478 research outputs found
Millennial Variability in an Idealized Ocean Model: Predicting the AMOC Regime Shifts
Get PDFA salient feature of paleorecords of the last glacial interval in the North Atlantic is pronounced millennial variability, commonly known as Dansgaard–Oeschger events. It is believed that these events are related to variations in the Atlantic meridional overturning circulation and heat transport. Here, the authors formulate a new low-order model, based on the Howard–Malkus loop representation of ocean circulation, capable of reproducing millennial variability and its chaotic dynamics realistically. It is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable. Accordingly, the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase. These indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system. Thus, monitoring particular indices of the ocean state could help predict a potential shutdown of the overturning circulation
10Be variations in polar ice cores
No full text10Be is produced in the atmosphere by cosmic radiation and has a half life of 1.5 · 106 years. The produced 10Be atoms are attached to aerosols and washed out by precipitation. The mean residence time in the atmosphere is about one year. Annual snow layers with their 10Be contents are ideally preserved in polar ice sheets.
With the new accelerator based mass-spectrometry technique it is possible to measure the natural 10Be concentrations in ice samples of 1 kg each. 10Be, with its short residence time and long half life, is very well suited for the study of variations of the interaction of cosmic radiation with the atmosphere. We measured firn samples from Dye 3 from the surface to about 100 m depth, representing the precipitation of the last 140 years. One sample per 2 years has been measured. The results are compared with the sunspot-number record. A clear indication of the 11 year cycle modulation has been obtained.
In addition the 10Be activities of ice samples from 1600 m-1900 m depth have been measured. Samples from this depth interval represent precipitation from about 13,000 yrs B.P. to 6000 yrs BP. The results are compared with results from Antarctica, published by Raisbeck. By measuring the 10Be/36Cl ratio, it should be possible to date ice older than about 50,000 yrs
The contribution of ice core studies to the understanding of environmental processes
No full textData obtained from the studies of polar ice cores supplement the records available from tree rings, peat bogs, lake and ocean sediments, and provide a relatively new data source to understand processes of the complex climatic and global cycles. The main sources of ice core data are stable and radioactive isotopes, soluble and particulate matter, and the composition of the gases occluded in the ice. Such information can be used to investigate the history and the variability of carbon dioxide and the climate system.
Temperature and other climatic data obtained from δ18O measurements of polar ice cores can be correlated with similar information obtained from carbonate lake sediments. Comparison of the δ18O profiles of the Dye 3 ice core and central European lake sediments show distinct similarities such as the identification of the Older Dryas-Bolling/Allerød-Younger Dryas-Preboreal sequence.
Measurements of the cosmic ray produced isotope 10Be on only 1 kg polar ice samples are possible by accelerator mass spectrometry. The resulting data reveals the 11-year solar modulation cycle and the Maunder Minimum of solar activity from 1645 to 1745 AD. The 10Be concentration values for the Maunder Minimum are a factor 1.6 higher than the average for the past 800 years. Using a carbon cycle model these 10Be variations can be compared to the 14C variations found in tree rings. The relatively good correlation suggests a common origin of the 10Be and 14C fluctuations and serves as a check of carbon cycle models. During the Wisconsin stage all of the Dye 3 ice core parameters measured to date (δ18O, CO2/air, SO4 −, NO3 −, Cl−, dust) show values fluctuating between two different boundary conditions. This suggests that the climate system existing at that time oscillated between a cold and a warm state, probably strongly influenced by different ocean circulations and ice cover. During the Wisconsin stage a cold system dominated; the transition to the Holocene is considered as the final transition to a warm state. Thereafter the boundary conditions did not allow the systems to switch back to a cold state
Data concerning statistical relation between obliquity and Dansgaard–Oeschger events
