Statistical analysis of tropical climate anomaly simulations

The statistical analysis of two atmospheric general circulation simulations using the ECHAM3 GCM in permanent January conditions are presented. The two simulations utilize different oceanic surface temperatures in the Atlantic as boundary conditions: the cold simulation has SST representing the anomalous cold conditions during the decade 1904-13 while the warm simulation has SST representative for the decade 1951-60 where anomalous warm conditions have been observed. The analysis concentrates on the simulated differences between both experiments within the tropical belt to test the working hypothesis whether changes in the deep tropical heating initiated by the anomalous SST are responsible for the anomalies in the flow and mass field. We present a method which extracts the significant and dynamically consistent signal of the total difference using a multivariate statistical test based on the amplitudes of an a-priori specified mode expansion. These expansion modes are defined from a variant of the Matsuno-Gill linearized reduced gravity model for the tropical atmosphere. The application of the method shows a clear and well defined tropical signal in the flow and mass field which can be understood as the reponse of the ECHAM3 model to a deep heating anomaly not in the vicinity of the anomalous SST but on the neighboring continents especially South America and with opposite sign in remote areas between Indonesia and the dateline. The signal can be summarized as an enhancement of the GCM's tropical East-West circulation with the ascending branch over South America in the warm simulation compared to the cold run.     

Dynamical Implications of Block Averaging

We show that traditional Reynolds (block) averaging produces turbulence statistics whose time evolution is incompatible with the Navier–Stokes equation. Specifically, the zero integral scale that block averaging always produces leads to a trivial (zero-equals-zero) solution of the Navier–Stokes equation for autocovariances. We suggest alternative methods for analyzing turbulence time series that do not always generate a zero integral scale and, as a result, yield autocovariances whose time evolutions are compatible with the Navier–Stokes equation.     

Five hundred years of gridded high-resolution precipitation reconstructions over Europe and the connection to large-scale circulation

We present seasonal precipitation reconstructions for European land areas (30°W to 40°E/30–71°N; given on a 0.5°×0.5° resolved grid) covering the period 1500–1900 together with gridded reanalysis from 1901 to 2000 (Mitchell and Jones 2005). Principal component regression techniques were applied to develop this dataset. A large variety of long instrumental precipitation series, precipitation indices based on documentary evidence and natural proxies (tree-ring chronologies, ice cores, corals and a speleothem) that are sensitive to precipitation signals were used as predictors. Transfer functions were derived over the 1901–1983 calibration period and applied to 1500–1900 in order to reconstruct the large-scale precipitation fields over Europe. The performance (quality estimation based on unresolved variance within the calibration period) of the reconstructions varies over centuries, seasons and space. Highest reconstructive skill was found for winter over central Europe and the Iberian Peninsula. Precipitation variability over the last half millennium reveals both large interannual and decadal fluctuations. Applying running correlations, we found major non-stationarities in the relation between large-scale circulation and regional precipitation. For several periods during the last 500 years, we identified key atmospheric modes for southern Spain/northern Morocco and central Europe as representations of two precipitation regimes. Using scaled composite analysis, we show that precipitation extremes over central Europe and southern Spain are linked to distinct pressure patterns. Due to its high spatial and temporal resolution, this dataset allows detailed studies of regional precipitation variability for all seasons, impact studies on different time and space scales, comparisons with high-resolution climate models as well as analysis of connections with regional temperature reconstructions. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Climate Variability-Observations, Reconstructions, and Model Simulations for the Atlantic-European and Alpine Region from 1500-2100 AD

A detailed analysis is undertaken of the Atlantic-European climate using data from 500-year-long proxy-based climate reconstructions, a long climate simulation with perpetual 1990 forcing, as well as two global and one regional climate change scenarios. The observed and simulated interannual variability and teleconnectivity are compared and interpreted in order to improve the understanding of natural climate variability on interannual to decadal time scales for the late Holocene. The focus is set on the Atlantic-European and Alpine regions during the winter and summer seasons, using temperature, precipitation, and 500 hPa geopotential height fields. The climate reconstruction shows pronounced interdecadal variations that appear to “lock” the atmospheric circulation in quasi-steady long-term patterns over multi-decadal periods controlling at least part of the temperature and precipitation variability. Different circulation patterns are persistent over several decades for the period 1500 to 1900. The 500-year-long simulation with perpetual 1990 forcing shows some substantial differences, with a more unsteady teleconnectivity behaviour. Two global scenario simulations indicate a transition towards more stable teleconnectivity for the next 100 years. Time series of reconstructed and simulated temperature and precipitation over the Alpine region show comparatively small changes in interannual variability within the time frame considered, with the exception of the summer season, where a substantial increase in interannual variability is simulated by regional climate models.     

