Manfred Mudelsee

REDFIT: estimating red-noise spectra directly from unevenly spaced paleoclimatic time series

Paleoclimatic time series are often unevenly spaced in time, making it difficult to obtain an accurate estimate of their red-noise spectrum. A Fortran 90 program (REDFIT) is presented that overcomes this problem by fitting a first-order autoregressive (AR1) process, being characteristic for many climatic processes, directly to unevenly spaced time series. Hence, interpolation in the time domain and its inevitable bias can be avoided. The program can be used to test if peaks in the spectrum of a time series are significant against the red-noise background from an AR1 process. Generated and paleoclimatic time series are used to demonstrate the capability of the program.

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago.

Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate, but the absolute changes in solar intensity over the range of decades to millennia are small and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone, whose northward migration stops near the southern shoreline of Arabia in the present climate. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th–U-dated stalagmite from Oman. The δ18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the δ18O record with the Δ14C record from tree rings, which largely reflects changes in solar activity. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.

No upward trends in the occurrence of extreme floods in central Europe

Extreme river floods have been a substantial natural hazard in Europe over the past centuries, and radiative effects of recent anthropogenic changes in atmospheric composition are expected to cause climate changes, especially enhancement of the hydrological cycle, leading to an increased flood risk. For the past few decades, however, observations from Europe do not show a clear increase in flood occurrence rate. Here we present longer-term records of winter and summer floods in two of the largest rivers in central Europe, the Elbe and Oder rivers. For the past 80 to 150 yr, we find a decrease in winter flood occurrence in both rivers, while summer floods show no trend, consistent with trends in extreme precipitation occurrence. The reduction in winter flood occurrence can partly be attributed to fewer events of strong freezing—following such events, breaking river ice at the end of the winter may function as a water barrier and enhance floods severely. Additionally, we detect significant long-term changes in flood occurrence rates in the sixteenth to nineteenth centuries, and conclude that reductions in river length, construction of reservoirs and deforestation have had minor effects on flood frequency.

The Mid-Pleistocene climate transition: onset of 100 ka cycle lags ice volume build-up by 280 ka

The Mid-Pleistocene Climate Transition (MPT) is the complex climatic change which brought the Late Pleistocene ice ages. We explore the MPT in the time and frequency domains by new methods of time series analysis. High-resolution oxygen isotope records reveal that the ice volume-related increase in δ18O mean (amplitude: 0.29 ± 0.05 (1 − σμ) ‰, transition midpoint: 922 ± 12 ka, duration: 40 ± 9 ka) significantly preceded the abrupt increase in the amplitude of the ∼ 100 ka cycle at 641 ± 9 ka. This finding can be quantitatively simulated using a simple ice-bedrock model in which, due to the additional ice, the calving threshold is exceeded. The simulated calving events prior to ∼ 650 ka are separated by ∼ 77 ka, whereas after ∼ 650 ka they occur pseudo-periodically with a mean period of nearly 100 ka. The cause for the delay of the ∼ 100 ka calving cycle was not a slow bedrock relaxation; rather, the coincidental combination of insolation, existing ice mass, and bedrock depression.

Timing and climatic impact of Greenland interstadials recorded in stalagmites from northern Turkey.

A 50 kyr-long exceptionally well-dated and highly resolved stalagmite oxygen (δ 18O) and carbon (δ 13C) isotope record from Sofular Cave in northwestern Turkey helps to further improve the dating of Greenland Interstadials (GI) 1, and 3–12. Timing of most GI in the Sofular record is consistent within ±10 to 300 years with the “iconic” Hulu Cave record. Larger divergences (>500 years) between Sofular and Hulu are only observed for GI 4 and 7. The Sofular record differs from the most recent NGRIP chronology by up to several centuries, whereas age offsets do not increase systematically with depth. The Sofular record also reveals a rapid and sensitive climate and ecosystem response in the eastern Mediterranean to GI, whereas a phase lag of ∼100 years between climate and full ecosystem response is evident. Finally, results of spectral analyses of the Sofular isotope records do not support a 1,470-year pacing of GI.

Extreme floods in central Europe over the past 500 years: Role of cyclone pathway ''Zugstrasse Vb''

