Different flavors of the Atlantic Multidecadal Variability   总被引：2，自引：2，他引：0  

Davide Zanchettin  Oliver Bothe  Wolfgang Müller  Jürgen Bader  Johann H. Jungclaus 《Climate Dynamics》2014,42(1-2):381-399

We investigate how differently-constructed indices for North Atlantic sea-surface temperatures (NASSTs) describe the “Atlantic Multidecadal Variability” (AMV) in a suite of unperturbed as well as externally-forced millennial (pre-industrial period) climate simulations. The simulations stem from an ensemble of Earth system models differing in both resolution and complexity. Different criteria exist to construct AMV indices capturing different aspects of the phenomenon. Although all representations of the AMV maintain strong multidecadal variability, they depict different characteristics of simulated low-frequency NASST variability, evolve differently in time and relate to different hemispheric teleconnections. Due to such multifaceted signatures in the ocean-surface as well as in the atmosphere, reconstructions of past AMV may not univocally reproduce multidecadal NASST variability. AMV features under simulated externally-forced pre-industrial climate conditions are not unambiguously distinguishable, within a linear framework, from AMV features in corresponding unperturbed simulations. This prevents a robust diagnosis of the simulated pre-industrial AMV as a predominantly internal rather than externally-forced phenomenon. We conclude that a multi-perspective assessment of multidecadal NASSTs variability is necessary for understanding the origin of the AMV, its physics and its climatic implications.  相似文献   

Seemingly disparate temperatures recorded in coexisting granulite facies lithologies     

Simon Schorn  Johann F.A. Diener 《Journal of Metamorphic Geology》2019,37(8):1049-1078

Phase equilibrium modelling of a conformable sequence of supracristal lithologies from the Bushmanland Subprovince of the Namaqua–Natal Metamorphic Complex (South Africa) reveals a disparity of some 60–70°C in estimated peak metamorphic temperature. Aluminous metapelites were equilibrated at ~770–790°C, whereas two‐pyroxene granulite and garnet–orthopyroxene–biotite gneiss record distinctly higher conditions of ~830–850°C. Semi‐pelite and Mg–Al‐rich gneisses yield poorly constrained estimates that span the range derived from other lithologies. All samples record peak pressure of ~5–6 kbar, and followed a roughly isobaric heating path from andalusite‐bearing greenschist/lower amphibolite facies conditions through a tight clockwise loop at near‐peak conditions, followed by near‐isobaric cooling. The disparity in peak temperatures appears to be robust, as the low‐variance assemblages in all samples reflect well‐known melting reactions that only occur over narrow temperature intervals. The stable coexistence of both products and reactants of these melting reactions indicates that they did not go to completion before metamorphism waned. Calculated pressure–enthalpy diagrams show that the melting reactions are strongly endothermic and therefore buffer temperature while heat is consumed by melting. Because the respective reactions occur at distinct P–T conditions and have different reactant assemblages, individual lithologies are thermally buffered at different temperatures and to different degrees, depending on the occurrence and abundance of reactant minerals. Our calculations show that all lithologies received essentially the same suprasolidus heat budget of 19 ± 1 kJ/mol, which led to the manifestation of lower peak temperatures in the more fertile and strongly buffered aluminous metapelites compared with more refractory rock types. If little to no thermal communication is assumed, this implies that lithology exerts a first‐order control over the heating path and the peak temperature that can be attained for a specific heat budget. Our results caution that the metamorphic conditions derived from pelitic granulites should not be assumed or extrapolated to larger sections of an orogenic crust that consist of other, more refractory lithologies.  相似文献   

Impact of tidal mixing with different scales of bottom roughness on the general circulation     

Exarchou  Eleftheria  von Storch  Jin Song  Jungclaus  Johann H. 《Ocean Dynamics》2012,62(10):1545-1563

