Showing papers by "Manfred Mudelsee published in 2011"

The leading mode of Indian Summer Monsoon precipitation variability during the last millennium

Abstract: [1] 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.

Aridity changes in the Temperate-Mediterranean transition of the Andes since AD 1346 reconstructed from tree-rings

Abstract: The Andes Cordillera acts as regional ''Water Towers'' for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0 isotherm, and changes in regional streamflow regimes. The Tem- perate-Mediterranean transition (TMT) zone of the Andes (35.5-39.5S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657 years and captures interannual to decadal scales of variability in late spring-early summer PDSI. These changes are related to the north-south oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprece- dented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Nino-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross-Amundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twen- tieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demon- strates the highly significant hindcast skill of A. chilensis as an aridity proxy.

Reconstructed streamflow for Citarum River, Java, Indonesia: linkages to tropical climate dynamics

Abstract: The Citarum river basin of western Java, Indonesia, which supplies water to 10 million residents in Jakarta, has become increasingly vulnerable to anthropogenic change. Citarum’s streamflow record, only ~45 years in length (1963-present), is too short for understanding the full range of hydrometeorological variability in this important region. Here we present a tree-ring based reconstruction of September–November Citarum streamflow (AD 1759–2006), one of the first such records available for monsoon Asia. Close coupling is observed between decreased tree growth and low streamflow levels, which in turn are associated with drought caused by ENSO warm events in the tropical Pacific and Indian Ocean positive dipole-type variability. Over the full length of record, reconstructed variance was at its weakest during the interval from ~1905–1960, overlapping with a period of unusually-low variability (1920–1960) in the ENSO-Indian Ocean dipole systems. In subsequent decades, increased variance in both the streamflow anomalies and a coral-based SST reconstruction of the Indian Ocean Dipole Mode signal the potential for intensified drought activity and related consequences for water supply and crop productivity in western Java, where much of the country’s rice is grown.

The Bootstrap in Climate Risk Analysis

Abstract: Climate risk is the probability of adverse effects from extreme values of variables in the climate system. Because climate changes, so can the various types of climate risk (floods, storms, etc.) change. This field is of strong socioeconomic relevance. Estimates of climate risk variations come from instrumental, proxy and documentary records of past climate extremes and projections of future extremes. Kernel estimation is a powerful statistical technique for quantifying trends in climate risk. It is not parametrically restricted and allows realistic, non-monotonic trends. The bootstrap is a computing-intensive statistical resampling method used here to provide a confidence band around the estimated risk curve. Confidence bands, like error bars, are essential for a reliable assessment whether changes and trends are significant or came by chance into the data. This methodology is presented using reconstructed flood records of the central European rivers Elbe, Oder and Werra over the past five centuries. Trends in flood risk differ among rivers and also between hydrological seasons. The scientific conclusion is that flood risk analysis has to take into account the high spatial variability from orographic rainfall, as well as different hydrological regimes in winter and summer. In an ideal co-operation between experts, quantitative knowledge with uncertainty ranges (like the estimated trends in climate risk) should form the deliverable from scientists to policy makers and decision takers.

A two-parameter stochastic process for Dansgaard–Oeschger events

Abstract: [1] Various climatic processes are thought to evolve as rapid, shift-like events, which points at the presence of nonlinear dynamics. Time series analysis of nonlinear processes, however, is not trivial, for example, because of the difficulty in coming up with a realistic random process as a viable null hypothesis. In this methodology paper we construct a basic two-parameter process of shift-like excursions in an excitable system with a threshold. We demonstrate that this stochastic process, in comparison with a specific one-parameter process, can better reproduce main features of the waiting time histogram of abrupt glacial climate events, the Dansgaard-Oeschger events, as seen in two paleoclimatic proxy records, the North Greenland Ice core Project (NGRIP) ice core and the Sofular stalagmite δ18O records. We use the two-parameter process to test some arguments that were proposed in the ongoing discussion of a possible solar role in triggering Dansgaard-Oeschger events. Using our approach, we suggest for future studies to generate time series of random events which can serve as a more plausible null hypothesis for Monte Carlo based statistical tests on the regularity of shift-like processes such as Dansgaard-Oeschger events.