Millennial-scale climate variability during Pleistocene Ice Ages, known as Dansgaard-Oeschger (DO) cycles, are characterised by abrupt transitions between Greenland cold stadials and warm interstadials, which coincide with gradual warming and cooling over Antarctica, respectively, via the bipolar seesaw. DO cycles are associated with reorganisations of the Atlantic Meridional Overturning Circulation (AMOC), but the mechanisms driving them remain unclear. In this study, from nudging experiments based on intrinsic millennial-scale AMOC variability in a complex climate model, we show that gradual changes in sea ice over the Southern Ocean induced by the bipolar seesaw act as a negative feedback to maintain the millennial-scale AMOC variability. Southern Ocean surface cooling during the interstadial phases enhances regional sea ice-related salt and freshwater fluxes, which eventually weakens the AMOC by strengthening the oceanic stratification over the North Atlantic by increasing and decreasing the salinity of Antarctic bottom water and Antarctic intermediate water, respectively. The Southern Ocean feedback becomes particularly important for DO cycles with long periodicities, such as those occurring during Marine Isotope Stages 5, 4, 2 and those appearing after major Heinrich events. Our results suggest that the Southern Ocean feedback helps drive the DO cycles, demonstrating the globally connected nature of these events.

How to cite: Sherriff-Tadano, S., Abe-Ouchi, A., Chan, W.-L., Mitsui, T., Oka, A., Obase, T., Kuniyoshi, Y., Romé, Y., and Buizert, C.:  Southern Ocean processes maintain Ice Age millennial-scale climate variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7757, https://doi.org/10.5194/egusphere-egu25-7757, 2025.

This paper examines the structure of glacial cycles, with a particular focus on the definition and complexity of cold stages in the stratigraphical record. The Middle and Late Pleistocene cold stages correspond to the classic orbitally-driven 100 ka glacial cycles. Closer examination of the global glacier-climate record also reveals that cold stages are structured within larger glacial cycles beyond the classic 100 ka pattern. ‘Mega’-glacial cycles closely correspond to 400 ka eccentricity cycles and the last two such cycles were bounded by MIS 19, 11 and 1. The last of these mega-cycles encompasses the Saalian Complex Stage in Europe, as well as the last cold stage (Weichselian Stage and equivalents) and has significant implications for how cold stages are defined.

The irregular pacing of quasi-100 ka glacial cycles is likely to represent an internal mechanism related to ice-sheet evolution through cold stages and their interaction with ocean and atmospheric circulation. Internal climate drivers also explain short-term climatic fluctuations within cold stages such as Dansgaard-Oeschger cycles and various other short-term interstadial-stadial transitions, such as the during the Late-glacial and the Younger Dryas Stadial, for example. Since the effects of global climate change are not manifested uniformly through time and space, such climatic effects result in diachronous boundaries in the geological record as well as spatial variability. This is especially characteristic of the Quaternary record where sediments and landforms record climate change over relatively short time intervals. This complexity, inherent in climate stratigraphy upon which the Quaternary stratigraphical record is built, poses challenges for regional and especially global correlations. In many studies cross-correlation within glacial cycles is achieved via the marine or ice-core records, especially for the last glacial cycle. However, whilst useful as records of time through the Quaternary, these records do not always reflect other processes on Earth. For example, it is now known that glacier behaviours around the world do not conform closely with the marine isotopic records, the latter being dominated by fluctuations in the Laurentide Ice Sheet over North America, overprinted by local factors.

Whilst orbital forces caused by the Earth’s interaction with other planetary orbits in our Solar System are pivotal in modulating and pacing climate change, the most important driver of the magnitude of climate change that we see in the Quaternary are largely internal factors. It is the coincidence of these drivers with orbital parameters that explain the structure and characteristics of glacial cycles. Whilst similarities between global climate patterns between glacial cycles are apparent, such as the saw-tooth pattern of change observed in marine isotope records, the complexity within these cycles differs within every cycle.  Thus, every glacial cycle is unique. This means that stratigraphical frameworks for subdividing, ordering and correlating structural elements of Pleistocene cold stages will also be unique for each glacial cycle and requires careful consideration and definition. This is especially important for correct correlation of intra-cold stage climatic stratigraphical events across regions and ultimately for comparison with global climate temporal frameworks such as the marine or ice-core records.  

How to cite: Hughes, P. and Gibbard, P.: Anatomy of a cold stage: deconstructing the structure of Pleistocene glacial cycles, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12509, https://doi.org/10.5194/egusphere-egu25-12509, 2025.

