Assimilation von ð13C-Daten aus marinen Sedimentbohrkernen in das LSG zur Rekonstruktion der Ozeanzirkulation während des letzten glazialen Maximums

Measurements of stable carbon isotope ratios (ôttC) from foraminiferal shells indicate that during the last glacial maximum (LGM) the Atlantic thermohaline circulation was significantly different from the present day circulation. It has been suggested that during the LGM the flow of North Atlantic Deep Water (NADW) ** reduced, while the com- pensating Antarctic Bottom Water (AABW) penetrated further northward. Assimilation of ô13C data by using the adjoint technique into a global ocean general circulation model (OGCM) can evaluate an optimal circulation patterns providing consistency between the dynamics of the model and the analysis of the ô13C data. In this study assimilation experiments with the Hamburg large scale geostrophic model (LSG) are examined to investigate the extent to which ô13C as a paleotracer constrains the simulation of the North Atlantic circulation during the LGM. The prescribed wind stress and 2 m air temperatures \ryere taken from the ice age ïesponse of an atmospheric general circulation model (AGCM). The first guess LGM salinity anomalies were calculated from salinity reconstructions adapted from stable oxygen isotope ratios (ô18O) in foraminiferal shells. The optimized model state was obtained by varying the salinities of the sea surface. This simulated glacial circulation is closer to the observed ô13C distribution than flow pattern of previous OGCMs of comparable resolution. The results of the assimilation experiments confirm that during the LGM the production of NADW was substantially reduced. An increased salinity difference between the tropical Atlantic and Pacific supports earlier suggestions of a higher water vapor transfer from the Atlantic to the Pacific. The glacial changes in the circulation indicate also an increased outflow from the Mediterranea¡r Sea as a consequence of increased surface salinities in this area. However, the optimized model simulation is only partially consistent with error estimates from the observations and large deviations still exist. Sensitivity experiments with respect to possible errors in the forcing boundary conditions show that circulation patterns range from an even stronger Atlantic 'conveyor belt' than the present day one to a nearly shut down. A simultaneous use of different tracers would possibly reduce these uncertainties significantly.Messungen der stabilen Isotopenverhältnisse (ô18c) von Foraminiferenschalen deuten darauf hin, daß sich die thermohaline Zirkulation während des letzten glazialen Maximums (LGM) signifikant von der heutigen unterschied. Es wurde aufgrund dieser Messungen angenommen, daß die Ausbreitung des nordatlantischen Tiefenwassers (NAD\M) während des LGM an Stärke und Ausdehnung abgenommen hat, während das kompensierende antarktische Bodenwasser (AABW) weiter nordwärts vorgedrungen ist. Die Assimilation von ô13C-Daten in ein globales Modell der allgemeinen Ozeanzirkulation (OGCM) ermöglicht bei Verwendung der adjungierten Methode die Bestimmung eines optimalen Zirkulationsmusters, das sowohl mit der Dynamik des verwendeten Modells als auch mit der Analyse der ð13C-Daten verträglich ist. In dieser Studie werden Assimilationsexperimente mit dem Hamburger großskalig-geostrophischen Ozeanzirkulationsmodell (LSG) durchgeführt, um die Zuverlässigkeit von ð13C als Paläotracer zur Bestimmung des nordatlantischen Tiefenwassers während des LGM zu untersuchen. Die vorgeschriebenen Windschubspannungen und 2 m-Lufttemperaturen wurden aus einer Eiszeit-Simulation eines allgemeinen Zirkulationsmodells der Atmosphäre (AGCM) entnommen. Die Salzgehaltsrandbedingungen im Anfangszustand wurden aus Salzgehaltsrekontruktionen berechnet; die stabilen Sauerstoffisotopenverhältnissen (ô18O) in Foraminiferenschalen angepafJt wurden. Der optimierte Zustand der Zirkulation wurde durch Variation der Salzgehalte an der Meeresoberfläche erreicht. Diese Zirkulation liegt dichter an den ô13C-Daten als die, die mit früheren OGCMs erzielt wurde. Die Ergebnisse der Assimilationsexperimente bestätigen, daß die Bildung von NAD\M während des LGM substantiell reduziert war. Ein erhöhter Salzgehaltsunterschied zwischen Atlantik und Pazifik unterstützt frühere Annahmen eines höheren Übertrags von Wasserdampf zwischen Atlantik und Pazifik. Die glazialen Änderungen in der Zirkulation weisen auch auf einen verstärkten ,,Overflow" vom Mittelmeer als Folge der erhöhten Salzgehalte in dieser Region hin. Ttotz der Übereinstimmungen sind die Modellsimulationen nur teilweise mit den aus den Daten berechneten Fehlern verträglich, und große Abweichungen existieren ímmèr noch. Sensitivitätsexperimente bezüglich möglicher Fehler in den- antreibenden Randbedingungen zeigen, daß die Zirkulationsmuster bei Beachtung von Meßfehlern sogar von einer stärkeren atlantischen Meridionalzirkulation als der rezenten bis zu deren nahezu totalem Zusammenbruch reichen. Eine simultane Verwendung von verschiedenen tacern würde diese Unsicherheiten möglicherweise signifikant reduzieren

