No full-text available
Request the article directly
from the author on ResearchGate.
Predictable components in global speleothem δ18O
Abstract
The earth's ice–ocean–atmosphere system is made up of subsystems which have different dynamics and which evolve at different timescales. Examples include the slow dynamics of ice sheet growth and melting, the tropical response to precessional cycles (∼21,000 years), and the fast dynamics of Dansgaard–Oeschger cycles (∼1500 years). Since dynamical systems evolve along characteristic trajectories, they are, to some extent, predictable. Further, it should be possible to decompose any dynamical system that is made up of subsystems with discrete dynamics and characteristic timescales, into time series which capture those discrete components. This study reviews five methods which can potentially achieve this, including: Optimal Persistence Analysis (OPA), Slow Feature Analysis (SFA), Principal Trend Analysis (PTA), Average Predictability Time Decomposition (APTD) and Forecastable Components Analysis (ForeCA). These methods produce sets of components that are in some way predictable, such that each component is more predictable than the next component, but each method uses a different measure of predictability. The five methods are applied to a global dataset of speleothem δ18O spanning the period 22–0 ka BP. The two leading predictable components are a monotonic trend, and a low-frequency oscillation with a periodicity of ∼21,000 years. The methods ForeCA and PTA cleanly separate these two components from higher-frequency signals. The third predictable component consists predominantly of a peak which ramps up during Heinrich Stadial 1, and falls thereafter. Furthermore, predictable components analysis can be used not only to investigate the predictability within a field, but can be extended to exploring the predictability between fields, such as between the northern hemisphere field and the southern hemisphere field. Predictable components analysis allows a better insight into the dynamical components of climate fields, and hence should be a useful tool for improving the interpretation of paleo-isotope records and other climate proxies.
... Furthermore, the use of multiple coeval speleothem records from different cave systems can be regarded as a potential (new) approach to synthesize the common signal of all of these records (e.g. δ 18 O) using for example principal component analysis (Deininger et al., 2016) or other statistical techniques (Rehfeld et al., 2013; Fischer, 2016). ...
... Assuming that the present-day mechanisms that determine the observed relationships between the wNAOi and temperature , precipitation and δ 18 O pw are relevant for the boreal winter of the entire Holocene, the evolution of the reconwww .clim-past.net/12/2127/2016/ Clim. Past, 12, 2127–2143, 2016 structed Holocene speleothem δ 18 O gradients can be interpreted as reflecting changes in atmospheric pressure patterns akin to predominantly negative wNAOi-like modes in the early Holocene until about ca. 8 ka and mainly positive wNAOi-like modes during mid and late Holocene winters (this assumption includes caveats that are discussed further below). For this assumption (NAO type forcing), the changes in the speleothem δ 18 O gradients are caused by changing temperature gradients (steeper west-to-east European temperature gradients in the early Holocene) and a changing precipitation history (increased precipitation and/or reduced amount of moisture in the atmosphere) linked to an NAO-like forcing. ...
Winter (October to March) precipitation δ18OP and δ18DP values in central Europe correlate with the winter NAO index (wNAOi), but the causal mechanisms remain poorly understood. Here we analyse the relationships between precipitation-weighted δ18OP and δ18DP datasets (δ18Opw and δ18Dpw) from European GNIP and ANIP stations and the wNAOi, with a focus on isotope gradients. We demonstrate that longitudinal δ18Opw and δ18Dpw gradients across Europe (continental effect) depend on the wNAOi state, with steeper gradients associated with more negative wNAOi states. Changing gradients reflect a combination of air temperature and variable amounts of precipitable water as a function of the wNAOi. The relationships between the wNAOi, δ18Opw and δ18Dpw can provide additional information from palaeoclimate archives such as European speleothems that primarily record winter δ18Opw. Comparisons between present-day and past European longitudinal δ18O gradients inferred from Holocene speleothems suggest that negative wNAO modes dominated the early Holocene, but positive wNAO modes were more common in the late Holocene.
... Furthermore, the use of multiple coeval speleothem records from different cave systems can be regarded as a potential (new) approach to synthesize the common signal of all of these records (e.g. δ 18 O) using for example principal component analysis (Deininger et al., 2016) or other statistical techniques (Rehfeld et al., 2013; Fischer, 2016). ...
... Assuming that the present-day mechanisms that determine the observed relationships between the wNAOi and temperature , precipitation and δ 18 O pw are relevant for the boreal winter of the entire Holocene, the evolution of the reconwww .clim-past.net/12/2127/2016/ Clim. Past, 12, 2127–2143, 2016 structed Holocene speleothem δ 18 O gradients can be interpreted as reflecting changes in atmospheric pressure patterns akin to predominantly negative wNAOi-like modes in the early Holocene until about ca. 8 ka and mainly positive wNAOi-like modes during mid and late Holocene winters (this assumption includes caveats that are discussed further below). For this assumption (NAO type forcing), the changes in the speleothem δ 18 O gradients are caused by changing temperature gradients (steeper west-to-east European temperature gradients in the early Holocene) and a changing precipitation history (increased precipitation and/or reduced amount of moisture in the atmosphere) linked to an NAO-like forcing. ...