No full textData presented are related to the research article entitled “Using Holo-Hilbert spectral analysis to quantify the modulation of Dansgaard-Oeschger events by obliquity” (J. Deng et al., 2018). The datasets in Deng et al. (2018) are analyzed on the foundation of ensemble empirical mode decomposition (EEMD) (Z.H. Wu and N.E. Huang, 2009), and reveal more occurrences of Dansgaard-Oeschger (DO) events in the decreasing phase of obliquity. Here, we report the number of significant high Shannon entropy (SE) (C.E. Shannon and W. Weaver, 1949) of 95% significance level of DO events in the increasing and decreasing phases of obliquity, respectively. First, the proxy time series are filtered by EEMD to obtain DO events. Then, the time-varying SE of DO modes are calculated on the basis of principle of histogram. The 95% significance level is evaluated through surrogate data (T. Schreiber and A. Schmitz, 1996). Finally, a comparison between the numbers of SE values that are larger than 95% significance level in the increasing and decreasing phases of obliquity, respectively, is reported. Keywords: Dansgaard–Oeschger events, Obliquity, Time-varying Shannon entropy, Surrogate dat
CO2 concentration in air extracted from Greenland ice samples
No full textThe principal aim of the analyses of the CO2 concentration in air extracted from ice samples is to reconstruct the CO2 concentration of the atmosphere during the last millenia. For this purpose ice from very cold regions is best suited. Ice samples from Dye 3, where the mean annual air temperature is −20°C and summer melting is frequent, are not very well suited from this point of view. The results of CO2 analyses give however very valuable information on a possible temperature effect on the CO2 concentration of air in the bubbles. The CO2 content show's seasonal variations with an annual maximum value in the summer melt layer. The annual minimum values correspond approximately to the estimated atmospheric CO2 concentrations. Based on this experience, in spite of the complications due to the melt features, we try to reconstruct the history of the CO2 concentrations of the atmosphere. of special interest are the fast climatic transitions in the course and especially at the end of the last glaciation which are represented in the ice core by changes of parameters like acidity, dust, and istopic ratios in short depth intervals.
In this respect we discuss the time lag between the climatic warming at the end of the last glaciation and the increase of the atmospheric CO2 concentration. Finally results of gas content and gas composition of two ice samples from the lowest, silty part of the ice core are discussed
Measurements of a kind of DC-conductivity on cores from Dye 3
No full textThe de-conductivity measured with two electrodes that move slowly over an ice sample shows generally a good correlation with the acidity of the melted ice sample. The method has been suggested and successfully applied by C. Hammer. In this paper the experimental setup and some physical aspects of the method are discussed; results obtained on the Dye 3 core and their climatological interpretation are discussed in another paper in this volume by C. Hammer. With the newly developed measuring device the current is measured as a function of depth and recorded by an XY-recorder as well as in digitized form on magnetic tape. The resolution of the two recording methods are compared.
Based on observations made during the measurements at Dye 3 the effect of ice-core temperature on electrical conductivity is discussed. This serves as a contribution to the understanding of the basic physical phenomena involved in the electrical conductivity of ice. Finally some possible explanations for the observed seasonal variations are suggested and discussed
Rapid subtropical North Atlantic salinity oscillations across Dansgaard-Oeschger cycles
No full textGeochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age, the Dansgaard–Oeschger cycles1, were coupled to fluctuations in North Atlantic meridional overturning circulation through its regulation of poleward heat flux2. The balance between cold meltwater from the north and warm, salty subtropical gyre waters from the south influenced the strength and location of North Atlantic overturning circulation during this period of highly variable climate3, 4, 5. Here we investigate how rapid reorganizations of the ocean–atmosphere system across these cycles are linked to salinity changes in the subtropical North Atlantic gyre. We combine Mg/Ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four Dansgaard–Oeschger cycles (spanning 45.9–59.2 kyr ago) to generate a seawater salinity proxy record from a subtropical gyre deep-sea sediment core. We show that North Atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials, covarying with inferred shifts in the Tropical Atlantic hydrologic cycle6 and North Atlantic overturning circulation. These salinity oscillations suggest a reduction in precipitation into the North Atlantic and/or reduced export of deep salty thermohaline waters during stadials. We hypothesize that increased stadial salinities preconditioned the North Atlantic Ocean for a rapid return to deep overturning circulation and high-latitude warming by contributing to increased North Atlantic surface-water density on interstadial transitions
Rapid subtropical North Atlantic salinity oscillations across Dansgaard-Oeschger cycles