Global Warming and the Summertime Evapotranspiration Regime of the Alpine Region

Changes of the summer evapotranspiration regime under increased levels of atmospheric greenhouse gases are discussed for three Alpine river basins on the basis of a new set of simulations carried out with a high-resolution hydrological model. The climate change signal was inferred from the output of two simulations with a state-of-the-art global climate model (GCM), a reference run valid for 1961–1990 and a time-slice simulation valid for 2071–2100 under forcing from the A2 IPCC emission scenario. In this particular GCM experiment and with respect to the Alpine region summer temperature was found to increase by 3 to 4 ^∘C, whereas precipitation was found to decrease by 10 to 20%. Global radiation and water vapor pressure deficit were found to increase by about 5% and 2 hPa, respectively. On this background, an overall increase of potential evapotranspiration of about 20% relative to the baseline was predicted by the hydrological model, with important variations between but also within individual basins. The results of the hydrological simulations also revealed a reduction in the evapotranspiration efficiency that depends on altitude. Accordingly, actual evapotranspiration was found to increase at high altitudes and to the south of the Alps, but to decrease in low elevation areas of the northern forelands and in the inner-Alpine domain. Such a differentiation does not appear in the GCM scenario, which predicts an overall increase in evapotranspiration over the Alps. This underlines the importance of detailed simulations for the quantitative assessment of the regional impact of climate change on the hydrological cycle.     

Influence of Site and Source Dependent Ground Motion Scenarios on the Seismic Safety of Long-span Bridges in Cologne, Germany

An overlap in frequency between the 1D resonance frequency of sediments and the vibrational frequencies of long-span bridges might lead to a strongly increased structural response of the latter. Interference of surface waves caused by reflections at dipping interfaces may introduce additional unfavourable amplifications. Therefore, the vulnerability of two bridges crossing the Rhine River in Cologne, Germany, was assessed using ground motion scenarios computed for four profiles crossing the Lower Rhine Embayment. Due to their vibrational frequencies being in the vicinity of resonant peaks in the response spectra, the Severinsbridge showed critical loading and the bridge Cologne-Deutz even exhibited grave failure according to the dynamic FE-simulations.     

Stress and strain during fault-controlled lithospheric extension—insights from numerical experiments

Two-dimensional finite element techniques are used to study the temporal evolution and spatial distribution of stress and strain during lithospheric extension. The thermomechanical model includes a pre-existing fault in the upper crust to account for the reactivation of older tectonic elements. The fault is described using contact elements which allow for independent meshing of hanging wall and foot wall as well as simulation of large differential displacements between the fault blocks. Numerical models are run for three different initial temperature distributions representing extension of weak, moderately strong and strong lithosphere and three different extension velocities. In spite of the simple geodynamic boundary conditions selected, i.e., wholesale extension at a constant rate, stress and strain vary substantially throughout the lithosphere. In particular, in case of the weak lithosphere model, lower crustal flow towards the locus of maximum upper crustal extension results in the formation of a lower crustal dome while maintaining a subhorizontal Moho relief. The core of the dome experiences hardly any internal deformation, although it is the part of the lower crust which is exhumed the most. Stress fields in the lower crustal dome vary significantly from the regional trend underlining mechanical decoupling of the lower crust from the rest of the lithosphere. These differences diminish if cooler temperatures and, hence, stronger rheologies are considered. Lithospheric strength also exerts a profound control on the basin architecture and the surface expressions of extension, i.e., rift flank uplift and basin subsidence. If the lower crust is sufficiently weak, its flow towards the region of extended upper crust can provide a threshold value for the maximum subsidence which can be achieved during the syn-rift stage. In spite of continuous regional extension, corresponding burial history plots show exponentially decreasing subsidence rates which would traditionally be interpreted in terms of lithospheric cooling during the post-rift stage. The models provide templates to genetically link the surface and sub-surface expressions of lithospheric extension, for which usually no contemporaneous observations are possible. In particular, they help to decipher the information on the physical state of the lithosphere at the time of extension which is stored in the architecture and subsidence record of sedimentary basins.     