... The temporal resolution of speleothem paleoclimate series ranges from sub-annual to centennial, and primarily depends on the karst and cave environment. Due to the high precision of uranium-series dating, speleothems provide opportunities to determine the timing 440 of regional hydrological response to global events and links to external forcing mechanisms (e.g., insolation changes) (Fischer, 2016). The different types of measurements made on speleothems-including δ 18 O, δ 13 C, and various trace elements-can be used to reconstruct past changes in the hydrological cycle. ...
Abstract. Reconstructions of global hydroclimate during the Common Era (CE; the past ~ 2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ<sup>18</sup>O) or hydrogen (δ<sup>2</sup>H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.6084/m9.figshare.11553162 (McKay and Konecky, 2020).
... Our study builds on the initial work of McDermott et al. (2011) on the scale of the European continent, but explicitly takes age model uncertainties into account and applies Principal Component Analysis (PCA) to retrieve quantitative measures of regional coherence and associated uncertainties. Recent studies have also used other methods to analyse compilations of multiple speleothems using inter-system recurrence networks (Feldhoff et al. 2012), network analysis (Rehfeld et al. 2013) and predictable components (Fischer 2016). The PCA methods employed in this study offer a powerful tool to investigate the spatio-temporal coherency (e.g. ...
Speleothem δ18O records provide valuable information about past continental environmental and cli- matic conditions, although their interpretation is often not straightforward. Here we evaluate a compilation of late Holocene speleothem δ18O records using a Monte Carlo based Principal Component Analysis (MC-PCA) method that accounts for uncertainties in individual speleothem age models and for the variable temporal resolution of each δ18O record. The MC-PCA approach permits not only the identi cation of temporally coherent changes in speleo- them δ18O; it also facilitates their graphical depiction and evaluation of their spatial coherency. The MC-PCA method was applied to 11 Holocene speleothem δ18O records that span most of the European continent (apart from the circum-Mediterranean region). We observe a common (shared) mode of speleothem δ18O variability that sug- gests millennial-scale coherency and cyclicity during thelast4.5ka.Thesechangesarelikelycausedbyvariability in atmospheric circulation akin to that associated with the North Atlantic Oscillation, re ecting meridionally shifted westerlies. We argue that these common large-scale vari- ations in European speleothem δ18O records are in phase with changes in the North Atlantic Ocean circulation indicated by the vigour of the Iceland Scotland Over ow Water (ISOW), the strength of the subpolar gyre (SPG) and an ocean stacked North Atlantic ice rafted debris (IRD) index. Based on a recent modelling study, we conclude that these changes in the North Atlantic circulation history may be caused by wind stress on the ocean surface driven by shifted westerlies. However, the mechanisms that ulti- mately force the westerlies remain unclear.
... PrCA is a particularly useful technique to explore Quaternary time series data where there may be multiple forcing factors over time, and in the quantification and analysis of signal versus noise in time series data. Code for the techniques highlighted by Fischer (2016) is freely available in the software package R (R Core Team, 2013). ...
Reconstructions of global hydroclimate during the Common Era (CE; the past ∼ 2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ 18 O) or hydrogen (δ 2 H) isotopic compositions of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 759 isotope records from the terrestrial and marine realms, including glacier and ground ice (210); speleothems (68); corals, sclerosponges, and mollusks (143); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and nonex-perts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate-model-simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis , model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593 (last access: 30 July 2020).
In this paper, we present a unique 9.5 m palaeo-lacustrine record of 771 palaeofloods which occurred over a period of 9.3 kyr in the Piànico–Sèllere Basin (southern Alps) during an interglacial period in the Pleistocene (sometime from 780 to 393 ka) and analyse its correlation, clustering, and cyclicity properties. We first examine correlations, by applying power-spectral analysis and detrended fluctuation analysis (DFA) to a time series of the number of floods per decade, and find weak long-range persistence: a power-spectral exponent βPS ≈ 0.39 and an equivalent power-spectral exponent from DFA, βDFA ≈ 0.25. We then examine clustering using the one-point probability distribution of the inter-flood intervals and find that the palaeofloods cluster in time as they are Weibull distributed with a shape parameter kW = 0.78. We then examine cyclicity in the time series of number of palaeofloods per year, and find a period of about 2030 years. Using these characterizations of the correlation, clustering, and cyclicity in the original palaeoflood time series, we create a model consisting of the superposition of a fractional Gaussian noise (FGN) with a 2030-year periodic component and then peaks over threshold (POT) applied. We use this POTFGN + Period model to create 2 600 000 synthetic realizations of the same length as our original palaeoflood time series, but with varying intensity of periodicity and persistence, and find optimized model parameters that are congruent with our original palaeoflood series. We create long realizations of our optimized palaeoflood model, and find a high temporal variability of the flood frequency, which can take values of between 0 and > 30 floods century⁻¹. Finally, we show the practical utility of our optimized model realizations to calculate the uncertainty of the forecasted number of floods per century with the number of floods in the preceding century. A key finding of our paper is that neither fractional noise behaviour nor cyclicity is sufficient to model frequency fluctuations of our large and continuous palaeoflood record, but rather a model based on both fractional noise superimposed with a long-range periodicity is necessary.