The current study deals with a parameterization of diapycnal diffusivity in an ocean model. The parameterization estimates the diapycnal diffusivity depending on the location of tidal-related energy dissipation over rough topography. The scheme requires a bottom roughness map that can be chosen depending on the scales of topographic features. Here, we implement the parameterization on an ocean general circulation model, and we examine the sensitivity of the modeled circulations to different spatial scales of the modeled bottom roughness. We compare three simulations that include the tidal mixing scheme using bottom roughness calculated at three different ranges of spatial scales, with the largest scale varying up to 200 km. Three main results are discussed. First, the dependence of the topographic spectra with depth, characterized by an increase in spectral energy over short length scales in the deep ocean, influences the vertical profile of the diffusivity. Second, the changes in diffusivities lead to different equilibrium solutions in the Atlantic meridional overturning circulation and bottom circulation. In particular, the lower cell of the Atlantic overturning and the bottom water transport in the Pacific Ocean are stronger for stronger diffusivities at the corresponding basins and depths, and the strongest when using the small-scale roughness map. Third, a comparison of the density fields of the three simulations with the density field of World Ocean Atlas dataset, from which the models are initialized, shows that among the simulations with three different roughness maps, the one using small-scale bottom roughness map has the smallest density bias.

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Sea ice in the Barents Sea: seasonal to interannual variability and climate feedbacks in a global coupled model   总被引：1，自引：0，他引：1  

Torben Koenigk  Uwe Mikolajewicz  Johann H. Jungclaus  Alexandra Kroll 《Climate Dynamics》2009,32(7-8):1119-1138

Sea ice variability in the Barents Sea and its impact on climate are analyzed using a 465-year control integration of a global coupled atmosphere–ocean–sea ice model. Sensitivity simulations are performed to investigate the response to an isolated sea ice anomaly in the Barents Sea. The interannual variability of sea ice volume in the Barents Sea is mainly determined by variations in sea ice import into Barents Sea from the Central Arctic. This import is primarily driven by the local wind field. Horizontal oceanic heat transport into the Barents Sea is of minor importance for interannual sea ice variations but is important on longer time scales. Events with strong positive sea ice anomalies in the Barents Sea are due to accumulation of sea ice by enhanced sea ice imports and related NAO-like pressure conditions in the years before the event. Sea ice volume and concentration stay above normal in the Barents Sea for about 2 years after an event. This strongly increases the albedo and reduces the ocean heat release to the atmosphere. Consequently, air temperature is much colder than usual in the Barents Sea and surrounding areas. Precipitation is decreased and sea level pressure in the Barents Sea is anomalously high. The large-scale atmospheric response is limited with the main impact being a reduced pressure over Scandinavia in the year after a large ice volume occurs in the Barents Sea. Furthermore, high sea ice volume in the Barents Sea leads to increased sea ice melting and hence reduced surface salinity. Generally, the climate response is smallest in summer and largest in winter and spring.  相似文献   

A GCM study of future climate response to aerosol pollution reductions     

Silvia Kloster  Frank Dentener  Johann Feichter  Frank Raes  Ulrike Lohmann  Erich Roeckner  Irene Fischer-Bruns 《Climate Dynamics》2010,34(7-8):1177-1194

We use the global atmospheric GCM aerosol model ECHAM5-HAM to asses possible impacts of future air pollution mitigation strategies on climate. Air quality control strategies focus on the reduction of aerosol emissions. Here we investigate the extreme case of a maximum feasible end-of-pipe abatement of aerosols in the near term future (2030) in combination with increasing greenhouse gas (GHG) concentrations. The temperature response of increasing GHG concentrations and reduced aerosol emissions leads to a global annual mean equilibrium temperature response of 2.18 K. When aerosols are maximally abated only in the Industry and Powerplant sector, while other sectors stay with currently enforced regulations, the temperature response is 1.89 K. A maximum feasible abatement applied in the Domestic and Transport sector, while other sectors remain with the current legislation, leads to a temperature response of 1.39 K. Increasing GHG concentrations alone lead to a temperature response of 1.20 K. We also simulate 2–5% increases in global mean precipitation among all scenarios considered, and the hydrological sensitivity is found to be significantly higher for aerosols than for GHGs. Our study, thus highlights the huge potential impact of future air pollution mitigation strategies on climate and supports the need for urgent GHG emission reductions. GHG and aerosol forcings are not independent as both affect and are influenced by changes in the hydrological cycle. However, within the given range of changes in aerosol emissions and GHG concentrations considered in this study, the climate response towards increasing GHG concentrations and decreasing aerosols emissions is additive.  相似文献   

Interdecadal variability of the meridional overturning circulation as an ocean internal mode   总被引：1，自引：1，他引：0  

Xiuhua Zhu  Johann Jungclaus 《Climate Dynamics》2008,31(6):731-741

The meridional overturning circulation (MOC) in the coupled ECHAM5/MPIOM exhibits variability at periods of near 30 years and near 60 years. The 30-year variability, referred to as interdecadal variability (IDV), exist in an ocean model driven by climatological atmospheric forcing, suggesting that it is maintained by ocean dynamics; the 60-year variability, the multidecadal variability (MDV), is only observed in the fully coupled model and therefore is interpreted as an atmosphere–ocean coupled mode. The coexistence of the 30-year IDV and the 60-year MDV provides a possible explanation for the widespread time scales observed in climate variables. Further analyses of the climatologically forced ocean model shows that, the IDV is related to the interplay between the horizontal temperature-dominated density gradients and the ocean circulation: temperature anomalies move along the cyclonic subpolar gyre leading to fluctuations in horizontal density gradients and the subsequent weakening and strengthening of the MOC. This result is consistent with that from less complex models, indicating the robustness of the IDV. We further show that, along the North Atlantic Current path, the sea surface temperature anomalies are determined by the slow LSW advection at the intermediate depth.  相似文献   

Monitoring Cenozoic climate evolution of northeastern Tibet: stable isotope constraints from the western Qaidam Basin,China     

Andrea B. Rieser  Ana-Voica Bojar  Franz Neubauer  Johann Genser  Yongjiang Liu  Xiao-Hong Ge  Gertrude Friedl 《International Journal of Earth Sciences》2009,98(5):1063-1075

Carbon and oxygen stable isotopic composition of Cenozoic lacustrine carbonates from the intramontane Qaidam Basin yields cycles of variable length and shows several distinct events driven by tectonics and climate changes. From Eocene to Oligocene, the over-all trend in the δ¹³C composition of lacustrine carbonates shows a shift toward higher values, possibly related to higher proportions of dissolved inorganic carbon transported to the lake or lower input of soil derived CO₂. At the same time, the δ¹⁸O composition of lacustrine carbonates is decreasing in accordance with the global cooling trend and northwards drifting of the whole region. During the Miocene, distinct isotopic events can be recognized, although their interpretation and linkage to a certain tectonic event remains difficult. These events may be related to uplift in the Himalayas, to the strongest phase of uplift in the Altyn Mountains, to pronounced subsidence of the Qaidam Basin or to the expansion of C4 plants on land. Generally cold, highly evaporative conditions can be deduced from enrichment of δ¹⁸O isotopic compositions during Pliocene and Quaternary times.  相似文献   

Distribution patterns and habitat characterization of Simuliidae (Insecta: Diptera) in a low-order sandstone stream (Weidlingbach, Lower Austria)     

Christian Scheder  Johann A. Waringer   《Limnologica》2002,32(3)

A total of 1504 larval and 31 pupal Simuliidae were caught from March 2000 to February 2001 at the Weidlingbach, a fourth order tributary of the Danube near Vienna, Austria, using a modified box sampler (sampling area = 2116 cm²) at 12 sampling stations from source to mouth. From the six species collected, Prosimulium tomosvaryi (Enderlein) and two species of the Simulium ornatum-group (S. trifasciatum Curtis and S. ornatum Meigen) accounted for 97.5% of the total. Based on head width, instars 1–7 were collected in the S. ornatum-group and instars 2–7 in P. tomosvaryi; from two of the remaining species [Simulium (Nevermannia) cryophilum (Rubzov) and S. (N.) vernum Macquart], only pupae were sampled. The S. ornatum-group was most abundant on coarse substrates (median = 55.9 mm) exposed to high water velocity (median = 55.9 cm/s; range = 9–83 cm/s); the latter was also true for P. tomosvaryi although it favoured smaller sediment grain sizes (median = 32.4 mm). Species richness and population density increased from source to mouth. At sampling sites near the source Simuliidae were completely lacking. In headwaters only P. tomosvaryi was present, whereas the S. ornatum-group and Simulium (Simulium) argyreatum Meigen was collected exclusively near the mouth.  相似文献   