[1] Anthropogenically induced climate change has been hypothesized to add to the risk of extreme river floods because a warmer atmosphere can carry more water. In the case of the central European rivers Elbe and Oder, another possibility that has been considered is a more frequent occurrence of a weather situation of the type ‘‘Zugstrasse Vb,’’ where a low-pressure system travels from the Adriatic region northeastward, carrying moist air and bringing orographic rainfall in the mountainous catchment areas (Erzgebirge, Sudeten, and Beskids). Analysis of long, homogeneous records of past floods allows us to test such ideas. M. Mudelsee and co-workers recently presented flood records for the middle parts of the Elbe and Oder, which go continuously back to A.D. 1021 and A.D. 1269, respectively. Here we review the reconstruction and assess the data quality of the records, which are based on combining documentary data from the interval up to 1850 and measurements thereafter, finding both the Elbe and Oder records to provide reliable information on heavy floods at least since A.D. 1500. We explain that the statistical method of kernel occurrence rate estimation can overcome deficiencies of techniques previously used to investigate trends in the occurrence of climatic extremes, because it (1) allows nonmonotonic trends, (2) imposes no parametric restrictions, and (3) provides confidence bands, which are essential for evaluating whether observed trends are real or came by chance into the data. We further give a hypothesis test that can be used to evaluate monotonic trends. On the basis of these data and methods, we find for both the Elbe and Oder rivers (1) significant downward trends in winter flood risk during the twentieth century, (2) no significant trends in summer flood risk in the twentieth century, and (3) significant variations in flood risk during past centuries, with notable differences between the Elbe and Oder. The observed trends are shown to be both robust against data uncertainties and only slightly sensitive to land use changes or river engineering, lending support for climatic influences on flood occurrence rate. In the case of winter floods, regional warming during the twentieth century has likely reduced winter flood risk via a reduced rate of strong river freezing (breaking ice at the end of winter may function as a water barrier and enhance a high water stage severely). In the case of summer floods, correlation analysis shows a significant, but weak, relation between flood occurrence and meridional airflow, compatible with a ‘‘Zugstrasse Vb’’ weather situation. The weakness of this relation, together with the uncertainty about whether this weather situation became more frequent, explains the absence of trends in summer flood risk for the Elbe and Oder in the twentieth century. We finally draw conclusions about flood disaster management and modeling of flood occurrence under a changed climate. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1620 Global Change: Climate dynamics (3309); 1655 Global Change: Water cycles (1836); 1821 Hydrology: Floods; KEYWORDS: climate change, extreme events, flood risk

A 780-year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman

[1] Meteorological records of monsoon rainfall in the Indian Ocean are generally less than 100 years long. The relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales, to determine what factors influence the intensity of rainfall on these timescales, or to place possible changes in the twentieth century into a broader historical context. Development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward. We have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern Arabia. Our results show that monsoon variation over the past century is not outside of the range of the past 800 years. Decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the Indian Ocean. Spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales. Decadal to interannual cycles in the record appear to originate in the tropical Pacific Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1620 Global Change: Climate dynamics (3309); 1699 Global Change: General or miscellaneous; KEYWORDS: Monsoon, paleoclimate, rainfall, Indian Ocean, speleothem, oxygen isotopes

Estimating Pearson's Correlation Coefficient with Bootstrap Confidence Interval from Serially Dependent Time Series

Pearsons correlation coefficient, rxy, is often used when measuring the influence of one time-dependent variable on another in bivariate climate time series. Thereby, positive serial dependence (persistence) and unknown data distributions impose a challenge for obtaining accurate confidence intervals for rxy. This is met by the presented approach, employing the nonparametric stationary bootstrap with an average block length proportional to the maximum estimated persistence time of the data. A Monte Carlo experiment reveals that this method can produce accurate (in terms of coverage) confidence intervals (of type bias-corrected and accelerated). However, since persistence reduces the number of independent observations, substantially more data points are required for achieving an accuracy comparable to a situation without persistence. The experiment further shows that neglecting serial dependence may lead to serious coverage errors. The presented method proves robust with respect to distributional shape (lognormal/normal) and time spacing (uneven/even). The method is used to confirm that a previous finding of a correlation between solar activity and Indian Ocean monsoon strength in early Holocene is valid. A further result is that the correlation between sunspot number and cosmogenic 10Be concentration vanishes after approximately 1870.

TAUEST: a computer program for estimating persistence in unevenly spaced weather/climate time series

Persistence is characteristic for weather and climate fluctuations (e.g. Wilks, 1995). Quantifying the memorizing ability from proxy records of past climates extends our understanding of natural weather and climate variability. The simplest yet successful persistence model is a first-order autoregressive (AR(1)) process where a fluctuation depends only on its own immediate past plus a random component (Gilman et al., 1963; Mann and Lees, 1996). Because the proxy data (from sediment or ice cores) are usually unevenly spaced in time, a simple estimation of the AR(1) model via the autocorrelation coefficient r has to be replaced by fitting the AR(1) model (Robinson, 1977)

Sub-Milankovitch climatic cycles in Holocene stalagmites from Sauerland, Germany

Calcitic stalagmites from caves in the Sauerland, Germany, prove the existence of sub-Milankovitch cycles in precipitation during the last 6000 yr. The δ18O record dated with Th/U is interpreted as an indicator of paleohumidity. Spectral analysis of δ18O from 6000 a BP up to the recent top of a stalagmite from the Atta cave yields statistically significant peaks at 1450, 117, 64 and 57 a. Additionally we find a good correlation of the stalagmite’s δ18O and Δ14C from European tree rings. The 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by Bond et al. [Science 278 (1997) 1257–1266; 294 (2001) 2130–2136] derived from the amount of ice rafted debris in deep sediments from the North Atlantic. Our results suggest that the centennial to millennial shifts observed in the North Atlantic are accompanied by synchronous shifts of the climate in Northern and Central Europe, which most probably can be attributed to solar irradiation variations.