurface ocean stratification leading to a mostly polar Atlantic with a shallow permanent thermocline. This shoaling possibly increased the rate of nutrient recycling causing higher biological surface ocean activity and the cooling promoted carbon storage. During the glacial, we assume an atmospheric forcing, such as equatorward displacement of the Hadley circulation, to steer the glacial polar water advance as mid-depth boundary currents in the northern and southern hemisphere to effectively spread the cold water through the entire mid-depth Atlantic.References:<ol><li>Bereiter et al.: Mean global ocean temperatures during the last glacial transition. Nature 553, 39-44 (2018).</li> <li>Ballarotta et al.: Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?, Clim. Past 9, 2669-2686 (2013).</li> <li>Adkins et al.: The Salinity, Temperature, and d18O of the Glacial Deep Ocean. Science 298, 1769-1773 (2002).</li> <li>Stocker and Johnsen: A minimum thermodynamic model for the bipolar seesaw, Paleoceanography 18, 1087 (2003).</li> </ol>

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Contrasting late-glacial paleoceanographic evolution between the upper and lower continental slope of the western South Atlantic

Climate of the Past ◽

10.5194/cp-16-1245-2020 ◽

2020 ◽

Vol 16 (4) ◽

pp. 1245-1261

Author(s):

Leticia G. Luz ◽

Thiago P. Santos ◽

Timothy I. Eglinton ◽

Daniel Montluçon ◽

Blanca Ausin ◽

...

Keyword(s):

Rio De Janeiro ◽

Planktonic Foraminifera ◽

South Atlantic ◽

Meridional Overturning Circulation ◽

Coastal Current ◽

Limited Information ◽

Last Deglaciation ◽

Glacial Cycle ◽

Last Glacial ◽

The Last Glacial

Abstract. The number of sedimentary records collected along the Brazilian continental margin has increased significantly in recent years, but relatively few are located in shallow waters and register paleoceanographic processes in the outer shelf–middle slope prior to 10–15 ka. For instance, the northward flow up to 23–24∘ S of cold and fresh shelf waters sourced from the Subantarctic region is an important feature of current hydrodynamics in the subtropical western South Atlantic Ocean, and yet limited information is available for the long-term changes of this system. Herein, we considered a suite of organic and inorganic proxies – alkenones-derived sea surface temperature (SST), δD-alkenones, δ18O of planktonic foraminifera, and ice-volume free seawater δ18OIVF−SW – in sediment from two cores (RJ-1501 and RJ-1502) collected off the Rio de Janeiro Shelf (SE Brazilian continental shelf) to shed light on SST patterns and relative salinity variations since the end of the last glacial cycle in the region and the implications of these processes over a broader spatial scale. The data indicate that, despite the proximity (∼40 km apart) of both cores, apparently contradictory climatic evolution occurred at the two sites, with the shallower (deeper) core RJ-1501 (RJ-1502) showing consistently cold (warm) and fresh (salt) conditions toward the Last Glacial Maximum (LGM) and last deglaciation. This can be reconciled by considering that the RJ-1501 core registered a signal from mid- to high latitudes on the upper slope off Rio de Janeiro represented by the influence of the cold and fresh waters composed of Subantarctic Shelf Water and La Plata Plume Water transported northward by the Brazilian Coastal Current (BCC). The data from core RJ-1502 and previous information for deep-cores from the same region support this interpretation. In addition, alkenone-derived SST and δ18OIVF−SW suggest a steep thermal and density gradient formed between the BCC and Brazil Current (BC) during the last climate transition which, in turn, may have generated perturbations in the air–sea heat flux with consequences for the regional climate of SE South America. In a scenario of future weakening of the Atlantic Meridional Overturning Circulation, the reconstructed gradient may become a prominent feature of the region.

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Orbital cycle-related benthic-pelagic fluctuations in Foraminifera during the last glacial-interglacial interval in the western South Atlantic

10.5194/egusphere-egu21-10245 ◽

2021 ◽

Author(s):

Jaime Yesid Suarez Ibarra ◽

Cristiane Fraga Frozza ◽

Sandro Monticelli Petró ◽

Pamela de Lara Palhano ◽

Maria Alejandra Gómez Pivel

Keyword(s):

