Late miocene climate modelling with ECHAM4/ML. The effects of the palaeovegetation on the Tortonian climate

In dieser Studie werden das Klima des Tortoniums (Spätes Miozän, 11 bis 7 Ma und insbesondere die Effekte der Paläovegetation auf das Klima mit dem komplexen atmosphärischen Zirkulationsmodell ECHAM4, welches an ein Mixed-Layer Ozeanmodell (ML) gekoppelt ist, untersucht. Frühere Tortonium-Simulationen berücksichtigen einen schwächeren paläoozeanischen Wärmetransport und eine angepaßte Paläoorographie, verwenden aber die rezente Vegetation (Steppuhn, 2002; Steppuhn et al., submitted; Steppuhn et al., in prep.). Für die vorliegende Tortonium-Simulation wird die Paläovegetation zusätzlich zu den bisher angepaßten Tortonium-Randbedingungen berücksichtigt. Eine auf Proxydaten basierende Rekonstruktion der Tortonium-Vegetation wird verwendet, um die Oberflächenparameter des Modells ECHAM4/ML anzupassen und damit eine Tortonium-Simulation durchzuführen. Dabei zeigt sich, daß die Paläovegetation signifikante Auswirkungen auf das Klima des Späten Miozäns hat. So wird durch die angepaßte Tortonium-Vegetation der meridionale Temperaturgradient im Vergleich zu heute reduziert. Der Vergleich mit Proxydaten belegt, daß eine adäquate Paläovegetation zu einer realistischeren Abbildung des Tortonium-Klimas im Modell ECHAM4/ML beiträgt. Mit den ECHAM4/ML-Modelldaten der Tortonium-Simulation wird des weiteren das Kohlenstoff-Kreislauf- und Vegetationsmodell CARAIB betrieben. Die von CARAIB simulierte Tortonium-Vegetation stimmt hinsichtlich der Grundmuster mit der Proxydaten-Rekonstruktion der Paläovegetation überein. Darüber hinaus zeigen Sensitivitätsexperimente mit CARAIB, daß Änderungen des atmosphärischen CO2-Gehalts für die Vegetation von größerer Bedeutung sind als die Unterschiede zwischen dem Tortonium- und dem heutigen Klima. Simulationen mit beiden Modellen, ECHAM4/ML und CARAIB, stimmen nicht vollständig mit Proxydaten überein, was letztlich zu der Schlußfolgerung führt, daß das Klima des Späten Miozäns noch immer nicht vollständig verstanden ist.In this study, the climate of the Tortonian (Late Miocene, 11 to 7 Ma) and particularly the effects of the palaeovegetation on the climate are investigated using the complex atmospheric general circulation model ECHAM4 coupled to a mixed-layer ocean model (ML). Previous Tortonian simulations consider an adjusted palaeocean heat transport and an adapted palaeorography, but use the Recent vegetation (Steppuhn, 2002; Steppuhn et al., submitted; Steppuhn et al., in prep.). For the present Tortonian simulation, the palaeovegetation is considered in addition to the previously adapted Tortonian boundary conditions. A proxy-based reconstruction of the Tortonian vegetation is used to adapt the surface parameters in the ECHAM4/ML model and a Tortonian climate simulation is performed. According to this Tortonian run, the palaeovegetation has significant effects on the Late Miocene climate. Due to the adapted Tortonian vegetation, the meridional temperature gradient is reduced as compared to nowadays. The comparison with proxy data demonstrates, that an appropriate palaeovegetation contributes to a more realistic representation of the Tortonian climate in the model ECHAM4/ML. With model results of the Tortonian run with ECHAM4/ML, the carbon cycle and vegetation model CARAIB is run. In its main patterns, the simulated Tortonian vegetation of the CARAIB model agrees with the proxy-based reconstruction of the palaeovegetation. CARAIB sensitivity experiments demonstrate that variations in the atmospheric CO2 are rather more important for the vegetation than differences between the Tortonian and today’s climate. However, simulations with both models, ECHAM4/ML and CARAIB, are not completely in accordance with proxy data. Therefore, it can be concluded, that the Late Miocene climate is still not completely understood