Global warming and unprecedented increase in CO2 urgently call for improved understanding of the climate system. The acceleration of ice sheet and sea ice melting, the weakening of the Atlantic Meridional Overturning Circulation (AMOC) to a tipping point for its collapse, and the observed anomalous distribution of heat in the North Atlantic region are severely contributing to an intensification of climate hazards (droughts, wildfires, storms and floods) in vulnerable regions. Understanding ocean-atmosphere dynamics and its role in precipitation distribution and intensification of hydroclimate hazards in highly vulnerable regions, as SW Europe, is a key priority.
We studied two severe glacial periods such as the Marine Isotopic Stages (MIS) 16 (621–676 ka) and MIS 12 (424–478 ka) and their terminations (TVII:  ~ 625 kyr and part of the TV: ~ 430 kyr, since there is a hiatuses). Each glaciation was characterised by expanding global ice-sheets from the time of glacial onset to the beginning of the deglaciation and are marked by instabilities of Northern Hemisphere ice sheets during the glacial inception, before the peak glaciation and during deglaciation. Northern Hemisphere ice sheets instabilities are however stronger after the peak glacial as revealed by maxima of ice-rafted detritus deposition in the North Atlantic. The major difference between the two glacials has been attributed to a more northern position of the Artic front during MIS 16 when compared to MIS 12. 
Here we present direct land-sea- comparison (including pollen, alkenone derived SST, % C37:4 and benthic 18O) of IODP Site U1385 (Expedition 339), SW Iberian margin, covering MIS 16, TVII, MIS 12 and partially TV. Both glacials are marked by 3 main episodes of semi-desert plants expansion (dry) intercalated by two episodes of heathland expansion (wet). Although these hydroclimate changes are mainly controlled by precession, the distribution of moisture in the north Atlantic region, including the western Iberian Peninsula and northern Hemisphere ice sheets is completely dependent on the position and intensity of both the Arctic front and polar jet stream. The comparison of Site U1385 Sea Surface Temperatures (SST) with other available North Atlantic records allows us to infer the position and intensity of the Jet Stream in each processional cycle. Furthermore, the driest Iberian Peninsula phases within MIS 16 and MIS 12 seem to have been amplified by pulses of meltwater from the Eurasian ice sheets as previously demonstrated for the last glacial period. 

How to cite: Naughton, F., Oliveira, D., Rodrigues, T., Desprat, S., Toucanne, S., Morales-del-Molino, C., Hodell, D., Alonso-Garcia, M., Abrantes, F., Gomes, S., and Sanchez Goñi, M. F.: Major Hydroclimate shifts in Europe during severe glacial periods (MIS 16 and MIS 12) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20191, https://doi.org/10.5194/egusphere-egu25-20191, 2025.

Due to the exceptionally long atmospheric lifetime of anthropogenic CO₂, anthropogenic emissions are expected to affect the timing of the next glacial cycle. This is because glacial inception depends not only on changes in solar insolation, but also on CO₂ concentration. Using the fast Earth system model CLIMBER-X, we conduct long-term transient coupled climate–carbon cycle–ice sheet simulations to explore how different levels of cumulative emissions influence the predicted timing of the next glacial inception. Our results show that assumptions about the pre-industrial state of the carbon cycle and the magnitude of cumulative emissions profoundly impact the predicted timing of inception. We find that historical carbon emissions are insufficient to delay the next glacial period, which would naturally occur around 50 kyr AP (kiloyears after present). Cumulative emissions exceeding 1000 PgC are likely to postpone glacial inception until 100 kyr AP, while emissions up to 5000 PgC would still lead to glacial inception within the next 200 kyr. Millennial-scale AMOC variability, particularly its weakening into Stadial conditions, is also shown to play a critical role in the exact timing of the onset of the next glaciation. Despite this, the simulated timing of glacial inception aligns reasonably well with the predicted timing, which was found using the critical insolation–CO₂ relation and a dedicated set of coupled climate–carbon cycle experiments. In these experiments, the predicted timing was identified as the point when the simulated atmospheric CO₂ concentration drops below the critical threshold required to trigger glacial inception, given a specific value of maximum summer insolation at 65°N. Our study underscores the long-term impact of anthropogenic CO₂ emissions on the Earth's climate, and offers new insights on the inherent predictability of glacial cycles.

How to cite: Kaufhold, C., Willeit, M., Munhoven, G., Klemann, V., and Ganopolski, A.: Magnitude of anthropogenic CO2 emissions and pre-industrial carbon cycle state as key factors which determine timing of the next glacial period, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10664, https://doi.org/10.5194/egusphere-egu25-10664, 2025.

We present results from the NBP03-01A-20PCA sedimentary record which was recovered from the outer continental margin of the central Ross Embayment. Sediments comprise mud, with sub-angular to sub-rounded fine to coarse sand, common pebbles and rare cobbles. A paleomagnetic age model indicates the succession has a basal age of c. 1.1 Ma with magnetic reversals at 4.21 m, 5.74 m, and 5.85 m depth correlated with C1n-C1r.1r-C1r.1n-C1r.2r geomagnetic reversals.