No full textGeochemical and sedimentological evidence suggest that the rapid climate warming oscillations of the last ice age, the Dansgaard–Oeschger cycles1, were coupled to fluctuations in North Atlantic meridional overturning circulation through its regulation of poleward heat flux2. The balance between cold meltwater from the north and warm, salty subtropical gyre waters from the south influenced the strength and location of North Atlantic overturning circulation during this period of highly variable climate3, 4, 5. Here we investigate how rapid reorganizations of the ocean–atmosphere system across these cycles are linked to salinity changes in the subtropical North Atlantic gyre. We combine Mg/Ca palaeothermometry and oxygen isotope ratio measurements on planktonic foraminifera across four Dansgaard–Oeschger cycles (spanning 45.9–59.2 kyr ago) to generate a seawater salinity proxy record from a subtropical gyre deep-sea sediment core. We show that North Atlantic gyre surface salinities oscillated rapidly between saltier stadial conditions and fresher interstadials, covarying with inferred shifts in the Tropical Atlantic hydrologic cycle6 and North Atlantic overturning circulation. These salinity oscillations suggest a reduction in precipitation into the North Atlantic and/or reduced export of deep salty thermohaline waters during stadials. We hypothesize that increased stadial salinities preconditioned the North Atlantic Ocean for a rapid return to deep overturning circulation and high-latitude warming by contributing to increased North Atlantic surface-water density on interstadial transitions
Abrupt changes in sea ice and dynamics of Dansgaard-Oeschger events
No full textChanges in sea ice are proposed as an important component in Dansgaard-Oeschger events; the abrupt climate change events that occurred repeatedly during the last ice age. Paleoclimatic reconstructions suggest an expansion of sea ice in the Nordic Seas during the cold stadial periods of the Dansgaard-Oeschger cycles. However, as the present configuration of the Nordic Seas does not allow for an extensive sea-ice cover in this region, the hydrography must have been different during glacial times. In fact, reconstructions show that the Nordic Seas hydrography during cold stadial periods was similar to the stratification of the Arctic Ocean today. However, the dynamic impacts of changing freshwater input and Atlantic water temperature on the Arctic stratification and sea ice are unclear. This study aims to assess the potential for Arctic-like stratification in the Nordic Seas during the last glacial period and the dynamics behind Dansgaard-Oeschger events, using models and theory. The results are presented in three papers. In the first paper, we develop a simple conceptual two-layer ocean model including sea ice representing the Nordic Seas during stadial times. Here, we find that a sea-ice cover is sensitive to changes in freshwater input, subsurface temperature, and the representation of vertical mixing. Abrupt changes in sea ice can occur with small changes to surface freshwater supply or Atlantic water temperatures. In the second paper we apply a three-dimensional eddy resolving numerical model to the same problem and find further support for the conclusions from Paper I; the stability of a sea-ice cover in the Nordic Seas is dependent on the background climate and large changes in stratification and sea ice occur with small changes in forcing. In addition, additional results presented in this dissertation (Sec. 6.2.1) show self-sustained oscillations in sea-ice cover without a change in forcing. From Paper II we learn that an extensive sea-ice cover and an Arctic-like stratification with a fresh surface layer and a halocline can exist in the Nordic Seas without an external freshwater supply. Under sufficient cold conditions, a halocline capped by sea ice emerges spontaneously due to redistribution of freshwater through sea-ice formation and melt. We find that an extensive sea-ice cover slows down the local overturning in the Nordic Seas; decreases the heat import to the basin; warms intermediate waters, and cools deep waters. In Paper III, the importance of background climate is further stressed. In this study, we move away from studying an Arctic-like stratification, and focus on sea-surface temperature variability in the region of the Nordic Seas and North Atlantic. We compile all available planktic foraminifera records from the North Atlantic with a sea-surface temperature reconstruction from the Dansgaard-Oeschger events. These are then combined with fully coupled climate model simulations using a proxy surrogate reconstruction method. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. However, forced runs from glacial times are needed to capture the amplitude of the temperature variability as seen in the proxy records. We suggest that sea-ice changes are important in extending the oceanic temperature signals to land. Combined, the three papers argue for an important role of the Nordic Seas during Dansgaard-Oeschger events, consistent with paleoclimatic reconstructions. Our results are also relevant for understanding potential future changes in Arctic sea-ice cover, and we argue that changes in Atlantic water temperature are of large importance
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