Abis B.,International Max Planck Research School on Earth System Modelling |
Abis B.,Max Planck Institute for Meteorology |
Brovkin V.,Max Planck Institute for Meteorology
Biogeosciences | Year: 2017
Previous analysis of the vegetation cover from remote sensing revealed the existence of three alternative modes in the frequency distribution of boreal tree cover: a sparsely vegetated treeless state, an open woodland state, and a forest state. Identifying which are the regions subject to multimodality, and assessing which are the main factors underlying their existence, is important to project future change of natural vegetation cover and its effect on climate. We study the link between the tree-cover fraction distribution and eight globally observed environmental factors: mean annual rainfall, mean minimum temperature, growing degree days above 0 °C, permafrost distribution, mean spring soil moisture, wildfire occurrence frequency, soil texture, and mean thawing depth. Through the use of generalised additive models, conditional histograms, and phase-space analysis, we find that environmental conditions exert a strong control over the tree-cover distribution, uniquely determining its state among the three dominant modes in 95% of the cases. Additionally, we find that the link between individual environmental variables and tree cover is different within the four boreal regions considered here, namely eastern North Eurasia, western North Eurasia, eastern North America, and western North America. Furthermore, using a classification based on rainfall, minimum temperatures, permafrost distribution, soil moisture, wildfire frequency, and soil texture, we show the location of areas with potentially alternative tree-cover states under the same environmental conditions in the boreal region. These areas, although encompassing a minor fraction of the boreal area (5%), correspond to possible transition zones with a reduced resilience to disturbances. Hence, they are of interest for a more detailed analysis of land-atmosphere interactions. © 2017 Author(s).
Hernandez-Deckers D.,International Max Planck Research School on Earth System Modelling |
Hernandez-Deckers D.,Max Planck Institute for Meteorology |
von Storch J.-S.,Max Planck Institute for Meteorology
Journal of Climate | Year: 2010
Increasing greenhouse gas concentrations warm the troposphere. However, it is not clear whether this implies changes in the energetics. To study the energetics responses to CO 2 increases, changes in the Lorenz energy cycle (LEC) are evaluated using output from the atmosphere-ocean ECHAM5/Max Planck Institute Ocean Model (MPI-OM). Equilibrium 2 × CO 2 experiments and 10-yr transient experiments with 3% increase per year are analyzed. Globally, doubling of CO 2 results in a decrease in the LEC strength-defined as the total conversion of available potential energy P into kinetic energy K-but also in an increase in the zonal-mean K. These global changes are a consequence of the strengthening of the LEC in the upper troposphere and the weakening of the cycle below. The two opposite responses result from the simulated warming pattern that shows the strongest warming in the upper tropical troposphere and in the lower troposphere at high latitudes. This warming structure causes changes in the horizontal temperature variance and in mean static stability, which increase zonal-mean P in the upper troposphere and decrease it below, triggering the two opposite responses via changes in baroclinic activity. In general, the lower-region weakening is stronger in the Northern Hemisphere, while the upper-region strengthening, and the increase of zonal-mean P and K, is stronger in the Southern Hemisphere. The former is more pronounced in the transient experiments but decreases in the stabilized 2 × CO 2 climate. © 2010 American Meteorological Society.
Voigt A.,International Max Planck Research School on Earth System Modelling |
Voigt A.,Max Planck Institute for Meteorology |
Marotzke J.,Max Planck Institute for Meteorology
Climate Dynamics | Year: 2010
We use the coupled atmosphere-ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters. © 2009 The Author(s).
Heinze M.,Max Planck Institute for Meteorology |
Heinze M.,International Max Planck Research School on Earth System Modelling |
Ilyina T.,Max Planck Institute for Meteorology
Climate of the Past | Year: 2015
The late Paleocene is characterized by warm and stable climatic conditions that served as the background climate for the Paleocene-Eocene Thermal Maximum (PETM, ∼55 million years ago). With respect to feedback processes in the carbon cycle, the ocean biogeochemical background state is of major importance for projecting the climatic response to a carbon perturbation related to the PETM. Therefore, we use the Hamburg Ocean Carbon Cycle model (HAMOCC), embedded in the ocean general circulation model of the Max Planck Institute for Meteorology, MPIOM, to constrain the ocean biogeochemistry of the late Paleocene. We focus on the evaluation of modeled spatial and vertical distributions of the ocean carbon cycle parameters in a long-term warm steady-state ocean, based on a 560 ppm CO2 atmosphere. Model results are discussed in the context of available proxy data and simulations of pre-industrial conditions. Our results illustrate that ocean biogeochemistry is shaped by the warm and sluggish ocean state of the late Paleocene. Primary production is slightly reduced in comparison to the present day; it is intensified along the Equator, especially in the Atlantic. This enhances remineralization of organic matter, resulting in strong oxygen minimum zones and CaCO3 dissolution in intermediate waters. We show that an equilibrium CO2 exchange without increasing total alkalinity concentrations above today's values is achieved. However, consistent with the higher atmospheric CO2, the surface ocean pH and the saturation state with respect to CaCO3 are lower than today. Our results indicate that, under such conditions, the surface ocean carbonate chemistry is expected to be more sensitive to a carbon perturbation (i.e., the PETM) due to lower CO32- concentration, whereas the deep ocean calcite sediments would be less vulnerable to dissolution due to the vertically stratified ocean. © 2015 Author(s).