Magnetic mineral concentration and micro IBRD data were considered a proxy for the proximity of the Ross Ice Shelf grounding zone and calving line to the core site. High magnetic mineral concentration and high terrigenous content correspond to sediments deposited beneath a floating ice shelf or from icebergs calved from that source. Time series analysis of these data indicate the advance and retreat of the Ross Ice Shelf—and by extension the West Antarctic Ice Sheet— were primarily paced by 41,000-year-long obliquity cycles until at least 400,000 years ago.

Insolation was predicted to control Antarctic ice volume; however, the frequency of glacial cycles inferred from global benthic foraminiferal oxygen isotopic and ice core records were originally interpreted to indicate that Antarctic ice volume variations were paced by 100,000-year-long (eccentricity) cycles from about 800,000 years ago. This interpretation was never confirmed from sedimentological reconstructions of ice margin advance and retreat cycles around Antarctica.

On seasonal timescales, ablation at the base of the western sector of the Ross Ice Shelf is controlled by the inflow of Ross Sea surface waters warmed by the summer sun. On orbital timescales, season length is suggested to control Antarctic temperature, with long summers and short winters leading to gradual warming and vice versa. We suggest that ablation, driven by circulation of warm summer surface waters under the ice shelf, is the mechanism by which long-term insolation control of sea surface temperatures produced the orbitally paced cycles observed here.

Our study reconciles the historical mismatch between high latitude insolation variations and glacial cycles inferred from distal records. We suggest that high-latitude insolation controlled Southern Ocean heat uptake and continued to be the main pacemaker of Antarctic glaciations well into the late Pleistocene. More sedimentary records from Antarctica and the Southern Ocean are required to reconstruct the true glacial history and whether different sectors of the margin are sensitive to different forcings.

Ohneiser, C., Hulbe, C.L., Beltran, C. et al. West Antarctic ice volume variability paced by obliquity until 400,000 years ago. Nat. Geosci. 16, 44–49 (2023). https://doi.org/10.1038/s41561-022-01088-w

How to cite: Ohneiser, C., Hulbe, C., Beltran, C., Condon, D., and Worthington, R.: West Antarctic ice volume variability paced by obliquity until 400,000 years ago, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3186, https://doi.org/10.5194/egusphere-egu25-3186, 2025.

In the Nordic seas, major warm and cold phases are thought to be related to a variable advection of warm surface waters and the occurrence or absence of deepwater formation, icebergs, sea-ice, and meltwater. The onset of the last glaciation – after the peak of MIS5e but well before MIS5d - was already accompanied by widespread ice drift, conditions which persisted almost continuously into the Holocene (~10 ka).

In this study we focused on the time period MIS5b into MIS3 (~90-60ka). With respect to the concept of a 100ky glacial-interglacial periodicity, this Middle Weichselian interval stands out in the arctic-subarctic ocean as it commenced with a deglacial phase that led to an intermittent but rather pronounced warming after ~85ka (MIS5a). By using benthic and planktic foraminiferal indicator species combined with stable isotope analyses, our data suggest that brief but intense warming episodes were responsible for ice growth and associated large sea-level drop recognized globally for glacial MIS4 (~70-65ka). Although occurring during times of decreasing summer insolation the massive size increase of surrounding ice sheets must have been facilitated through high moisture supply transferred into the polar region. MIS4 terminates in a major deglaciation (~H6) implying that the transition into MIS3 was again forced by a major warming. While this warming is difficult to verify in the Nordic seas through conventional proxies, the world-wide decrease in foraminiferal carbon isotopes at this time may indicate the involvement of the carbon system.

How to cite: Bauch, H. and Bingham, G.: Ice-ocean interactions and climate dynamics across the Middle Weichselian (~90-60ka), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9244, https://doi.org/10.5194/egusphere-egu25-9244, 2025.

Glacial inceptions and terminations are driven by orbital forcing and non-linear responses of the carbon cycle and ice sheets. Traditional theory primarily points to peak summer insolation at 65°N to explain changes in Northern Hemisphere ice sheet extent, a process amplified by atmospheric CO2 concentration. In the meantime, several studies of the last deglaciation find that Southern Hemisphere temperatures and atmospheric CO2 concentration increase earlier than temperatures in the Northern Hemisphere. Hence, Northern Hemisphere ice sheets may be more strongly coupled to surface temperature in the Southern Hemisphere than in the Northern Hemisphere. We expand this analysis for the entire last glacial cycle, with regional reconstructions of Sea Surface Temperatures. We find that the Southern Hemisphere leads the Northern Hemisphere by 3kyr for timescales above 10kyr. In addition, variations in the Southern Hemisphere show significantly larger amplitudes and correlate with peak summer insolation at 65°N. We test a range of mechanisms using CLIMBER-X and LOVECLIM simulations, including direct effects of orbital forcing and AMOC variability, to explain these results. These findings bring a new understanding of the role of the bipolar seesaw and the orbital forcing to explain temperature variability on timescales above 10kyr. Finally, our study provides new insights into the triggers of glacial inception and termination.