Recent studies have proposed that millennial-scale reorganization of the ocean-atmosphere circulation drives increased upwelling in the Southern Ocean, leading to rising atmospheric carbon dioxide levels and ice age terminations. Southward migration of the global monsoon is thought to link the hemispheres during deglaciation, but vital evidence from the southern sector of the vast Australasian monsoon system is yet to emerge. Here we present a (230)thorium-dated stalagmite oxygen isotope record of millennial-scale changes in Australian-Indonesian monsoon rainfall over the last 31,000 years. The record shows that abrupt southward shifts of the Australian-Indonesian monsoon were synchronous with North Atlantic cold intervals 17,600-11,500 years ago. The most prominent southward shift occurred in lock-step with Heinrich Stadial 1 (17,600-14,600 years ago), and rising atmospheric carbon dioxide. Our findings show that millennial-scale climate change was transmitted rapidly across Australasia and lend support to the idea that the 3,000-year-long Heinrich 1 interval could have been critical in driving the last deglaciation.
Annually laminated stalagmites provide both a precise chronology and a
paleoclimate proxy. The rate of annual growth of stalagmites is recorded
in changes of calcite fabric, annual fluxes of fluorescent organic
matter or annual variations in trace element composition. The processes
governing stalagmite growth are the flux of water, the CO2 saturation of
drip water relative to the cave atmosphere, and the temperature.
Although these processes are well understood, the water flux and
chemistry also depend on the specific hydrogeological flow routing of
individual stalagmites. Therefore, although past climates are recorded
in the growth lamina thickness, the climatic signal is perturbed by
noise related to local hydrologic factors. To separate local from global
factors, we used geostatistical tools to analyze annual growth rate data
from eleven stalagmites located on four continents. Variogram analyses
permit the quantification of the signal content contained within the
growth rate records. The information content ranges from 23 to 87%.
Analysis of the growth derivative shows an anticorrelation at a lag of 1
year, meaning that an increase in growth rate tends to be systematically
followed by a decrease in growth rate and vice versa. We call this
behavior "flickering" growth. We demonstrate that the decadal-scale
climate signal preserved in laminated stalagmites is degraded by the
variability in the processes necessary to generate the annual laminae.
The regional monsoons of the world have long been viewed as seasonal atmospheric circulation reversal—analogous to a thermally-driven land-sea breeze on a continental scale. This conventional view of monsoons is now being integrated at a global scale and accordingly, a new paradigm has emerged which considers regional monsoons to be manifestations of global-scale seasonal changes in response to overturning of atmospheric circulation in the tropics and subtropics, and henceforth, interactive components of a singular Global Monsoon (GM) system. The paleoclimate community, however, tends to view ‘paleomonsoon’ (PM), largely in terms of regional circulation phenomena. In the past decade, many high-quality speleothem oxygen isotope (δ18O) records have been established from the Asian Monsoon and the South American Monsoon regions that primarily reflect changes in the integrated intensities of monsoons on orbital-to-decadal timescales. With the emergence of these high-resolution and absolute-dated records from both sides of the Equator, it is now possible to test a concept of the ‘Global-Paleo-Monsoon’ (GPM) on a wide-range of timescales. Here we present a comprehensive synthesis of globally-distributed speleothem δ18O records and highlight three aspects of the GPM that are comparable to the modern GM: (1) the GPM intensity swings on different timescales; (2) their global extent; and (3) an anti-phased inter-hemispheric relationship between the Asian and South American monsoon systems on a wide range of timescales.
Empirical Orthogonal Function (EOF) Analysis is commonly used in the climate sciences and elsewhere to describe, reconstruct, and predict highly dimensional data fields. When data contain a high percentage of missing values (i.e. “gappy”), alternate approaches must be used in order to correctly derive EOFs. The aims of this paper are to assess the accuracy of several EOF approaches in the reconstruction and prediction of gappy data fields, using the Galapagos Archipelago as a case study example. EOF approaches included least-squares estimation via a covariance matrix decomposition (LSEOF), “Data Interpolating Empirical Orthogonal Functions” (DINEOF), and a novel approach called “Recursively-Subtracted Empirical Orthogonal Functions” (RSEOF). Model-derived data of historical surface Chlorophyll a concentrations and sea surface temperature, combined with a mask of gaps from historical remote sensing estimates, allowed for the creation of “true” and “observed” fields by which to gauge the performance of EOF approaches. Only DINEOF and RSEOF were found to be appropriate for gappy data reconstruction and prediction. DINEOF proved to be the superior approach in terms of accuracy, especially for noisy data with a high estimation error, although RSEOF may be preferred for larger data fields due to its relatively faster computation time.
Recent analysis of 38 globally distributed paleoclimatic records covering Marine Isotope Stage 3 (MIS 3) 60–26 ka demonstrated that the two leading empirical orthogonal functions (EOFs) explaining the data are the Greenland ice-core signal (''northern'' signal) and the Antarctic ice-core signal (''southern'' signal). Here singular spectral analysis (SSA) is used to show that millennial-scale variability of each of these two leading EOFs is characterized by two independent modes. The two modes of each EOF share similar relative distributions of variance, identical spectra, and, where each mode has spectral power, coherency spectra, which are significantly above the null hypothesis level at 95% confidence. The only difference between the modes of the northern and southern signals is that they are phase shifted. The phasing and long response time of the low-frequency mode, combined with its relationship to atmospheric CO 2 and sea level, are consistent with coupled changes in the ocean, ice sheets, atmosphere, and carbon cycle, whereas the phasing and short response time of the high-frequency mode are consistent with an atmospheric transmission likely induced by changes in hemispheric sea ice distributions and attendant feedbacks.