Windinduzierte interannuale Variabilität in der Warmwassersphäre von 1981 bis 1987 - Teil II: FLUktuation im Kohlenstoffkreislauf

Die Reaktion der bodennahen Luftströmung auf Änderungen der Rauhigkeit sowie op- tischer und thermischer Eigenschaften einer nahezu ebenen Oberfläche wird mit Hilfe numerischer Simulationen untersucht. Die physikalischen Prozesse, die die Entwicklung interner Grenzschichten bestimmen, werden eingehend beschrieben. Insbesondere wer- den untersucht' die Bedeutung der turbulenten Diffusion bei -der Entwicklunginterner Grenzschichten, die Reaktion des turbulenten Austauschkoeffizienten auf Anderungen der Bodeneigenschaften, der Einfluß des Druckfeldes auf die bodennahe Strömung strom- auf von einer Rauhigkeitsänderung, der Haushalt der turbulenten kinetischen Energie innerhalb einer internen Grenzschicht, die Entwicklung interner Massengrenzschichten, der Aufbau interner Grenzschichten in einer nichtneutral geschichteten Atmosphäre, die horizontale und vertikale Ausdehnung von Nachlaufgrenzschichten stromab von isolier- ten Rauhigkeitsstreifen, das Verschmelzen mehrerer interner Grenzschichten sowie die Entwicklung einer einhüllenden internen Grenzschicht. Auf der detaillierten Untersuchung der horizontal inhomogenen, bodennahen Luftströ- mung aufbauend, wird ein Verfahren zur Berechnung räumlich gemittelte, bodenna- her Impuls-, Energie- und Stoffflüsse in einem größerskaligen Strömungsmodell entwik- kelt, das die Inhomogenität einer Oberfläche nicht aufzulösen gestattet. Ferner wird die Größenordnung der sogenannten mesoturbulenten Transporte abgeschätzt, also der Transporte, die durch Luftbewegungen hervorgerufen werden, die sich nicht als Mi- kroturbulenz erfassen lassen und deren charakteristische Ausdehnung kleiner als die Gittermaschenweite eines numerischen Modells ist.The atmospheric surface layer over inhomogeneous terrain The response of the atmospheric surface-layer flow to changes in roughness, optical and thermal conditions of an almost flat surface are investigated by use of numerical simulations. The physical processes, which dominate the development of internal boun- dary layers, are described in detail. Attention is foccussed on: the role of turbulent diffusion in the development of internal boundary layers, the respond of the eddy visco- sity to changes of surface conditions, the influence of pressure on the surface-layer flow upstream of a change in roughness, the budget of turbulent kinetic energy within an internal boundary layer, the development of internal boundary layers of constituents, the horizontal and vertical extent of wakes downstream of isolated roughness strips, the blending of multiple internal boundary layers, and the development of enveloping internal boundary layers. Based on the detailed investigation of the horizontally inhomogeneous surface-layer How, a method for computation of spatially-averaged surface fluxes of momentum, energy, and constituents in a larger scale model, which does not resolve the inhomogeneity of the surface, is developed. Furthermore, the magnitude of the so-called mesoturbulent transports, i.e., transports originated by motions at scales larger than micro turbulence, but smaller than the grid size of a numerical model, are estimated