How to cite: Baudouin, J.-P., Willeit, M., Weitzel, N., Jonkers, L., Mulitza, S., and Rehfeld, K.: Southern Hemisphere leads global temperature changes during the last Glacial Cycle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16252, https://doi.org/10.5194/egusphere-egu25-16252, 2025.

The connection between abrupt high-latitude warming during the last glacial period and rapid temperature/hydrologic changes at lower latitudes has revealed strong inter-hemispheric teleconnections in the ocean–atmosphere system. How this connection influenced climate in the midcontinent of North America is unclear because climate archives with sufficient time resolution are scarce. This study investigates paleoclimate changes ~70–45 thousand years ago (ka) across multiple timescales (sub-annual to millennial) from speleothems in southern Wisconsin and Minnesota (USA). To do this we use fluorescent imaging (confocal laser microscopy) and a secondary ion mass spectrometer (SIMS) to collect high-resolution δ¹⁸O measurements (5-10µm) across individual stalagmite growth bands. Annual growth bands are 20-80 µm in width, allowing for subannual sampling resolution. Abrupt climate changes that were previously identified in a stalagmite from this region (Batchelor et al. 2023) are recorded as several large (2.0 to 3.0‰), negative δ¹⁸O excursions that occurred between 61–55 ka, coinciding with Dansgaard–Oeschger (DO) events 17–14. Here we extend the record from our prior work by 10 ka from a higher accumulation rate speleothem. Our high-resolution δ¹⁸O SIMS data show similar large (1 to 1.5‰) δ¹⁸O shifts during DO events 13 and 12 that occur in less than 30 years. When comparing the intra-annual δ¹⁸O gradient measured in different fluorescent bands, we observe a weaker gradient during DO interstadials, and a stronger gradient during DO stadials suggesting that seasonal changes in the hydrological system are also occurring during these events. Our results demonstrate large, abrupt temperature and hydrological responses to DO events during the last glacial period in midcontinent North America and indicate a strong seasonal climate response to DO warming and cooling. At the meeting we will present our published and unpublished measurements and related modeling simulations.

How to cite: Marcott, S., Reusche, M., Batchelor, C., Orland, I., He, F., Edwards, R. L., and Dutton, A.: Decadal to seasonal changes in oxygen isotopes across Dansgaard-Oeschger events from speleothems in the mid-continent of North America, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1918, https://doi.org/10.5194/egusphere-egu25-1918, 2025.

A global set of pollen records extending through Dansgaard-Oeschger (D-O) cycles during Marine Isotope Stage 3 was combined with LOVECLIM model simulations to infer patterns of land temperature and albedo dynamics, using a 3D variational data assimilation technique. The calculated global land albedo feedback was unexpectedly large and positive, suggesting a potential role (alongside oceanic mechanisms) in the causation of these cycles. A simple zero-dimensional climate model, equipped with a capacitor standing for ocean heat storage, and an inductor standing for the non-linear feedback, illustrates a mechanism that can generate ‘fast-slow’ oscillations similar to those observed, when the equilibrium point lies in a certain range ­– while tending to converge to the equilibrium point when it lies outside that range. This critical range corresponds to a state of the Earth system where the gain of the non-linear feedback is ≥ 1. The equilibrium point is sensitive to orbital forcing, suggesting a possible mechanism by which the Earth system could enter and exit the oscillatory state.

How to cite: Liu, M., Zhu, Z., Chavez, E., Prentice, I. C., and Harrison, S. P.: Land albedo feedback may have shaped the ‘fast-slow’ pattern of Dansgaard-Oeschger cycles, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5403, https://doi.org/10.5194/egusphere-egu25-5403, 2025.