Internal variability of the Asian monsoon system and the relationship amongst its sub-systems, the Indian and East Asian Summer Monsoon, are not sufficiently understood to predict its responses to a future warming climate. Past environmental variability is recorded in Palaeoclimate proxy data. In the Asian monsoon domain many records are available, e.g. from stalagmites, tree-rings or sediment cores. They have to be interpreted in the context of each other, but visual comparison is insufficient. Heterogeneous growth rates lead to uneven temporal sampling. Therefore, computing correlation values is difficult because standard methods require co-eval observation times, and sampling-dependent bias effects may occur. Climate networks are tools to extract system dynamics from observed time series, and to investigate Earth system dynamics in a spatio-temporal context. We establish paleoclimate networks to compare paleoclimate records within a spatially extended domain. Our approach is based on adapted linear and nonlinear association measures that are more efficient than interpolation-based measures in the presence of inter-sampling time variability. Based on this new method we investigate Asian Summer Monsoon dynamics for the late Holocene, focusing on the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the recent period of warming in East Asia. We find a strong Indian Summer Monsoon (ISM) influence on the East Asian Summer Monsoon during the MWP. During the cold LIA, the ISM circulation was weaker and did not extend as far east. The most recent period of warming yields network results that could indicate a currently ongoing transition phase towards a stronger ISM penetration into China. We find that we could not have come to these conclusions using visual comparison of the data and conclude that paleoclimate networks have great potential to study the variability of climate subsystems in space and time.
Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius–Clapeyron relationship. Here it is argued that a different nonlinearity related to moisture advection by the atmospheric circulation is more important. When perihelion changes from one hemisphere’s summer solstice to the other’s in an idealized aquaplanet atmospheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into “thermodynamic” and “dynamic” components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land–sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precession-induced seasonal humidity changes correlate with the seasonal cycle in low-level convergence. This interpretation is confirmed using simulations in which the Clausius–Clapeyron relationship is explicitly linearized. The thermodynamic mechanism also operates in simulations with an idealized representation of land, although in these simulations the dynamic component of the precipitation changes is also important, adding to the thermodynamic precipitation changes in some latitudes and offsetting it in others.
The hydrological balance of the Black Sea is governed by riverine input and by the exchange with the Mediterranean Sea through the shallow Bosporus Strait. These sources have distinctly different oxygen isotope (delta18O) signatures. Therefore, the delta18O of Black Sea water directly reflects the presence or absence of a connection with the Mediterranean Sea, as well as hydrological changes in the vast watersheds of the Black and Caspian seas. However, the timing of late to middle Pleistocene water intrusions to the Black Sea is poorly constrained in sedimentary sequences. Here we present a stacked speleothem delta18O record from Sofular Cave in northern Turkey that tracks the isotopic signature of Black Sea surface water, and thus allows a reconstruction of the precise timing of hydrological shifts of the Black Sea. Our record, which extends discontinuously over the last 670,000 years, suggests that the connection between the Black Sea and Mediterranean Sea has been open for a significant period at least twelve times since 670,000yr ago, more often than previously suggested. Distinct minima in the Sofular delta18O record indicate at least seven intervals when isotopically depleted freshwater from the Caspian Sea entered the Black Sea. Our data provide precisely dated evidence for a highly dynamic hydrological history of the Black Sea.
The covariation of carbon dioxide (CO(2)) concentration and temperature in Antarctic ice-core records suggests a close link between CO(2) and climate during the Pleistocene ice ages. The role and relative importance of CO(2) in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO(2) during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO(2) concentrations is an explanation for much of the temperature change at the end of the most recent ice age.
Variations in speleothem oxygen-isotope values (δ18O) result from a complicated interplay of environmental controls and processes in the ocean, atmosphere, soil zone, epikarst, and cave system. As such, the controls on speleothem δ18O values are extremely complex. An understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem δ18O as paleoclimate and paleoenvironmental proxies, and is best complemented by study of site-specific cave processes such as infiltration, flow routing, drip seasonality and saturation state, and cave microclimate, among others. This review is a process-based summary of the multiple controls on δ18O in the atmosphere, soil, epikarst, and speleothem calcite, illustrated with case studies. Primary controls of δ18O in the atmosphere include temperature and relative humidity through their role in the multiple isotope “effects”. Variability and modifications of water δ18O values in the soil and epikarst zones are dominated by evaporation, mixing, and infiltration of source waters. The isotopically effective recharge into a cave system consists of those waters that participate in precipitation of CaCO3, resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state. Recent modeling, experimental, and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid CaCO3, and have implications for the ‘Hendy’ test. To assist interpretation of speleothem δ18O time series, quantitative and semi-quantitative δ18O-climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records. Finally, several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of δ18O to reconstruct past climate changes in regions that have been typically poorly represented in paleoclimate records, such as tropical and subtropical terrestrial locations. The new approach to speleothem paleoclimatology emphasizes climate teleconnections between regions and attribution of forcing mechanisms. Such investigations allow paleoclimatologists to infer regional to global-scale climate dynamics.
Marine records of sediment oxygen isotope compositions show that the Earth's climate has gone through a succession of glacial and interglacial periods during the past million years. But the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep-water temperature affect the recorded isotopic composition. Separating these two effects would require long records of either sea level or deep-ocean temperature, which are currently not available. Here we use a coupled model of the Northern Hemisphere ice sheets and ocean temperatures, forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores, to quantify both contributions simultaneously. We find that the ice-sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima, suggesting that strong ocean cooling preceded slow ice-sheet build-up. The model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 degrees N, ice volume and global sea level. We find that during extreme glacial stages, air temperatures were 17 +/- 1.8 degrees C lower than present, with a 120 +/- 10 m sea level equivalent of continental ice present.
Invariant features of temporally varying signals are useful for analysis and classification. Slow feature analysis (SFA) is a new method for learning invariant or slowly varying features from a vectorial input signal. It is based on a nonlinear expansion of the input signal and application of principal component analysis to this expanded signal and its time derivative. It is guaranteed to find the optimal solution within a family of functions directly and can learn to extract a large number of decorrelated features, which are ordered by their degree of invariance. SFA can be applied hierarchically to process high-dimensional input signals and extract complex features. SFA is applied first to complex cell tuning properties based on simple cell output, including disparity and motion. Then more complicated input-output functions are learned by repeated application of SFA. Finally, a hierarchical network of SFA modules is presented as a simple model of the visual system. The same unstructured network can learn translation, size, rotation, contrast, or, to a lesser degree, illumination invariance for one-dimensional objects, depending on only the training stimulus. Surprisingly, only a few training objects suffice to achieve good generalization to new objects. The generated representation is suitable for object recognition. Performance degrades if the network is trained to learn multiple invariances simultaneously.
During the last glacial period, large millennial-scale temperature oscillations--the 'Dansgaard/Oeschger' cycles--were the primary climate signal in Northern Hemisphere climate archives from the high latitudes to the tropics. But whether the influence of these abrupt climate changes extended to the tropical and subtropical Southern Hemisphere, where changes in insolation are thought to be the main direct forcing of climate, has remained unclear. Here we present a high-resolution oxygen isotope record of a U/Th-dated stalagmite from subtropical southern Brazil, covering the past 116,200 years. The oxygen isotope signature varies with shifts in the source region and amount of rainfall in the area, and hence records changes in atmospheric circulation and convective intensity over South America. We find that these variations in rainfall source and amount are primarily driven by summer solar radiation, which is controlled by the Earth's precessional cycle. The Dansgaard/Oeschger cycles can be detected in our record and therefore we confirm that they also affect the tropical hydrological cycle, but that in southern subtropical Brazil, millennial-scale climate changes are not as dominant as they are in the Northern Hemisphere.
The analysis of univariate or multivariate time series provides crucial information to describe, understand, and predict climatic variability. The discovery and implementation of a number of novel methods for extracting useful information from time series has recently revitalized this classical field of study. Considerable progress has also been made in interpreting the information so obtained in terms of dynamical systems theory. In this review we describe the connections between time series analysis and nonlinear dynamics, discuss signal- to-noise enhancement, and present some of the novel methods for spectral analysis. The various steps, as well as the advantages and disadvantages of these methods, are illustrated by their application to an important climatic time series, the Southern Oscillation Index. This index captures major features of interannual climate variability and is used extensively in its prediction. Regional and global sea surface temperature data sets are used to illustrate multivariate spectral methods. Open questions and further prospects conclude the review.
The Asian summer monsoon dynamics at the orbital scale are a subject of considerable debate. The validity of Asian speleothem δ(18)O records as a proxy for summer monsoon intensity is questioned together with the ultimate forcing and timing of the monsoon. Here, using the results of a 150,000-year transient simulation including water isotopes, we demonstrate that Asian speleothem δ(18)O records are not a valid proxy for summer monsoon intensity only at the orbital timescale. Rather, our results show that these records reflect annual variations in hydrologic processes and circulation regime over a large part of the Indo-Asian region. Our results support the role of internal forcing, such as sea surface temperature in the equatorial Pacific, to modulate the timing of monsoon precipitation recorded in paleo-proxies inside the Asian region.
Recent studies of the Last Glacial period Indo-Australian summer monsoon
(IASM) have revealed links to both northern and southern hemisphere high
latitude climate as well as to regional ocean conditions. Particular
interest has been paid to the monsoon response to Heinrich events, with
variability explained by meridional shifts in positioning of the
intertropical convergence zone (ITCZ), but this model has not been
adequately tested. In addition, the shorter-lived Dansgaard/Oeschger
(D/O) events have not been detected (beyond D/O-1, the
Bølling/Allerød) in land-based records from the
Indo-Pacific, despite their prominent expression in stalagmites from
southern Asia, raising questions about the sensitivity of the IASM to
these events. Here we present a Southern Hemisphere stalagmite oxygen
isotopic time series from Ball Gown Cave (BGC), tropical northern
Australia, located on the margins of the modern austral summer ITCZ,
that spans 40-31 and 27-8 ka. Elevated IASM rainfall
coincides with Heinrich stadials and the Younger Dryas, while decreased
rainfall characterizes D/O interstadials, a response that is anti-phased
with sites spanning the Indo-Pacific Warm Pool and with Chinese records
of the East Asian summer monsoon. The BGC time series thus reveals a
precipitation dipole consistent with a southward (northward) migration
of the ITCZ during periods of high northern latitude cooling (warming)
as the primary driver of millennial-scale IASM variability during the
Last Glacial period. Our record indicates a strengthening of the IASM
after the Younger Dryas period, likely as a result of rising sea level
and sea surface temperatures, breaking the link with the high latitudes.