Paleonutrient data analysis of the glacial Atlantic using an adjoint ocean general circulation model

In this study we assimilate measurements of stable carbon isotope compositions delta(13)C and the ratio of cadmium to calcium (Cd/Ca) concentrations from marine sediments into an ocean general circulation model to reconstruct the flow field of the deep-sea during the last glacial maximum (LGM) 21,000 years ago. The results of the assimilation confirm that the southward how of North Atlantic Deep Water was shallower during the LGM and this was accompanied by a strong source of glacial North Atlantic Intermediate Water. The optimized glacial flow field in the Southern Ocean is consistent with the Cd/Ca measurements but can not explain delta(13)C changes, suggesting a breakdown of the glacial phosphate-delta(13)C relationship

On the sensitivity of an ocean general circulation model to glacial boundary conditions

Several studies in the last two decades measuring stable isotope ratios 613C from foraminifera shells pointed out that during the last glacial maximum (LGM) the Atlantic thermohaline circulation was significantly different from the present day circulation, indicating a shallower and reduced North Atlantic Deep Water (NADW), while the compensating Antarctic Bottom Water (AABW) penetrated further northward. Here, we show a 3 D OGCM response to glacial wind stress and air temperature derived from an atmospheric GCM and salinity reconstructions adapted from 6180 measurements in foraminifera shells. The OGCM includes a simplified biogeochemical cycle and reproduces the main features of the past 6130 distribution with a reduction of the Atlantic ’conveyor belt’ by about the half at 30 08. Sensitivity experiments with respect to possible errors in the reconstructed salinity boundary fields show circulation patterns in the Atlantic ranging from an even stronger than the present day one to a nearly shut down of the ’conveyor belt’ circulation. Additionally, a sensitivity experiment with respect to uncertainties of the wind field in order of the glacial—interglacial amplitude shows that Atlantic overturning circulation is not severely affected

A model-model and data-model comparison for the early Eocene hydrological cycle

A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and <i>P</i> − <i>E</i> distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. <br><br> We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric paleo-CO₂, resulting in elevated temperatures, although the effects of differences in paleogeography and ice sheets are also important in some models. For a given CO₂ level, globally averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (d<i>P</i>∕d<i>T</i>) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa, and the Peri-Tethys, which may represent targets for future proxy acquisition. <br><br> A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that GCMs generally underestimate precipitation rates at high latitudes, although a possible seasonal bias of the proxies cannot be excluded. Models which warm these regions, either via elevated CO₂ or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO₂ are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates

A Model-data Comparison for a Multi-model Ensemble of Early Eocene Atmosphere-ocean Simulations: EoMIP

The early Eocene (~55 to 50 Ma) is a time period which has been explored in a large number of modelling and data studies. Here, using an ensemble of previously published model results, making up EoMIP – the Eocene Modelling Intercomparison Project – and syntheses of early Eocene terrestrial and sea surface temperature data, we present a self-consistent inter-model and model–data comparison. This shows that the previous modelling studies exhibit a very wide inter-model variability, but that at high CO2, there is good agreement between models and data for this period, particularly if possible seasonal biases in some of the proxies are considered. An energy balance analysis explores the reasons for the differences between the model results, and suggests that differences in surface albedo feedbacks, water vapour and lapse rate feedbacks, and prescribed aerosol loading are the dominant cause for the different results seen in the models, rather than inconsistencies in other prescribed boundary conditions or differences in cloud feedbacks. The CO2 level which would give optimal early Eocene model–data agreement, based on those models which have carried out simulations with more than one CO2 level, is in the range of 2500 ppmv to 6500 ppmv. Given the spread of model results, tighter bounds on proxy estimates of atmospheric CO2 and temperature during this time period will allow a quantitative assessment of the skill of the models at simulating warm climates. If it is the case that a model which gives a good simulation of the Eocene will also give a good simulation of the future, then such an assessment could be used to produce metrics for weighting future climate predictions