Albite dissolution kinetics as a function of distance from equilibrium: Implications for natural feldspar weathering

Determining the kinetics of many geologic and engineering processes involving solid/fluid interactions requires a fundamental understanding of the Gibbs free energy dependency of the system. Currently, significant discrepancies seem to exist between kinetic datasets measured to determine the relationship between dissolution rate and Gibbs free energy. To identify the causes of these discrepancies, we have combined vertical scanning interferometry, atomic force microscopy, and scanning electron microscopy techniques to identify dissolution mechanisms and quantify dissolution rates of albite single crystals over a range of Gibbs free energy (−61.1 < ΔG < −10.2 kJ/mol). During our experiments, both a previously dissolved albite surface exhibiting etch pits and a pristine surface lacking dissolution features were dissolved simultaneously within a hydrothermal, flow-through reactor. Experimental results document an up to 2 orders of magnitude difference in dissolution rate between the differently pretreated surfaces, which are dominated by different dissolution mechanisms. The rate difference, which persists over a range of solution saturation state, indicates that the dissolution mechanisms obey different Gibbs free energy dependencies. We propose that this difference in rates is the direct consequence of a kinetic change in dissolution mechanism with deviation from equilibrium conditions. The existence of this kinetic “switch” indicates that a single, continuous function describing the relationship between dissolution rate and Gibbs free energy may be insufficient. Finally, we discuss some of the potential consequences of our findings on albite’s weathering rates with a particular focus on the sample’s history.     

Initial responses of carbonate-rich shelf sediments to rising atmospheric pCO2 and “ocean acidification”: Role of high Mg-calcites

Carbonate-rich sediments at shoal to shelf depths (<200 m) represent a major CaCO₃ reservoir that can rapidly react to the decreasing saturation state of seawater with respect to carbonate minerals, produced by the increasing partial pressure of atmospheric carbon dioxide (pCO₂) and “acidification” of ocean waters. Aragonite is usually the most abundant carbonate mineral in these sediments. However, the second most abundant (typically ∼24 wt%) carbonate mineral is high Mg-calcite (Mg-calcite) whose solubility can exceed that of aragonite making it the “first responder” to the decreasing saturation state of seawater. For the naturally occurring biogenic Mg-calcites, dissolution experiments have been used to predict their “stoichiometric solubilities” as a function of mol% MgCO₃. The only valid relationship that one can provisionally use for the metastable stabilities for Mg-calcite based on composition is that for the synthetically produced phases where metastable equilibrium has been achieved from both under- and over-saturation. Biogenic Mg-calcites exhibit a large offset in solubility from that of abiotic Mg-calcite and can also exhibit a wide range of solubilities for biogenic Mg-calcites of similar Mg content. This indicates that factors other than the Mg content can influence the solubility of these mineral phases. Thus, it is necessary to turn to observations of natural sediments where changes in the saturation state of surrounding waters occur in order to determine their likely responses to the changing saturation state in upper oceanic waters brought on by increasing pCO₂. In the present study, we investigate the responses of Mg-calcites to rising pCO₂ and “ocean acidification” by means of a simple numerical model based on the experimental range of biogenic Mg-calcite solubilities as a function of Mg content in order to bracket the behavior of the most abundant Mg-calcite phases in the natural environment. In addition, observational data from Bermuda and the Great Bahama Bank are also presented in order to project future responses of these minerals. The numerical simulations suggest that Mg-calcite minerals will respond to rising pCO₂ by sequential dissolution according to mineral stability, progressively leading to removal of the more soluble phases until the least soluble phases remain. These results are confirmed by laboratory experiments and observations from Bermuda. As a consequence of continuous increases in atmospheric CO₂ from burning of fossil fuels, the average composition of contemporary carbonate sediments could change, i.e., the average Mg content in the sediments may slowly decrease. Furthermore, evidence from the Great Bahama Bank indicates that the amount of abiotic carbonate production is likely to decline as pCO₂ continues to rise.