The Atlantic meridional overturning circulation affects the latitudinal
distribution of heat, and is a key component of the climate system.
Proxy reconstructions, based on sedimentary
231Pa/230Th ratios and the difference between
surface- and deep-water radiocarbon ages, indicate that during the last
glacial period, the overturning circulation was reduced during
millennial-scale periods of cooling. However, much debate exists over
the robustness of these proxies. Here we combine proxy reconstructions
of sea surface and air temperatures and a global climate model to
quantitatively estimate changes in the strength of the Atlantic
meridional overturning circulation during the last glacial period. We
find that, relative to the Last Glacial Maximum, the overturning
circulation was reduced by approximately 14Sv during the cold Heinrich
event 1. During the Younger Dryas cold event, the overturning
circulation was reduced by approximately 12Sv, relative to the preceding
warm interval. These changes are consistent with qualitative estimates
of the overturning circulation from sedimentary
231Pa/230Th ratios. In addition, we find that the
strength of the overturning circulation during the Last Glacial Maximum
and the Holocene epoch are indistinguishable within the uncertainty of
the reconstruction.
The discrepancy between central Greenland borehole temperatures and the isotopic composition of Last Glacial Maximum ice can be explained by a shift in the delta18O-T relationship for the hydrological cycle linked to cooler tropical temperatures. This concept is illustrated using a simple Rayleigh distillation model. An estimate for alpha=Deltadelta18O/DeltaT (LGM-Holocene) of -0.370/00/°C is determined with a simple graphical technique.
This paper proposes a new method for diagnosing predictability on multiple time scales without time averaging. The method finds components that maximize the average predictability time (APT) of a system, where APT is defined as the integral of the average predictability over all lead times. Basing the predictability measure on the Mahalanobis metric leads to a complete, uncorrelated set of components that can be ordered by their contribution to APT, analogous to the way principal components decompose variance. The components and associated APTs are invariant to nonsingular linear transformations, allowing variables with different units and natural variability to be considered in a single state vector without normalization. For prediction models derived from linear regression, maximizing APT is equivalent to maximizing the sum of squared multiple correlations between the component and the time-lagged state vector. The new method is used to diagnose predictability of 1000-hPa zonal velocity on time scales from 6 h to decades. The leading predictable component is dominated by a linear trend and presumably identifies a climate change signal. The next component is strongly correlated with ENSO indices and hence is identified with seasonal-to-interannual predictability. The third component is related to annular modes and presents decadal variability as well as a trend. The next few components have APTs exceeding 10 days. A reconstruction of the tropical zonal wind field based on the leading seven components reveals eastward propagation of anomalies with time scales consistent with the Madden - Julian oscillation. The remaining components have time scales less than a week and hence are identified with weather predictability. The detection of predictability on these time scales without time averaging is possible by virtue of the fact that predictability on different time scales is characterized by different spatial structures, which can be optimally extracted by suitable projections.
This paper introduces the average predictability time (APT) for characterizing the overall predictability of a system. APT is the integral of a predictability measure over all lead times. The underlying predictability measure is based on the Mahalanobis metric, which is invariant to linear transformation of the prediction variables and hence gives results that are independent of the (arbitrary) basis set used to represent the state. The APT is superior to some integral time scales used to characterize the time scale of a random process because the latter vanishes in situations when it should not, whereas the APT converges to reasonable values. The APT also can be written in terms of the power spectrum, thereby clarifying the connection between predictability and the power spectrum. In essence, predictability is related to the width of spectral peaks, with strong, narrow peaks associated with high predictability and nearly flat spectra associated with low predictability. Closed form expressions for the APT for linear stochastic models are derived. For a given dynamical operator, the stochastic forcing that minimizes APT is one that allows transformation of the original stochastic model into a set of uncoupled, independent stochastic models. Loosely speaking, coupling enhances predictability. A rigorous upper bound on the predictability of linear stochastic models is derived, which clarifies the connection between predictability at short and long lead times, as well as the choice of norm for measuring error growth. Surprisingly, APT can itself be interpreted as the "total variance" of an alternative stochastic model, which means that generalized stability theory and dynamical systems theory can be used to understand APT. The APT can be decomposed into an uncorrelated set of components that maximize predictability time, analogous to the way principle component analysis decomposes variance. Part II of this paper develops a practical method for performing this decomposition and applies it to meteorological data.
