The last ice age was repeatedly punctuated by millennial scale intervals of extreme cold in the Northern Hemisphere. These cold periods, known as stadials, are also times of ice calving and glacial melt, sending debris-laden icebergs into the North Atlantic Ocean, and raising questions about the mechanisms of ice sheet instability. Here, we provide new high resolution marine temperature reconstructions from the northeast north Atlantic (ODP 980 55°29.1’N 14°42.1’W) that show that, whilst persistent cold stadial temperatures might be inferred from abundances of the polar foraminifera Neogloboquadrina pachyderma (% N. pachyderma), Mg/Ca temperature reconstructions from the same planktic species and Globigerina bulloides indicate gradual subsurface warming by as much as 3ᵒC over the course of a stadial. We explain this apparent discrepancy by turning to the seasonal influences on these temperature proxies, suggesting that very high % N. pachyderma reflects preferential survival of this species in polar waters with extensive sea ice, while Mg/Ca reveals relatively mild subsurface conditions during the summer growing season. Modelling shows that warming summers in the high latitude subsurface may be explained by persistent influence of warm waters from lower latitudes, in combination with a lack of winter heat loss due to the insulating effect of sea ice and climbing atmospheric CO₂. This accumulation of heat at critical depths for marine terminating glaciers underlines the influence of warming seawater on ice sheet stability and could provide an additional source of heat to trigger abrupt interstadial warming.

How to cite: Littley, E., Burke, A., Shankle, M., Gray, W., Zhang, X., Sun, Y., Roberts, W., Rome, Y., Ivanovic, R., and Rae, J.: Persistent shallow subsurface warming during North Atlantic Stadials drives ice-destabilisation and rapid climate change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19161, https://doi.org/10.5194/egusphere-egu24-19161, 2024.

New multiproxy records informing in unprecedented detail on hydrographic changes in the northernmost Nordic Seas during Dansgaard-Oechger events, will be presented. An extensive sea ice cover and a homogenous intermediate water mass have been suggested to characterize the Nordic Seas during stadials, based on records from the southern Nordic Seas. Combined planktic and benthic oxygen and carbon isotopes informing on water mass stratification and ventilation, supplemented by planktic Mg/Ca, planktic and benthic foraminiferal abundance, Ice Rafted Debris, diatom, and biomarker data suggests, however, less stable stadial water column conditions in the Fram Strait. The stadial to interstadial transitions are characterized by a series of short living changes in productivity, water mass characteristics and IRD deposition. The new records from the Fran Strait will be see in context of comparable records from other sites in the Nordic Seas. Implications of the new results will be discussed.

How to cite: Risebrobakken, B., Blindheim, D. I., Tisserand, A., Wong, W., Kryk, A., and Bąk, M.: Stadial to interstadial hydrographic changes in the northern Nordic Seas , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12366, https://doi.org/10.5194/egusphere-egu24-12366, 2024.

A generic nine-box ocean model (GNOM) is developed to study the stability and variability of the Atlantic Meridional Overturning Circulation (AMOC). Using this model, it is shown that centennial to millennial time scale self-sustained oscillations in AMOC strength and associated climate characteristics resembling temporal dynamics of observed Dansgaard-Oeschger events appear within a certain range of the boundary conditions (temperature and freshwater flux). Adding white noise to the system makes such oscillations more robust, i.e. they occur within a larger area in the phase space of temperature-freshwater forcing. However, regardless of the model’s parameters and boundary conditions, the typical periodicity of such oscillations is about 1000 years, which is much shorter than the typical recurrence time of observed  Dansgaard-Oeschger events. Only after adding an interaction with an additional component, which has a longer internal time scale and mimics the response of the surrounding ice sheets, do much longer self-sustained oscillations of AMOC arise.  

How to cite: Ganopolski, A.: Dansgaard-Oeschger events in a box ocean model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1582, https://doi.org/10.5194/egusphere-egu24-1582, 2024.