The Atlantic meridional overturning circulation (AMOC) plays a crucial role in shaping climate conditions over the North Atlantic region and beyond, and its instability explains past abrupt climate transitions and millennial-scale glacial climate variability.
Here we use the fast Earth system model CLIMBER-X to explore the stability of the AMOC when faced with combined changes in atmospheric CO₂ concentrations and FWF in the North Atlantic under different configurations of the Northern Hemisphere (NH) ice sheets.
We find four different AMOC states associated with qualitatively different convection patterns. Apart from an Off AMOC state with no North Atlantic deep-water formation and a Modern-like AMOC with deep water forming in the Labrador and Nordic seas as observed at present, we find a Weak AMOC state with convection occurring south of 55°N and a Strong AMOC state characterized by deep-water formation extending into the Arctic. Generally, the equilibrium strength of the AMOC increases with increasing CO₂ and decreases with increasing FWF.
Our results show that Dansgaard-Oeschger (DO) oscillations originate from convective instability leading to transitions between the Weak and Modern AMOC states when the surface buoyancy flux integrated over the northern North Atlantic (>55°N) changes sign. Under typical mid-glacial conditions in terms of NH ice sheet size and atmospheric CO₂ the model produces pronounced internal millennial-scale climate variability that resembles observed DO oscillations.

How to cite: Willeit, M. and Ganopolski, A.: Generalized stability landscape of the Atlantic Meridional Overturning Circulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9702, https://doi.org/10.5194/egusphere-egu25-9702, 2025.

The Last Glacial Period was characterised by repeated millennial-scale climate oscillations which are commonly referred as the Dansgaard-Oeschger (D-O) variability. During some of the coldest D-O stadials, massive icebergs discharged into the North Atlantic Ocean leading to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC). These events are known as Heinrich events. In this study, we perform North Atlantic freshwater hosing experiments under 49 ka boundary conditions to simulate Heinrich 5 using the Australian Earth System Model ACCESS-ESM1.5 (CMIP6/PMIP4 model). We investigate the Southern Hemisphere and Australian climate response to an AMOC shutdown. The simulated temperature and precipitation changes are compared with available proxy reconstructions of Heinrich stadials, with a focus on the Southern Hemisphere. These idealised simulations provide insight into the processes linking changes in North Atlantic climate to the Southern Hemisphere, and particularly the Australian region.

How to cite: Du, Y., Brown, J., Menviel, L., Saini, H., Drysdale, R., and Hutchinson, D.: ACCESS-ESM1.5 model simulations of AMOC shutdown during Heinrich 5: impacts on Southern Hemisphere hydroclimate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-384, https://doi.org/10.5194/egusphere-egu25-384, 2025.

The western Pacific warm pool, being the greatest source of heat and water vapor, has a significant impact on global climate change. Furthermore, it has unique properties that distinguish it from other seas, such as the barrier layer, which can govern ENSO via the thermohaline stratification structure. However, it is unclear what the tropical Pacific Ocean's state was throughout the deglacial period and how it influenced global warming. Here we reconstructed the temperature and salinity evolution history of the upper water in the core area of ​​the barrier layer area by collecting sediment core samples from the KX97322-4 station (0ºS, 159.24ºE, 2362 m water depth) of the Ontong-Java station. Based on the analysis of δ¹⁸O and Mg/Ca of two planktonic foraminifera (Globigerinoides ruber and Trilobatus sacculifer) living in different layers of the mixed layer, we reconstructed the temperature and salinity changes in the upper and lower layers of the mixed layer in this region over the past 140,000 years. The vertical temperature and salinity comparison reveals that the barrier layer has tended to strengthen over the two most recent deglaciations. Combined with the reconstruction findings of additional stations, we discovered that the barrier layer was more broadly established during the second termination period, which may explain why MIS5e is warmer than MIS1. This phenomena is linked to increased rainfall in the Indonesian Sea and decreased rainfall in the open sea area of the warm pool. Based on present ocean measurements, it is assumed that an increase in rainfall in the warm pool during the deglacial will result in a deepening of the vertical salinity stratification of the surface water, hence reinforcing the barrier layer. Our findings indicate that lateral halocline intrusion primarily controls the barrier layer on a long time scale, potentially increasing the impact of ENSO on the La Niña-like condition throughout the deglaciation period. During the Holocene, the weakening of the barrier layer correlates to an El Niño-like condition. Reconstructing seawater δ¹⁸O at 185 sites in the Indo-Pacific area revealed that rainfall variations in the western and eastern regions of the warm pool exhibited opposing tendencies. The westward movement of the rain belt may have contributed to the weakening of the barrier layer over the Holocene. Our findings give fresh evidence and a mechanical explanation for long-scale ENSO-like phenomena, particularly during the fast warming deglacial.

How to cite: Zhang, S., Yu, Z., Holbourn, A., Wang, Y., Gong, X., Chang, F., and Li, T.: The role of thermohaline structure in upper waters of the western Pacific warm pool during deglaciation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13110, https://doi.org/10.5194/egusphere-egu25-13110, 2025.

How to cite: Romé, Y., Ivanovic, R., Gregoire, L., Swingedouw, D., Sherriff-Tadano, S., and Börner, R.: Explaining the chain of events of the last deglaciation through the convection-advection oscillator mechanism, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1714, https://doi.org/10.5194/egusphere-egu25-1714, 2025.