Peters K.,Max Planck Institute for Meteorology |
Peters K.,University of Hamburg |
Peters K.,International Max Planck Research School on Earth System Modelling |
Quaas J.,Max Planck Institute for Meteorology |
Bellouin N.,UK Met Office
Atmospheric Chemistry and Physics | Year: 2011
We present a method for deriving the radiative effects of absorbing aerosols in cloudy scenes from satellite retrievals only. We use data of 2005-2007 from various passive sensors aboard satellites of the "A-Train" constellation. The study area is restricted to the tropical- and subtropical Atlantic Ocean. To identify the dependence of the local planetary albedo in cloudy scenes on cloud liquid water path and aerosol optical depth (AOD), we perform a multiple linear regression. The OMI UV-Aerosolindex serves as an indicator for absorbing-aerosol presence. In our method, the aerosol influences the local planetary albedo through direct- (scattering and absorption) and indirect (Twomey) aerosol effects. We find an increase of the local planetary albedo (LPA) with increasing AOD of mostly scattering aerosol and a decrease of the LPA with increasing AOD of mostly absorbing aerosol. These results allow us to derive the direct aerosol effect of absorbing aerosols in cloudy scenes, with the effect of cloudy-scene aerosol absorption in the tropical- and subtropical Atlantic contributing (+21.2 ± 11.1)×-10-3 Wm-2 to the global top of the atmosphere radiative forcing. © 2011 Author(s).
Veira A.,Max Planck Institute for Meteorology |
Veira A.,International Max Planck Research School on Earth System Modelling |
Kloster S.,Max Planck Institute for Meteorology |
Schutgens N.A.J.,University of Oxford |
Kaiser J.W.,Max Planck Institute for Chemistry
Atmospheric Chemistry and Physics | Year: 2015
Wildfires represent a major source for aerosols impacting atmospheric radiation, atmospheric chemistry and cloud micro-physical properties. Previous case studies indicated that the height of the aerosol-radiation interaction may crucially affect atmospheric radiation, but the sensitivity to emission heights has been examined with only a few models and is still uncertain. In this study we use the general circulation model ECHAM6 extended by the aerosol module HAM2 to investigate the impact of wildfire emission heights on atmospheric long-range transport, black carbon (BC) concentrations and atmospheric radiation. We simulate the wildfire aerosol release using either various versions of a semi-empirical plume height parametrization or prescribed standard emission heights in ECHAM6-HAM2. Extreme scenarios of near-surface or free-tropospheric-only injections provide lower and upper constraints on the emission height climate impact. We find relative changes in mean global atmospheric BC burden of up to 7.9±4.4 % caused by average changes in emission heights of 1.5-3.5 km. Regionally, changes in BC burden exceed 30-40 % in the major biomass burning regions. The model evaluation of aerosol optical thickness (AOT) against Moderate Resolution Imaging Spectroradiometer (MODIS), AErosol RObotic NETwork (AERONET) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations indicates that the implementation of a plume height parametrization slightly reduces the ECHAM6-HAM2 biases regionally, but on the global scale these improvements in model performance are small. For prescribed emission release at the surface, wildfire emissions entail a total sky top-of-atmosphere (TOA) radiative forcing (RF) of -0.16±0.06 W m-2. The application of a plume height parametrization which agrees reasonably well with observations introduces a slightly stronger negative TOA RF of -0.20±0.07 W m-2. The standard ECHAM6-HAM2 model in which 25 % of the wildfire emissions are injected into the free troposphere (FT) and 75 % into the planetary boundary layer (PBL), leads to a TOA RF of -0.24±0.06 W m-2. Overall, we conclude that simple plume height parametrizations provide sufficient representations of emission heights for global climate modeling. Significant improvements in aerosol wildfire modeling likely depend on better emission inventories and aerosol process modeling rather than on improved emission height parametrizations. © Author(s) 2015.