Greenland ice core records feature Dansgaard–Oeschger (D-O) events, which are abrupt warming episodes followed by gradual cooling during ice age climate. The three climate models used in this study (CCSM4, MPI-ESM, and HadCM3) show spontaneous self-sustained D-O-like oscillations (albeit with differences in amplitude, duration and shape) in a remarkably similar, narrow window of carbon dioxide (CO2) concentration, roughly 185-230 ppm. This range matches atmospheric CO2 during Marine Isotopic Stage 3 (MIS 3: between 27.8 – 59.4 thousand of years BP, hereafter ka), a period when D-O events were most frequent. Insights from the three climate models point to NA sea-ice coverage as a key ingredient behind D-O type oscillations, which acts as a tipping point. No other climate property (NA salinity, Atlantic Meridional Overturning Circulation, Global mean Ocean temperature and Global mean temperature) is found to directly determine whether D-O type behaviour can occur in all three models.

How to cite: Malmierca Vallet, I., Sime, L. C., Valdes, P. J., Klockmann, M., Vettoretti, G., and Slattery, J.: Spontaneous Dansgaard–Oeschger type oscillations in three models: the impact of CO2, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5094, https://doi.org/10.5194/egusphere-egu24-5094, 2024.

Greenland ice core records feature Dansgaard–Oeschger (D-O) events; abrupt warming episodes followed by a gradual cooling phase during mid-glacial periods. Here, we analyse spontaneous self-sustained D-O type oscillations reproduced in four climate models: COSMOS, CCSM4, MPI-ESM and HadCM3. We assess model performance against several metrics: rate of Antarctic warming during D-O stadials, timing of change point in Antarctic temperature with respect to the mid-point of the Greenland transition, duration of the D-O event and Intertropical Convergence Zone (ITC) position. The four models suggest consistently that the amplitude and spatial expression of D-O event temperature anomalies are dominated by coupled changes in the Atlantic Meridional Overturning Circulation (AMOC) and North Atlantic sea ice extent.

How to cite: Goswami, S. and Malmierca-Vallet, I.: Dansgaard-Oeschger events in climate models - criteria for model assessment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22546, https://doi.org/10.5194/egusphere-egu24-22546, 2024.

Dansgaard-Oeschger (D-O) climate variability during the last glaciation was first evidenced in ice cores and marine sediments, and is also recorded in various terrestrial paleoclimate archives in Europe. The relative synchronicity across Greenland, the North Atlantic and Europe implies a tight and fast coupling between those regions, most probably effectuated by an atmospheric transmission mechanism. In this study, we investigated the atmospheric changes during Greenland interstadial (GI) and stadial (GS) phases based on regional climate model simulations using two specific periods, GI-10 and GS-9 both around 40 ka, as boundary conditions. Our simulations accurately capture the changes in temperature and precipitation as reconstructed by the available proxy data. Moreover, the simulations depict an intensified and southward shifted eddy-driven jet during the stadial period. Ultimately, this affects the near-surface circulation towards more southwesterly and cyclonic flow in western Europe during the stadial period, explaining much of the seasonal climate variability recorded by the proxy data, including oxygen isotopes, at the considered proxy sites.

How to cite: Stadelmaier, K. H., Ludwig, P., Pinto, J. G., and Újvári, G.: Changes in atmospheric dynamics over Dansgaard-Oeschger climate oscillations around 40 ka and its impact on Europe, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1337, https://doi.org/10.5194/egusphere-egu24-1337, 2024.

Dansgaard-Oeschger (DO) events are a dominant mode of millennial-scale climate variability during the last glacial period. While the influences of DO events are most pronounced in the North Atlantic region, their impacts are detectable on the global scale. In Antarctica, their imprint primarily manifests as a muted and anti-phased response, which can be explained by the bipolar seesaw mechanism. However, the ice core record revealed an additional in-phase component, suggesting the presence of atmospheric teleconnections.