Deep Water ◽

Bottom Water ◽

South Atlantic ◽

Water Masses ◽

Meridional Overturning Circulation ◽

Biological Pump ◽

Sea Floor ◽

Last Glacial ◽

Surface Productivity ◽

The Last Glacial

Paleoceanographic studies reconstructing surface paleoproductivity and benthic conditions allow us to measure the effectiveness of the biological pump, an important mechanism in the global climate system. In order to assess surface productivity changes and their effect on the sea-floor environment, a multiproxy paleoceanographic analysis was conducted on the core SAT-048A (1542 m.b.s.l.), recovered from the continental slope of the southernmost Brazilian continental margin, western South Atlantic. We assessed sea surface productivity using different planktonic foraminiferal proxies: (1) the relative abundances of the species Globigerina bulloides and Globigerinita glutinata and (2) the &#948;13C signal of shells of the species Globigerinoides ruber ruber. To assess the organic matter (OM) flux to the seafloor, the foraminiferal planktonic:benthic ratio and the &#948;13C signal of shells of the benthic foraminifer Uvigerina spp. were used. To study dissolution effects occurring at the sea-floor, the Fragmentation Intensity (i.e., the proportion of fragments and broken foraminiferal shells), the number of planktonic foraminiferal tests per gram of dry sediment, and the CaCO3 and Sand contents of the sediment were measured. Superimposed on the climate-induced changes related to the last glacial-interglacial transition, the reconstruction indicates paleoproductivity changes synchronized with the precessional cycle. From the reconstructed data, it was possible to identify the glacial and postglacial stages: surface productivity, flux to the seafloor, and dissolution rates of planktonic foraminiferal tests where high during the glacial and low during the postglacial. Furthermore, within the glacial, enhanced productivity was associated with higher insolation values, which can be explained by increased NE summer winds that strengthened the Brazil Current transport and, in turn, promoted meandering and upwelling of the nutrient rich South Atlantic Central Water. Changes in the Atlantic Meridional Overturning Circulation and the reorganization of bottom water masses may change the CO32- saturation levels and, consequently, influence carbonate preservation. However, the &#948;13C values from shells of Uvigerina spp. are different from present-day &#948;13C values from dissolved inorganic carbon for the Upper Circumpolar Deep Water and the North Atlantic Deep Water, which is likely linked to varying OM fluxes. Future studies (e.g., &#949;Nd in benthic Foraminifera) must quantify the effect of the reorganization of the bottom water masses on the dissolution of the planktonic foraminiferal tests, to better understand the effect of the biological pump removing carbon from the seawater and its subsequent sequestration in the seafloor sediments.

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A Heuristic Model of Dansgaard–Oeschger Cycles. Part I: Description, Results, and Sensitivity Studies

Journal of Climate ◽

10.1175/jcli-d-12-00672.1 ◽

2014 ◽

Vol 27 (12) ◽

pp. 4337-4358 ◽

Cited By ~ 10

Author(s):

Hansi A. Singh ◽

David S. Battisti ◽

Cecilia M. Bitz

Keyword(s):

Sea Ice ◽

Large Scale ◽

Oscillatory Behavior ◽

Meridional Overturning Circulation ◽

Heuristic Model ◽

Overturning Circulation ◽

Nordic Seas ◽

Last Glacial ◽

Sensitivity Studies ◽

The Last Glacial

Abstract A simple model for studying the Dansgaard–Oeschger (D-O) cycles of the last glacial period is presented, based on the T. Dokken et al. hypothesis for D-O cycles. The model is a column model representing the Nordic seas and is composed of ocean boxes stacked below a one-layer sea ice model with an energy-balance atmosphere; no changes in the large-scale ocean overturning circulation are invoked. Parameterizations are included for latent heat polynyas and sea ice export from the column. The resulting heuristic model was found to cycle between stadial and interstadial states at times scales similar to those seen in the proxy observational data, with the presence or absence of perennial sea ice in the Nordic seas being the defining characteristic for each of these states. The major discrepancy between the modeled oscillations and the proxy record is in the length of the interstadial phase, which is shorter than that observed. The modeled oscillations were found to be robust to parameter changes, including those related to the ocean heat flux convergence (OHFC) into the column. Production of polynya ice was found to be an essential ingredient for such sustained oscillatory behavior. A simple parameterization of natural variability in the OHFC enhances the robustness of the modeled oscillations. The authors conclude by discussing the implications of such a hypothesis for the state of the Nordic seas today and its state during the Last Glacial Maximum and contrasting the model to other hypotheses that invoke large-scale changes in the Atlantic meridional overturning circulation for explaining millennial-scale variability in the climate system. An extensive time-scale analysis will be presented in the future.

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A South Atlantic island record uncovers shifts in westerlies and hydroclimate during the last glacial