Goessling H.F.,Max Planck Institute for Meteorology |
Goessling H.F.,International Max Planck Research School on Earth System Modelling |
Goessling H.F.,Alfred Wegener Institute for Polar and Marine Research |
Reick C.H.,Max Planck Institute for Meteorology
Atmospheric Chemistry and Physics | Year: 2013
Atmospheric water vapour tracers (WVTs) are an elegant tool to determine source-sink relations of moisture "online" in atmospheric general circulation models (AGCMs). However, it is sometimes desirable to establish such relations "offline" based on already existing atmospheric data (e.g. reanalysis data). One simple and frequently applied offline method is 2-D moisture tracing. It makes use of the "well-mixed" assumption, which allows for treating the vertical dimension integratively. Here we scrutinise the "well-mixed" assumption and 2-D moisture tracing by means of analytical considerations in combination with AGCM-WVT simulations. We find that vertically well-mixed conditions are seldom met. Due to the presence of vertical inhomogeneities, 2-D moisture tracing (i) neglects a significant degree of fast-recycling, and (ii) results in erroneous advection where the direction of the horizontal winds varies vertically. The latter is not so much the case in the extratropics, but in the tropics this can lead to large errors. For example, computed by 2-D moisture tracing, the fraction of precipitation in the western Sahel that originates from beyond the Sahara is ∼40%, whereas the fraction that originates from the tropical and Southern Atlantic is only ∼4%. According to full (i.e. 3-D) moisture tracing, however, both regions contribute roughly equally, showing that the errors introduced by the 2-D approximation can be substantial. © 2013 Author(s).
Griewank P.J.,Max Planck Institute for Meteorology |
Griewank P.J.,International Max Planck Research School on Earth System Modelling |
Notz D.,Max Planck Institute for Meteorology
Journal of Geophysical Research: Oceans | Year: 2013
We study gravity drainage using a new 1-D, multiphase sea ice model. A parametrization of gravity drainage based on the convective nature of gravity drainage is introduced, whose free parameters are determined by optimizing model output against laboratory measurements of sea ice salinity evolution. Optimal estimates of the free parameters as well as the parametrization performance remain stable for vertical grid resolutions from 1 to 30 mm. We find a strong link between sea ice growth rate and bulk salinity for constant boundary conditions but only a weak link for more realistic boundary conditions. We also demonstrate that surface warming can trigger brine convection over the whole ice layer. Over a growth season, replacing the convective parametrization with constant initial salinities leads to an overall 3% discrepancy of stored energy, thermal resistance, and salt release. We also derive from our convective parametrization a simplified, numerically cheap and stable gravity-drainage parametrization. This parametrization results in an approximately 1% discrepancy of stored energy, thermal resistance, and salt release compared to the convective parametrization. A similarly low discrepancy to our complex parametrization can be reached by simply prescribing a depth-dependent salinity profile. ©2013. American Geophysical Union. All Rights Reserved.
Pithan F.,Max Planck Institute for Meteorology |
Pithan F.,International Max Planck Research School on Earth System Modelling |
Medeiros B.,U.S. National Center for Atmospheric Research |
Mauritsen T.,Max Planck Institute for Meteorology
Climate Dynamics | Year: 2014
Temperature inversions are a common feature of the Arctic wintertime boundary layer. They have important impacts on both radiative and turbulent heat fluxes and partly determine local climate-change feedbacks. Understanding the spread in inversion strength modelled by current global climate models is therefore an important step in better understanding Arctic climate and its present and future changes. Here, we show how the formation of Arctic air masses leads to the emergence of a cloudy and a clear state of the Arctic winter boundary layer. In the cloudy state, cloud liquid water is present, little to no surface radiative cooling occurs and inversions are elevated and relatively weak, whereas surface radiative cooling leads to strong surface-based temperature inversions in the clear state. Comparing model output to observations, we find that most climate models lack a realistic representation of the cloudy state. An idealised single-column model experiment of the formation of Arctic air reveals that this bias is linked to inadequate mixed-phase cloud microphysics, whereas turbulent and conductive heat fluxes control the strength of inversions within the clear state. © 2013 Springer-Verlag Berlin Heidelberg.
Schilling J.,University of Hamburg |
Freier K.P.,University of Hamburg |
Freier K.P.,International Max Planck Research School on Earth System Modelling |
Hertig E.,University of Augsburg |
Scheffran J.,University of Hamburg
Agriculture, Ecosystems and Environment | Year: 2012
Our study links environmental impacts of climate change to major socio-economic and agricultural developments in North Africa. We jointly investigate climate projections, vulnerability, impacts, and options for adaptation. Precipitation in North Africa is likely to decrease between 10 and 20%, while temperatures are likely to rise between 2 and 3. °C by 2050. This trend is most pronounced in the north-western parts of northern Africa as our own model results suggest. The combination of decreasing supply and strong population growth aggravates the stressed water situation in the region. We further compare the vulnerabilities, adaptive capacities and conflict implications of climate change in Algeria, Egypt, Libya, Morocco, and Tunisia. Climate change will likely have the strongest effect on Morocco where the agricultural sector is of high importance for the country's economy and particularly for poor people. Our analysis of climate impacts and adaptation options in Morocco suggests that the agricultural incentives used in the past are inadequate to buffer drought effects. To increase resilience against climate change, agricultural policies should shift from maximizing agricultural output to stabilizing it. Our bio-economic model results further suggest a considerable potential of replacing firewood by electric energy to sustain pastoral productivity. © 2012 Elsevier B.V..