Here, we study the propagation of DO-type signals from the North Atlantic to the Southern Hemisphere in a set of simulations with the general circulation model HadCM3, including an isotope-enabled run. The simulations show spontaneous DO-type oscillations under glacial boundary conditions, which allows studying the timing and interaction of atmospheric and oceanic processes under a continuously varying background state. Consistent with the ice core record, we find a muted and anti-phased temperature response over Antarctica that lags the North Atlantic by ~300 years, and global sea surface temperature patterns in agreement with the bipolar seesaw mechanism. Additionally, we identify a robust Southern Hemisphere mode in phase with the North Atlantic. This mode is associated with hydroclimate changes in the tropical Pacific, which modulate the Southern Hemisphere atmospheric circulation, and, thereby, impact Antarctic temperatures. Notably, the dominant millennial-scale circulation pattern exhibits zonal asymmetries that do not resemble the leading modes of inter-annual variability. Preliminary analyses indicate a good agreement of the simulated millennial-scale variability in oxygen isotopes with a global compilation of speleothem and ice core records.

How to cite: Trombini, I., Weitzel, N., Valdes, P., and Rehfeld, K.: Interhemispheric teleconnections as drivers of Southern Hemisphere climate in simulations of spontaneous Dansgaard-Oeschger-type events, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9642, https://doi.org/10.5194/egusphere-egu24-9642, 2024.

Dansgaard–Oeschger (DO) warming events occurred throughout the last glacial period. Greenland ice cores show a rapid warming during each stadial to interstadial transition, alongside abrupt loss of sea ice and major reorganisation of the atmospheric circulation. Other records also indicate simultaneous abrupt changes to the oceanic circulation. Recently, an advanced Bayesian ramp fitting method has been developed and used to investigate time lags between transitions in these different climate elements, with a view to determining the relative order of these changes. Here, we subject this method to a critical review. Using ice core data, climate model output, and carefully synthesised data representing DO warming events, we demonstrate that the method suffers from noise-induced bias of up to 15 years. This bias means that the method will tend to yield transition onsets that are too early, and we find that the estimated timings of noisier transitions are more strongly biased. Further investigation of DO warming event records in climate models and ice core data reveals that the bias is on the same order of magnitude as potential timing differences between the abrupt transitions of different climate elements. Additionally, we find that higher-resolution records would not reduce this bias. We conclude that time lags of less than 20 years cannot be reliably detected, as we cannot exclude the possibility that they result solely from the bias. This prevents the unambiguous determination of the temporal phasing of DO warming events.

How to cite: Slattery, J., Sime, L. C., Muschitiello, F., and Riechers, K.: The Temporal Phasing of Rapid Dansgaard–Oeschger Warming Events Cannot Be Reliably Determined, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6161, https://doi.org/10.5194/egusphere-egu24-6161, 2024.

Terrestrial and marine proxy records suggest that climate variability was higher during the last glacial cycle than today. This is particularly true in the North Atlantic region where this enhanced variability is associated with changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Despite prolonged efforts to explain an oscillatory behaviour of glacial AMOC strength, it is still not fully understood; moreover, studies often describe its mechanism qualitatively or attempt to identify its drivers using methods that do not imply causation.

In this study we use the isotope-enabled Earth system model iCESM1.2 to simulate the glacial climate under conditions representing the Marine Isotope Stage 3 (about 38 thousand years before present). Our results show the AMOC oscillating with a period of roughly 500 years and an amplitude of 4 Sv (1 Sv = 10⁶ m³s^-1). Surface air temperature varies by about 1-2°C in Northern Europe, 4°C in Greenland, and up to 15°C over the North Atlantic Ocean where sea ice cover varies the most. Based on a supervised machine learning method (causal discovery), we find causal links between AMOC and North Atlantic Ocean salinity and meridional salt transport. We show how changes in the salinity of water advected into key deep-water forming areas feed back to the AMOC, thus driving the oscillation.