Regional climate model experiment to investigate the Asian monsoon in the Late Miocene

Peer reviewe

Late Miocene model boundary condtions

Here we present Late Miocene (~11-7 Ma) model boundary conditions used by Knorr et al. [2011] in two files (Paleogeography.nc and Surface_conditions.nc). These boundary conditions relate to Figure 1 in the corresponding study. The file Paleogeography.nc (0.5°x 0.5° resolution) contains the orography (positive values and zero) and bathymetry (negative values). The file Surface_conditions.nc contains land surface conditions. Besides the glacier mask these surface conditions (3.75°x 3.75° resolution) contain the vegetation distribution that is represented by specifying different land surface parameters including, surface albedo, surface roughness length, field capacity of soil, forest ratio, leaf area index, fractional vegetation cover, and soil data flags (Food and Agriculture Organization soil map) [cf. Hagemann et al., 1999]. For further details regarding the paleogeography reconstruction and proxy-based reconstruction of the Late Miocene vegetation we would like to refer to Micheels et al. [2007], Micheels et al. [2011] and the references therein

Asynchronous responses of East Asian and Indian summer monsoons to mountain uplift shown by regional climate modelling experiments

has been demonstrated in climate models that both the Indian and East Asian summer monsoons (ISM and EASM) are strengthened by the uplift of the entire Asian orography or Tibetan Plateau (TP) (i.e. bulk mountain uplift). Such an effect is widely perceived as the major mechanism contributing to the evolution of Asian summer monsoons in the Neogene. However, geological evidence suggests more diachronous growth of the Asian orography (i.e. regional mountain uplift) than bulk mountain uplift. This demands a re-evaluation of the relation between mountain uplift and the Asian monsoon in the geological periods. In this study, sensitivity experiments considering the diachronous growth of different parts of the Asian orography are performed using the regional climate model COSMO-CLM to investigate their effects on the Asian summer monsoons. The results show that, different from the bulk mountain uplift, the regional mountain uplift can lead to an asynchronous development of the ISM and EASM. While the ISM is primarily intensified by the thermal insulation (mechanical blocking) effect of the southern TP (Zagros Mountains), the EASM is mainly enhanced by the surface sensible heating of the central, northern and eastern TP. Such elevated surface heating can induce a low-level cyclonic anomaly around the TP that reduces the ISM by suppressing the lower tropospheric monsoon vorticity, but promotes the EASM by strengthening the warm advection from the south of the TP that sustains the monsoon convection. Our findings provide new insights to the evolution of the Asian summer monsoons and their interaction with the tectonic changes in the Neogene

Late Miocene vegetation reconstruction with the CARAIB model

Climatic outputs from the atmospheric general circulation model ECHAM4 coupled to a mixed layer ocean model are used as inputs to the CARAIB global vegetation model to reconstruct the distribution of vegetation and the biosphere carbon stocks over the continents during the Late Miocene (Tortonian). The results show significant changes in the geographical distribution of vegetation during the Late Miocene compared to the present with a reduction of desert areas and an expansion of tropical seasonal forests, which reached temperate latitudes. These changes in vegetation distribution are accompanied by a moderate increase of the total biosphere carbon stock by 159Gt. Sensitivity tests to atmospheric CO2 have also been performed with the vegetation model only, i.e., while keeping constant all climatic variables to their reference Tortonian state. These tests point out the potential importance Of CO2 fertilization both regarding vegetation distribution and biosphere carbon storage. The impact of an atmospheric CO2 decrease (from 280 to 200ppmv) or increase (from 280 to 560ppmv) on the vegetation distribution appears to be at least as large as that of the climate change between the Tortonian and the present, while in terms of carbon storage the impact of atmospheric CO2 is far much larger than the climatic one. Although the actual response of vegetation to CO2 fertilization may be much smaller than its theoretical response in the model, these results emphasize the need to consider atmospheric CO2 as an important parameter for palaeovegetation reconstructions. (c) 2006 Elsevier B.V. All rights reserved