During the last 20,000 years, the climate of the Earth evolved from a state much colder than today with large ice sheets covering North America and Northwest Eurasia to its present state. The fully-interactive simulation of this transition represented a hitherto unsolved challenge for state-of-the-art climate models. We use a novel coupled comprehensive atmosphere-ocean-vegetation-ice sheet-solid earth model to simulate this transient climate evolution, referred to as the last deglaciation. The model considers dynamical changes of ice sheets (shape and extent) as well as changes in the land-sea mask and river routing. The model also contains a dynamical iceberg component. An ensemble of eight transient model simulations realistically captures the key features of the last deglaciation, as depicted by proxy estimates.

In addition, our model simulates a series of abrupt climate changes, which can be attributed to different drivers. Sudden weakenings of the Atlantic meridional overturning circulation during the glacial state and the first half of the deglaciation are caused by Heinrich-event like ice-sheet surges, which are part of the model’s internal variability. We show that the timing of these surges depends on the initial state and the model parameters, illustrating the stochastic nature of the events. Abrupt events during the second half of the deglaciation are caused by a long-term shift in the sign of the Arctic freshwater budget, by changes in the opening of ocean passages and/or by abrupt changes in the river routing. In contrast to the Heinrich-event like ice-sheet surges, the abrupt events of the second half of the deglaciation are deterministic, as they occur as inherent features of the deglaciation.

How to cite: Mikolajewicz, U., Kapsch, M.-L., Schannwell, C., Six, K. D., Ziemen, F. A., Bagge, M., Baudouin, J.-P., Erokhina, O., Gayler, V., Klemann, V., Meccia, V. L., Mouchet, A., and Riddick, T.:  Deglaciation and abrupt events in a coupled comprehensive atmosphere--ocean--ice sheet--solid earth model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8507, https://doi.org/10.5194/egusphere-egu25-8507, 2025.

The last deglaciation (Termination I, T-I) in the terrestrial mid latitudes of the Southern Hemisphere is well represented by records from lakes and alpine glaciers, which together provide information on regional vegetation, fire and temperature change. However, much less is known about variations in hydroclimate across this interval. We present a high-resolution, multi-proxy time series from a Tasmanian stalagmite spanning T-I. Coherent patterns in stable oxygen and carbon isotopes, growth rates, initial uranium isotope ratios and trace elements (Mg and Sr) reveal multiple, millennial-scale episodes of regional hydrological change during the deglaciation. Periods with a more positive moisture balance, inferred from low carbon isotope ratios and low Mg/Ca and Sr/Ca, occur when the climate was cooler, and coincide with lower growth rates and oxygen isotope ratios. Converse excursions reflect warmer periods with a reduced water balance. These swings in hydroclimate occur at a higher frequency than the classical millennial-scale deglaciation model of early warming - Antarctic Cold Reversal – late warming.

The stalagmite hydroclimate record shows a strong similarity to iceberg-rafted debris fluxes off the Antarctic Peninsula, with periods of increased iceberg flux coinciding with increased effective moisture over northern Tasmania. This suggests that enhanced iceberg and meltwater release altered regional ocean and atmospheric circulation, bringing cooler (and potentially wetter) conditions to Tasmania. Results from climate-model freshwater hosing experiments show that during such episodes, sea-ice cover increases and negative surface-temperature anomalies are present over the Indian, Atlantic and SW Pacific sectors of the Southern Ocean. Although rainfall itself does not increase, water balance over Tasmania is substantially enhanced during these events, consistent with the speleothem data. There is also some evidence that the patterns of retreat (re-advance) of some NZ glaciers may coincide with reduced (enhanced) moisture balance over Tasmania, implying that NZ glacier history through T-I may be more nuanced than previously proposed.

How to cite: Drysdale, R., MacGregor, C., Cooley, S., Hellstrom, J., Jasper, C., Treble, P., Passelergue, M., Eberhard, R., Kearns, R., Larcher, R., Demeny, A., Sun, Y., Zhang, X., and Lohmann, G.: Southern Hemisphere mid-latitude hydroclimate during Termination I: links to Antarctic iceberg discharge, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8625, https://doi.org/10.5194/egusphere-egu25-8625, 2025.

Freshwater forcing (FWF) is recognized as a primary driver of abrupt Atlantic Meridional Overturning Circulation (AMOC) changes during significant paleoclimate variations. Numerous hosing experiments have been conducted to examine the sensitivity of AMOC to FWF; however, most of research effort was on the AMOC shutdown with few examining its recovery. Therefore, the processes underlying AMOC recovery remain uncertain.

This study employs version II of the TraCE-21K simulation (TraCE-21K-II) using the Community Climate System Model version 3 (CCSM3) to investigate key factors and mechanisms behind the two-phase AMOC recovery, shown in the TraCE-21K-II simulation, following the sudden cessation of FWF input at the end of the Younger Dryas (YD). Our findings reveal a strong correlation between AMOC strength and sea surface salinity (SSS) patterns in the North Atlantic, influenced differently by FWF and sea ice cover during various paleoclimate periods. Before the initial AMOC recovery, SSS is dominated by FWF input, with sea ice mostly covering high-latitude regions throughout the YD. As the FWF ceases, the interaction of warm, salty subtropical water with cold, fresh northern water induces AMOC strength oscillations. Eventually, persistent northward flow of warm, salty water leads to sea ice collapse, resulting in a sharp SSS increase and triggering the second recovery phase. These results highlight the critical role of SSS-related processes in AMOC variation and argue that the AMOC recovery may not be solely influenced by the cessation of FWF. Investigating these mechanisms further could prompt a reassessment of our current knowledge of AMOC dynamics and their implications for future climate projections.

How to cite: Lee, S.-Y., Lin, Y.-S., and He, F.: The role of sea ice in AMOC recovery in TraCE-21KII simulation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2027, https://doi.org/10.5194/egusphere-egu25-2027, 2025.

According to CMIP6 projections, all socio-economic scenarios show a rapidly warming climate with substantial polar amplification that will lead the Arctic becoming ice free during summer. This absence of summer sea ice acts as a positive feedback for Arctic warming and impacts regional to global scale (Bruhwiler et al., 2021).

The last interglacial, ~127,000 years ago, is a past warm climate state with significantly reduced prevalence of Arctic sea ice. This period provides a valuable out-of-sample test for climate models with which future projections are computed and may help us to better understand processes and climate patterns related to a blue Arctic.

The Last Interglacial (~127,000 years ago) is a period when orbital parameters caused much increased boreal high-latitude summer insolation forcing. The fourth phase of the Paleoclimate Modelling Intercomparison Project (PMIP4) identified in the lig127k simulation substantial model-spread of simulated minimum annual Arctic sea ice conditions (Kageyama et al., 2020; Sime et al., 2023). To enable a better understanding of the origin of model-model discord the paleoclimate science community has proposed simulation abrupt-127k (Sime et al., in prep.) as part of the FastTrack portfolio of the seventh interation of the Climate Modelling Intercomparison Project (CMIP7, Dunne et al., 2024). While simulation abrupt-127k inherits orbital and greenhouse gas parameters of PMIP4 simulation lig127k, its layout follows the approach of CMIP simulation abrupt-4xCO2, where the initial scientific focus is on a comparably short period (~100 model years) after model initialisation rather than on the quasi-equilibrated climate as in PMIP4 simulation lig127.

This presentation will outline the rationale and utility of CMIP7 FastTrack simulation abrupt-127k to a) increase the model ensemble from the classical PMIP to the wider CMIP framework; b) focus on processes and feedbacks that translate modified climate forcing into Arctic climate towards refining our understanding of the apparent model-model discord found in lig127k; c) enhance analysis of simulated sea ice conditions and dynamics based on the standardized protocol for sea-ice related climate model outputs by the Sea-Ice Model Intercomparison Project (SIMIP; Notz et al., 2016).

How to cite: Kageyama, M., Stepanek, C., Sime, L., Diamond, R., Brierley, C., Schroeder, D., and Malmierca-Vallet, I.: The CMIP7-PMIP FastTrack abrupt-127k simulation : an opportunity to test Arctic sea ice representation in climate models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20249, https://doi.org/10.5194/egusphere-egu25-20249, 2025.

The Last Interglacial (127,000–116,000 years before present) is characterized by higher Arctic surface temperatures relative to the pre-industrial era. During this period, the Arctic experienced significant summer sea-ice reduction, with some studies suggesting the possibility of an ice-free Arctic Ocean at the end of summers. These changes in sea-ice cover were primarily driven by increased summer solar insolation caused by orbital configuration at that time, but they may also have been influenced by shifts in oceanic conditions. Paleoceanographic studies generally assume modern-like circulation patterns when interpreting proxy data. However, the stability of oceanic circulation under Last Interglacial forcing and its potential impact on sea-ice loss remain poorly understood.

Using simulations from the last Coupled Model Intercomparison Project (CMIP6/PMIP4), we analysed differences in surface ocean circulation between the Last Interglacial and pre-industrial periods, and their effect on annual sea-ice variations. Our results indicate an anomalous cyclonic circulation over Greenland and the surrounding seas, which leads to intensified Baffin and Labrador Currents west of Greenland and a weakened East Greenland Current. These changes affect sea-ice drift and water transports outside the Arctic Basin. Furthermore, models generally simulate a strengthening of the North Atlantic subpolar gyre during the Last Interglacial, associated with increased Atlantic inflows south of Greenland and towards the Nordic Seas. Despite these changes, we found no consistent evidence of "Atlantification" of the Arctic during the Last Interglacial. This contrasts with future CO2 emission scenarios, where similar reductions in annual sea-ice volume are accompanied by Atlantification.

How to cite: Sicard, M., de Boer, A. M., Coxall, H. K., Koenigk, T., Karami, M. P., Navarro-Labastida, R. G., Jakobsson, M., and O'Regan, M.: Ocean circulation shifts and sea-ice decline in the North Atlantic-Arctic sector during the Last Interglacial, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12091, https://doi.org/10.5194/egusphere-egu25-12091, 2025.

There is spatial and temporal variability in the density of sea ice reconstructions available around Antarctica through the Holocene and the preceding Last Glacial Maximum. This study provides the first continuous sea ice reconstruction for the Princess Ragnhild Coast (20 °E and 34 °E), spanning the last ~50,000 years (Marine Isotope Stage 3, the Last Glacial Maximum, the deglaciation and the Holocene). We use snow petrel (Pagodroma nivea) stomach-oil deposits as archives of the sea ice environment in which the seabirds forage. For our study site, the snow petrels integrate information spanning the Atlantic and Indian Ocean sectors of the Southern Ocean, within ~1000 km of the coastline.  Our region of study currently has no Holocene reconstructions and only inferred Last Glacial Maximum sea ice reconstructions (Lhardy et al., 2021; Crosta et al., 2022).

Here, we use bulk stable isotopes, fatty acid ratios and elemental data to infer that sea ice in the Princess Ragnhild Coast area had a limited expansion during the Last Glacial Maximum. In agreement with previously inferred limits (Lhardy et al., 2021) our data suggests that the sea ice limits remained within ~500km of the continent edge over our time interval of interest. We also see a large negative trend in nitrogen isotopes between the Last Glacial Maximum and Holocene, in agreement with trends seen in marine sediment cores (Ai et al., 2021) although we suggest that diet largely remains the same throughout.

How to cite: Cole, Y., Hodgson, D., Bentley, M., Honan, E. M., and McClymont, E.: Fluctuating sea ice margins over the last ~50 kyr from the Sør Rondane mountains, East Antarctica, from a novel biological archive, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19820, https://doi.org/10.5194/egusphere-egu25-19820, 2025.

Paleo-records such as marine sediments and ice cores are commonly used to extend our knowledge about past sea-ice cover during the period prior to instrumental observations. Several studies (Spolaor et al., 2016, Saiz Lopez and Von Glasow, 2012) have identified bromine in ice cores as a potential proxy for past sea ice conditions. During polar springtime, in fact, the photochemical recycling of bromine is extremely efficient over first year sea ice (FYSI), resulting in enhanced concentrations of inorganic gas phase bromine (e.g. BrO) compared to the ocean surface, multi-year sea ice or snow-covered land. This process is known as “bromine explosion” and is detected by satellite sensors and in-situ observations from early March
to late May. After emission, the BrO plume is frequently carried for several days by high-latitude cyclones in the lower troposphere until it reaches land and falls in the form of bromine enriched snow compared to seawater Br/Na ratio.  Here, we present the first statistical validation of this proxy using satellite sea ice observations. By combining bromine enrichment (relative to seawater, Br_enr) records from three Greenlandic ice cores with satellite sea ice imagery over a span of three decades, we demonstrate its efficacy. During the satellite era (1984–2016), Brenr values in the ice cores show significant correlations with first-year sea ice formed in the Baffin Bay and Labrador Sea, confirming that gas-phase bromine enrichment processes, which predominantly occur over sea ice surfaces, are the primary drivers of the Brenr signal in ice cores. Furthermore, to evaluate Br_enr’s ability to capture historical sea ice variability, we compare 20th-century Arctic sea ice historical records and proxy data with reconstructions derived from an autoregressive–moving-average (ARMA) model. The results show overall strong agreement. While further improvements are needed—such as site-specific calibrations and detailed studies on bromine transport dynamics—this study introduces a novel quantitative method for reconstructing past seasonal sea ice variability using bromine enrichment in ice cores

How to cite: Scoto, F., Maffezzoli, N., Saiz-López, A., Cuevas, C. A., Gagliardi, A., Varin, C., and Spolaor, A.: A novel method for sea ice reconstructions using satellite calibration of bromine enrichment records in Arctic ice cores, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15866, https://doi.org/10.5194/egusphere-egu25-15866, 2025.