Our findings from applying causal discovery outline the mechanism of a salt-oscillator in a fully coupled model, and indicate the potential of this method to identify causal drivers that trigger variations in AMOC strength on different time scales too.

How to cite: Kovacs, T., Galytska, E., Prange, M., Paul, A., and Schulz, M.: Causal drivers of North Atlantic glacial climate variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20201, https://doi.org/10.5194/egusphere-egu24-20201, 2024.

How to cite: Kjær, H. A., Vinther, B., Svensson, A., Rasmussen, S., Blunier, T., Erhardt, T., Harlan, M., vallelonga, P., Maffezoli, N., Gkinis, V., Sowers, T., Menking, A., deCampo, A., Morris, V., Vaughn, B., Buizert, C., and Steffensen, J. P.: Spatial patterns of DO events in Greenland ice core chemistry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17661, https://doi.org/10.5194/egusphere-egu24-17661, 2024.

Glacial periods were punctuated by abrupt millennial scale climate changes, such as Dansgaard Oeschger events, Boeling-Allerod and Younger Dryas. Although glacial abrupt climate changes were shown to have a strong link to the Atlantic Meridional overturning circulation (AMOC) changes and the glacial background climate, simulating the stability and millennial change of AMOC and climate with fully coupled ocean-atmosphere GCM have been challenging. Here we present many cases of millennial scale climate variability with our Atmospheric Ocean coupled GCM, MIROC4m. A series of long transient experiments (> 10, 000 years) were performed systematically with different steady glacial conditions (CO2 level, obliquity, precession, meltwater, ice sheet size), to study the dependence of the sweet spot of millennial scale variability on the background climate and summarize the results as phase diagrams. We chose the model version which we simulate LGM AMOC weaker and shallower than the AMOC under Pre-Industrial condition. A reasonable sweet-spot of oscillation exists when the Northern Hemisphere ice sheets exist even without freshwater perturbation. In the sweet spot, self-sustained oscillation with bipolar seesaw pattern and shift between interstadial and stadial occur, with interval between abrupt events ranging from 1000 years to more than 5000 years depending on the background condition, while an abrupt shift from stadial to interstadial mode occurs in about 100 years. The sweet spot exists when the CO2 level is between 260ppm and 185ppm, depending largely on the obliquity but marginally on the precession and ice sheet size. When the obliquity or the CO2 amount is large (small), the AMOC is in a strong (weak) stable mode of about 18 (10) Sv (Sverdrup). Many aspects of the sweet spot, i.e., the duration of interstadial is longer systematically when the CO2 or obliquity is larger and the relation between the duration of interglacial and Antarctica air temperature, are very much in agreement with the ice core analysis and the deep-sea sediment.

How to cite: Abe-Ouchi, A., Chan, W.-L., Sherriff-Tadano, S., Obase, T., Kuniyoshi, Y., Mitsui, T., and Buizert, C.: Dansgaard Oeschger event and its dependence on background glacial climate condition simulated in a coupled Atmosphere-Ocean GCM, MIROC, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20346, https://doi.org/10.5194/egusphere-egu24-20346, 2024.

The Dansgaard-Oeschger (DO) events are characteriszed by pronounced abrupt changes in climatic conditions in high northern latitudes. While they have first been discovered in Greenland ice core records, their imprints are global and can be deteced in proxy archives across the globe. They can be considered as the archetype of abrupt climate changes for which empirical evidence exists, but have not yet been fully explained. In this talk I will present some recent results from analyzing ice-core derived time series with a focus on dating uncertainties, from concentual models proposing unerling physical mechanisms, and from comparing spatial patterns of the impacts of the DO events on global atmospheric circulation patterns. 

How to cite: Boers, N.: Dansgaard-Oeschger events - time series analysis and modelling across the hierarchy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10896, https://doi.org/10.5194/egusphere-egu24-10896, 2024.