Stratigraphie und Tektonik der Schuppenzone Elbas     

Dr. Johann Bodechtel 《International Journal of Earth Sciences》1964,53(1):25-41

Zusammenfassung Die Serien der Schuppenzone Elbas zeigen eine durchlaufende stratigraphische Folge vom Paläozoikum bis ins Tertiär. Ein intensiver ostvergenter Falten- und Schuppenbau führt zu einer mehrmaligen Wiederholung bestimmter Gesteinsserien. Es sind 5 größere Schuppen oder Schuppenzonen auszuscheiden, die dem im Süden sich heraushebenden Autochthon überschoben sind. Die syntektonisch aufgedrungenen Granite haben das tektonische Geschehen beeinflußt. Das benachbarte Festland und das übrige toskanische Archipel zeigen denselben Baustil wie die Insel.

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Imbricated zones of the Isle of Elba display a continuous sequence from Paleozoic to Tertiary beds in several repetitions. Five larger thrust-zones override an autochthonous Eastern unit which appears (tilted up) in the South. Syntectonic granites. The tectonic style corresponds to the neighbouring continental situation.

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Résumé Les séries de la zone des écailles de l'île d'Elbe montrent une succession stratigraphique continue allant du Paléozoïque jusqu'au Tertiaire. Une structure en écailles et en plis fortement déversés vers l'est amène de nombreuses répétitions de certaines séries de roches. On peut distinguer 5 écailles et zones d'écaillés majeures qui sont refoulées sur l'autochtone affleurant au Sud. Les granites, mis en place syntectoniquement, ont influencé la tectonique. Le continent avoisinant et le reste de l'archipel toscan montrent le même style structural que les îles.

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Mineral equilibrium constraints on the feasibility of diffusive H2O-fluxed melting in the continental crust     

Lorena A. Tafur  Johann F. A. Diener 《Journal of Metamorphic Geology》2020,38(7):719-742

Generation of granitic melt is believed to occur predominantly by melting through the breakdown of hydrous minerals. However, melting due to the influx of H₂O has been recognized in anatectic amphibolite facies tonalitic grey gneisses, metagreywackes and low-P metapelites, and has consequently been proposed as an alternative mechanism for the generation of granitic melt. Melting induced by H₂O addition is recognized from voluminous melt production at relatively low temperature, where hydrous minerals are stable and anhydrous minerals are preferentially consumed during melting. Mineral equilibrium modelling to determine the P–T conditions, melt volumes, melting reactions and viable H₂O sources reveals that the process is not restricted to specific compositions or P–T conditions, although lower pressure and lithologies with a low hydrous mineral content are more favourable. Melting reactions in all lithologies primarily consume quartz and feldspars to yield 5–6 mol.% melt for each mol.% of H₂O added. remains constant at ~0.70 to 0.77 during progressive melting as long as alkali feldspar is present. Once alkali feldspar is exhausted, plagioclase becomes the main reactant, producing more tonalitic melt compositions with gradually higher . Our results demonstrate that, at the site of melting, melting is driven by diffusion of H₂O into the target rock along chemical potential gradients, rather than the advective flow of a mechanically distinct water-rich fluid phase. Melting will initiate and proceed as long as a gradient exists between the H₂O source and target lithology. Our calculations show that an ordinary magma, such as an I-type magma with typical H₂O content, has a high enough to be a viable H₂O source, allowing diffusive H₂O-fluxed melting to produce melt proportions and fertility comparable to that of dehydration melting. However, high degrees of partial melting require a considerable amount of H₂O, which necessitates a continuously advecting H₂O source such as a magma conduit or melt-bearing shear zone. A magmatic H₂O source at emplacement level will undergo a similar amount of crystallization as the melt fraction produced in the target rock such that there will be no net melt production. Considering that shear-zone hosted magma conduits are localized features, diffusive H₂O-fluxed melting is likely to only be viable in a small fraction of the anatectic orogenic crust. Although it may play an important role in locally raising melt volumes and modifying magma chemistry through mingling and hybridization, it does not appear to, of itself, be able to generate significant volumes of granitic melt.  相似文献