Changes in terrestrial carbon storage during interglacials: a comparison between Eemian and Holocene

International audienceA complex earth system model (atmosphere and ocean general circulation models, ocean biogeochemistry and terrestrial biosphere) was used to perform transient simulations of two interglacial sections (Eemian, 128?113 ky B.P., and Holocene, 9 ky B.P.-present). The changes in terrestrial carbon storage during these interglacials were studied with respect to changes in the earth's orbit. The effect of different climate factors for the changes in carbon storage were studied in offline experiments in which the vegetation model was forced with only temperature, hydrological parameters, radiation, or CO2 concentration from the transient runs. Although temperature caused the largest anomalies in terrestrial carbon storage, the increase in storage due to forest expansion and increased photosynthesis in the high latitudes was nearly balanced by the decrease due to increased respiration. Large positive effects on carbon storage came from an enhanced monsoon circulation in the subtropics between 128 and 121 ky B.P. and between 9 and 6 ky B.P., and from increases in incoming radiation during summer for 45° to 70° N compared to a control run with present-day insolation. Compared to this control run, the net effect of these changes was a positive carbon storage anomaly of about 200 Pg C for 125 ky B.P. and 7 ky B.P., and a negative anomaly around 150 Pg C for 116 ky B.P. Although the net increases for Eemian and Holocene were rather similar, the causes of this differ substantially. The decrease in terrestrial carbon storage during the experiments was the main driver of an increase in atmospheric CO2 concentration for both the Eemian and the Holocene

Dynamics of the terrestrial biosphere, climate and atmospheric CO₂ concentration during interglacials: a comparison between Eemian and Holocene

A complex earth system model (atmosphere and ocean general circulation models, ocean biogeochemistry and terrestrial biosphere) was used to perform transient simulations of two interglacial sections (Eemian, 128&ndash;113 ky B.P., and Holocene, 9 ky B.P.&ndash;present). The changes in terrestrial carbon storage during these interglacials were studied with respect to changes in the earth&apos;s orbit. The effects of different climate factors on changes in carbon storage were studied in offline experiments in which the vegetation model was forced only with temperature, hydrological parameters, radiation, or CO₂ concentration from the transient runs. <br><br> The largest anomalies in terrestrial carbon storage were caused by temperature changes. However, the increase in storage due to forest expansion and increased photosynthesis in the high latitudes was nearly balanced by the decrease due to increased respiration. Large positive effects on carbon storage were caused by an enhanced monsoon circulation in the subtropics between 128 and 121 ky B.P. and between 9 and 6 ky B.P., and by increases in incoming radiation during summer for 45&deg; to 70&deg; N compared to a control simulation with present-day insolation. <br><br> Compared to this control simulation, the net effect of these changes was a positive carbon storage anomaly in the terrestrial biosphere of about 200 Pg C for 125 ky B.P. and 7 ky B.P., and a negative anomaly around 150 Pg C for 116 ky B.P. Although the net increases for Eemian and Holocene were rather similar, the magnitudes of the processes causing these effects were different. The decrease in terrestrial carbon storage during the experiments was the main driver of an increase in atmospheric CO₂ concentration during both the Eemian and the Holocene

Uncertainties in the modelled CO2 threshold for Antarctic glaciation

A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ∼780 ppmv is based on the study of DeConto and Pollard (2003) using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene-Oligocene transition ∼34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from 7 different climate models with Eocene boundary conditions (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS-ER). These climate simulations are sourced from a number of independent studies, and as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter