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Hannover, Germany

Sinha A.,California State University, Dominguez Hills | Stott L.,University of Southern California | Berkelhammer M.,University of Southern California | Cheng H.,Xian Jiaotong University | And 6 more authors.
Quaternary Science Reviews | Year: 2011

Proxy reconstructions of precipitation from central India, north-central China, and southern Vietnam reveal a series of monsoon droughts during the mid 14th-15th centuries that each lasted for several years to decades. These monsoon megadroughts have no analog during the instrumental period. They occurred in the context of widespread thermal and hydrologic climate anomalies marking the onset of the Little Ice Age (LIA) and appear to have played a major role in shaping significant regional societal changes at that time. New tree ring-width based reconstructions of monsoon variability suggest episodic and widespread reoccurrences of monsoon megadroughts continued throughout the LIA. Although the El-Niño Southern Oscillation (ENSO) plays an important role in monsoon variability, there is no conclusive evidence to suggest that these megadroughts were associated with anomalous sea surface temperature anomalies that were solely the result of ENSO-like variability in the tropical Pacific. Instead, the causative mechanisms of these megadroughts may reside in protracted changes in the synoptic-scale monsoon climatology of the Indian Ocean. Today, the intra-seasonal monsoon variability is dominated by 'active' and the 'break' spells - two distinct oscillatory modes of monsoon that have radically different synoptic scale circulation and precipitation patterns. We suggest that protracted locking of the monsoon into the " break-dominated" mode - a mode that favors reduced precipitation over the Indian sub-continent and SE Asia and enhanced precipitation over the equatorial Indian Ocean, may have caused these exceptional droughts. Impetus for periodic locking of the monsoon into this mode may have been provided by cooler temperatures at the extratropical latitudes in the Northern Hemisphere which forced the mean position of the Inter-Tropical Convergence Zone (ITCZ) further southward in the Indian Ocean. © 2010 Elsevier Ltd.

Sarnthein M.,University of Kiel | Balmer S.,University of Kiel | Grootes P.M.,University of Kiel | Mudelsee M.,Climate Risk Analysis
Radiocarbon | Year: 2015

This article presents a compilation of planktic and benthic14C reservoir ages for the Last Glacial Maximum (LGM) and early deglacial from 11 key sites of global ocean circulation in the Atlantic and Indo-Pacifc Ocean. The ages were obtained by14C plateau tuning, a robust technique to derive both an absolute chronology for marine sediment records and a high-resolution record of changing reservoir/ventilation ages (Δ14C values) for surface and deep waters by comparing the suite of planktic14C plateaus of a sediment record with that of the atmospheric14C record. Results published thus far have used as atmospheric14C reference U/Th-dated corals, the Cariaco planktic record, and speleothems. We have now used the varve-counted atmospheric14C record of Lake Suigetsu terrestrial macrofossils to recalibrate the boundary ages and reservoir ages of the seven published records directly to an atmospheric14C record. In addition, the results for four new cores and further planktic results for four published records are given. Main conclusions from the new compilation are the following: (1) The Suigetsu atmospheric14C record on its varve-counted timescale refects all14C plateaus, their internal structures, and relative length previously identifed, but implies a rise in the average14C plateau age by 200–70014C yr during the LGM and early deglacial times. (2) Based on different14C ages of coeval atmospheric and planktic14C plateaus, marine surface water Δ14C may have temporarily dropped to an equivalent of ~0 yr in low-latitude lagoon waters, but reached >250014C yr both in stratifed subpolar waters and in upwelled waters such as in the South China Sea. These values differ signifcantly from a widely assumed constant global planktic Δ14C value of 400 yr. (3) Suites of deglacial planktic Δ14C values are closely reproducible in14C records measured at neighboring core sites. (4) Apparent deep-water14C ventilation ages (equivalents of benthic Δ14C), deduced from the sum of planktic Δ14C and coeval benthic-planktic14C differences, vary from 500 up to >5000 yr in LGM and deglacial ocean basins. © 2015 by the Arizona Board of Regents on behalf of the University of Arizona.

Berkelhammer M.,University of Southern California | Sinha A.,California State University, Dominguez Hills | Mudelsee M.,Climate Risk Analysis | Mudelsee M.,Alfred Wegener Institute for Polar and Marine Research | And 3 more authors.
Earth and Planetary Science Letters | Year: 2010

The instrumental record of Indian Summer Monsoon (ISM) precipitation displays two complete manifestations of an inferred multidecadal cycle. Hitherto, no precipitation-sensitive proxy record from the Indian subcontinent has had the necessary resolution and length to adequately assess whether this observed feature is an inherent aspect of the ISM system on longer timescales. Here we present compelling evidence for persistence of this cycle using a millennial length (AD 600-1550) and sub-annually resolved speleothem oxygen isotope record (δ18O) from Dandak Cave in east-central India. The record displays a high degree of correlation with a speleothem-based Asian monsoon reconstruction from Wanxiang Cave in north-central China on annual to decadal timescales showing the regional significance of these findings. The observed period in our monsoon reconstruction is similar to that associated with the Gleissberg solar cycle and multidecadal sea surface temperature variability in the north Atlantic (AMO), both of which are often cited as the prominent pacemakers of ISM variability on multidecadal timescales. We document transient coherence between ISM precipitation amount and solar variability that persists exclusively in the century prior to and during the Medieval Climate Anomaly (nominally, AD 950-1300). The non-stationary nature of the SFV-monsoon relationship presented here may be evidence of the time-varying influence of tropical ocean-atmosphere dynamics on the solar-monsoon link; however, it is not possible to show with significance that this period of coherence is anything more than an artifact of two timeseries with similar spectra. We therefore, are inclined to interpret our record as evidence of a minimal role of solar variability in driving persistent multidecadal variability of the ISM. Multidecadal SST variability in the North Atlantic remains as the likely alternative driver for persistence of this cycle. Regardless of the causative mechanism(s), the amplitude and regional signature of the observed cycle in ISM precipitation highlight its societal importance with respect to forecasting ISM precipitation on multidecadal timescales. © 2009 Elsevier B.V. All rights reserved.

Sinha A.,California State University, Dominguez Hills | Berkelhammer M.,University of Colorado at Boulder | Stott L.,University of Southern California | Mudelsee M.,Climate Risk Analysis | And 4 more authors.
Geophysical Research Letters | Year: 2011

The "internally" generated intraseasonal variability of the Indian Summer Monsoon is characterized by intermittent periods of enhanced ("active") and deficient ("break") precipitation, which produce a quasi east-west precipitation dipole over the Indian subcontinent. Here we present multicentennial-length and near annually-resolved reconstructions of monsoon precipitation, inferred from absolute-dated and instrumentally calibrated speleothem oxygen isotope records from regions (central and northeast India) that have diametric responses to active-break monsoon circulation patterns. On centennial timescales (AD 1400-2008), precipitation variability from these two regions exhibit opposing behavior, oscillating between periods with a persistently "active-dominated" (AD ∼1700 to 2007) and a "break-dominated" (AD 1400 to ∼1700) regime. The switch between these regimes occurs abruptly (within decades) at a time (AD ∼ 1650-1700) when a proxy record of upwelling intensity from the Arabian Sea suggest an abrupt increase in the monsoon winds. On the basis of these observations, we hypothesize that the frequency distribution of active-break periods varies on centennial timescales, implying a leading role of internal dynamics in governing the ISM response to slowly-evolving changes in the external boundary conditions. Copyright 2011 by the American Geophysical Union.

Olafsdottir K.B.,Climate Risk Analysis | Olafsdottir K.B.,University of Bremen | Mudelsee M.,Climate Risk Analysis
Mathematical Geosciences | Year: 2014

Estimation of Pearson's correlation coefficient between two time series, in the evaluation of the influences of one time-dependent variable on another, is an often used statistical method in climate sciences. Data properties common to climate time series, namely non-normal distributional shape, serial correlation, and small data sizes, call for advanced, robust methods to estimate accurate confidence intervals to support the correlation point estimate. Bootstrap confidence intervals are estimated in the Fortran 90 program PearsonT (Mudelsee, Math Geol 35(6):651-665, 2003), where the main intention is to obtain accurate confidence intervals for correlation coefficients between two time series by taking the serial dependence of the data-generating process into account. However, Monte Carlo experiments show that the coverage accuracy of the confidence intervals for smaller data sizes can be substantially improved. In the present paper, the existing program is adapted into a new version, called PearsonT3, by calibrating the confidence interval to increase the coverage accuracy. Calibration is a bootstrap resampling technique that performs a second bootstrap loop (it resamples from the bootstrap resamples). It offers, like the non-calibrated bootstrap confidence intervals, robustness against the data distribution. Pairwise moving block bootstrap resampling is used to preserve the serial dependence of both time series. The calibration is applied to standard error-based bootstrap Student's t confidence intervals. The performance of the calibrated confidence interval is examined with Monte Carlo simulations and compared with the performance of confidence intervals without calibration. The coverage accuracy is evidently better for the calibrated confidence intervals where the coverage error is acceptably small already (i.e., within a few percentage points) for data sizes as small as 20. © 2014 International Association for Mathematical Geosciences.

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