This chapter reviews that the Late Wisconsinan North American ice sheet complex consisted of three major ice sheets: (1) the Laurentide Ice Sheet, which was centred on the Canadian Shield, but also expanded across the Interior Plains to the west and south, (2) the Cordilleran Ice Sheet, which inundated the western mountain belt between the northernmost co-terminus United States and Beringia, and (3) the Innuitian Ice Sheet, which covered most of the Canadian Arctic Archipelago north of about 7°N latitude. The ice cover over Newfoundland and the Maritime Provinces of Canada is usually referred to as the Appalachian Ice Complex, because ice flowed out from local centres rather than from the Canadian Shield. All of the peripheral ice sheets were confluent at the Last Glacial Maximum (LGM) with the Laurentide Ice Sheet, and the Greenland Ice Sheet was confluent with the Innuitian Ice Sheet. The nucleus of this complex, the Laurentide, comprised three major sectors, the Labrador Sector, the Keewatin Sector, and the Baffin Sector, named for areas of inception mid probable areas of outflow at LGM and located respectively east, west and north of Hudson Bay. The chapter presents revised maps of North American deglaciation at 500-year and finer resolution. These maps represent an updating of a series prepared nearly two decades ago for the INQUA 1987 Congress.
A new statistical optimization method is used to identify components of surface air temperature and precipitation on six continents that are predictable in multiple climate models on multiyear time scales. The components are identified from unforced "control runs" of the Coupled Model Intercomparison Project phase 3 dataset. The leading predictable components can be calculated in independent control runs with statistically significant skill for 3-6 yr for surface air temperature and 1-3 yr for precipitation, depending on the continent, using a linear regression model with global sea surface temperature (SST) as a predictor. Typically, lag-correlation maps reveal that the leading predictable components of surface air temperature are related to two types of SST patterns: persistent patterns near the continent itself and an oscillatory ENSO-like pattern. The only exception is Europe, which has no significant ENSO relation. The leading predictable components of precipitation are significantly correlated with an ENSO-like SST pattern. No multiyear predictability of land precipitation could be verified in Europe. The squared multiple correlations of surface air temperature and precipitation for nonzero lags on each continent are less than 0.4 in the first year, implying that less than 40% of variations of the leading predictable component can be predicted from global SST. The predictable components describe the spatial structures that can be predicted on multiyear time scales in the absence of anthropogenic and natural forcing, and thus provide a scientific rationale for regional prediction on multiyear time scales.
This paper documents the low-frequency (i.e.. decadal) variations of surface temperature for the period 1899-1998 in observations, and in simulations conducted as part of the Intergovernmental Panel oil Climate Change (IPCC) Fourth Assessment Report (AR4). The space-time structure of low-frequency variations is extracted using optimal persistence analysis, which is a technique that linearly decomposes a vector time series into a set of uncorrelated components, ordered such that the first component maximizes the decor relation time, the second maximizes the decorrelation time subject to being uncorrelated with the first, and so on. The results suggests that only the first two optimal persistence patterns (OPPs) in the observation-based record are statistically distinguishable from white noise. These two components can reproduce the spatial structure of local linear trends over various multidecadal periods, indicating that they give an efficient representation of the observed change in surface temperature. In contrast, most silmulations suggests the existence of a single physically significant OPP, all with qualitatively similar time series but each with somewhat different spatial structure. The leading OPP computed from the full model grid is surprisingly consistent with the leading OPP computed from the observation based grid with missing data masked out, suggesting that the observation-based grid does not pose a serious harrier to extracting the dominant low-frequency variations in the global climate system. The regions in which the leading optimal persistence patterns agree in their predictions of warming coincides with the regions in which warming has in fact been observed to occur.
The paper presents a new self-consistent method to infer missing data from oceanographic data series and to extract the relevant empirical orthogonal functions. As a by-product, the new method allows for the detection of the number of statistically significant EOFs by a cross-validation procedure for a complete or incomplete dataset, as well as the noise level and interpolation error. Since the proposed filling and analysis method does not need a priori information about the error covariance structure, the method is self-consistent and parameter free.
We introduce a geometric method for identifying the coupling direction
between two dynamical systems based on a bivariate extension of recurrence
network analysis. Global characteristics of the resulting inter-system
recurrence networks provide a correct discrimination for weakly coupled
R\"ossler oscillators not yet displaying generalised synchronisation.
Investigating two real-world palaeoclimate time series representing the
variability of the Asian monsoon over the last 10,000 years, we observe
indications for a considerable influence of the Indian summer monsoon on
climate in Eastern China rather than vice versa. The proposed approach can be
directly extended to studying $K>2$ coupled subsystems.
Trends are very important in climate research and are ubiquitous in the climate system. Trends are usually estimated using simple linear regression. Given the complexity of the system, trends are expected to have various features such as global and local characters. It is therefore important to develop methods that permit a systematic decomposition of climate data into different trend patterns and remaining no-trend patterns. Empirical orthogonal functions and closely related methods, widely used in atmospheric science, are unable in general to capture trends because they are not devised for that purpose. The present paper presents a novel method capable of systematically capturing trend patterns from gridded data. The method is based on an eigenanalysis of the covariance/correlation matrix obtained using correlations between time positions of the sorted data, and trends are associated with the leading nondegenerate eigenvalues. Application to simple low-dimensional time series models and reanalyses data are presented and discussed. Copyright © 2006 Royal Meteorological Society.
Laminated stalagmites, observed in either ultra-violet or visible light or recognized via trace elements, are now widely recognized as a common deposition form. Annually laminated stalagmites should be expected in caves which have an overlying climate that has a strong seasonality, similar climate zones to where trees grow with distinct annual rings. Continuous laminated stalagmite chronologies (up to several thousand years) should be expected where some mixing of stored water occurs. Such stalagmites can be used to reconstruct climate, particularly through variations in lamina width. Such climate records would be relatively damped by mixing of ‘event’ water with ‘stored’ groundwater, constraining the amount of high-frequency climate signals contained in the stalagmite, but relatively long continuous lamina sequences permit the preservation of low frequency, centennial scale, climate signals. This contrasts with numerous tree ring climate records, which are frequently limited in preserving multi-centennial trends, due to the necessary removal of age related noise from relatively short tree segments. Laminated stalagmites and tree rings should therefore to some degree provide complementary climate information. Appropriate methods for compiling stalagmite layer chronologies and climatologies are presented.
I introduce Forecastable Component Analysis (ForeCA), a novel dimension
reduction technique for temporally dependent signals. Based on a new
forecastability measure, ForeCA finds an optimal transformation to separate a
multivariate time series into a forecastable and an orthogonal white noise
space. I present a converging algorithm with a fast eigenvector solution.
Applications to financial and macro-economic time series show that ForeCA can
successfully discover informative structure, which can be used for forecasting
as well as classification. The R package ForeCA
(http://cran.r-project.org/web/packages/ForeCA/index.html) accompanies this
work and is publicly available on CRAN.
Speleothems are now regarded as valuable archives of climatic conditions on the continents, offering a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores. They are ideal materials for precise U-series dating, yielding ages in calendar years, thereby circumventing the radiocarbon calibration problems associated with most other continental records. Stable isotope studies in speleothems have shifted away from attempting to provide palaeo-temperature reconstructions to the attainable goal of providing precise estimates for the timing and duration of major O isotope-defined climatic events characterised by high signal to noise ratios (e.g. glacial/interglacial transitions, Dansgaard–Oeschger oscillations, the ‘8200-year’ event). Unlike the marine records, speleothem data sets are not ‘tuned’, and their independent chronology offers opportunities to critically assess leads and lags in the climate system, that in turn can provide important insights into forcing and feedback mechanisms. Improved procedures for the extraction and measurement of stable isotope ratios in fluid inclusions trapped in speleothems are likely to provide, in the near future, a much enhanced basis for the quantitative interpretation of O isotope ratios in speleothem calcite. The latter developments open up once again the tantalising prospect of palaeo-temperature estimates, but more importantly perhaps, provide a direct test for a new generation of general circulation models whose hydrological cycles will incorporate the ‘water isotopes’. The literature is reviewed briefly to provide for the reader a sense of the current state-of-the-art, and to provide some pointers for future research directions.
Speleothems are primarily studied in order to generate archives of climatic change and results have led to significant advances in identifying and dating major shifts in the climate system. However, the climatological meaning of many speleothem records cannot be interpreted unequivocally; this is particularly so for more subtle shifts and shorter time periods, but the use of multiple proxies and improving understanding of formation mechanisms offers a clear way forward.An explicit description of speleothem records as time series draws attention to the nature and importance of the signal filtering processes by which the weather, the seasons, and longer-term climatic and other environmental fluctuations become encoded in speleothems. We distinguish five sources of variation that influence speleothem geochemistry, i.e. atmospheric, vegetation/soil, karstic aquifer, primary speleothem crystal growth and secondary alteration, and give specific examples of their influence. The direct role of climate diminishes progressively through these five factors.We identify and review a number of processes identified in recent and current work that bear significantly on the conventional interpretation of speleothem records, for example:(1)speleothem geochemistry can vary seasonally and hence a research need is to establish the proportion of growth attributable to different seasons and whether this varies over time;(2)whereas there has traditionally been a focus on monthly mean δ18O data of atmospheric moisture, current work emphasizes the importance of understanding the synoptic processes that lead to characteristic isotope signals, since changing relative abundance of different weather types might control their variation on the longer-term;(3)the ecosystem and soil zone overlying the cave fundamentally imprint the carbon and trace element signals and can show characteristic variations with time;(4)new modelling on aquifer plumbing allows quantification of the effects of aquifer mixing;(5)recent work has emphasized the importance and seasonal variability of CO2-degassing leading to calcite precipitation upflow of a depositional site on carbon isotope and trace element composition of speleothems;(6)although much is known about the chemical partitioning between water and stalagmites, variability in relation to crystal growth mechanisms and kinetics is a research frontier;(7)aragonite is susceptible to conversion to calcite with major loss of chemical information, but the controls on the rate of this process are obscure.Analytical factors are critical in generating high-resolution speleothem records. A variety of methods of trace element analysis is available, but standardization is a common problem with the most rapid methods. New stable isotope data on Irish stalagmite CC3 compares rapid laser-ablation techniques with the conventional analysis of micromilled powders and ion microprobe methods. A high degree of comparability between techniques for δ18O is found on the millimeter to centimeter scale, but a previously described high-amplitude oxygen isotope excursion around 8.3 ka is identified as an analytical artefact related to fractionation of the laser-analysis associated with sample cracking. High-frequency variability of not less than 0.5‰ may be an inherent feature of speleothem δ18O records.