In Supplement to Leduc Guillaume Schneider Ralph R Kim Jung Hyun Lohmann Gerrit Holocene and Eemian Sea Surface Temperature Trends As Revealed By Alkenone and Mg Ca Paleothermometry Quaternary Science Reviews 29 989 1004 Doi 10 1016 J Quascirev 2010 01 004, Apr 21, 2010
In this paper the relationship between the variability of the seasonal Diurnal Temperature Range ... more In this paper the relationship between the variability of the seasonal Diurnal Temperature Range (DTR) over Europe and the atmospheric circulation has been investigated. The spatiotemporal variability of the seasonal DTR over Europe and the influence of largescale atmospheric circulation on DTR have been examined by means of Empirical Orthogonal Function (EOF) analysis and composite map analysis. Based on the EOF analysis is found that the leading modes of seasonal DTR variability are seasondependent. The character of the links between the DTR variability and atmospheric circulation features also essential seasonality. The spatial structure of the leading EOFs shows some similarity from one season to another, but their temporal evolution differs significantly. It is found that during winter the first three EOFs of the DTR field are related to the North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO) and the Scandinavian Pattern (SCA). During summer the DTR variab...
The deep water formation in the Labrador Sea is simulated with the Finite-Element Sea-Ice Ocean M... more The deep water formation in the Labrador Sea is simulated with the Finite-Element Sea-Ice Ocean Model (FESOM) in a regionally focused, but globally covered model setup. The model has a regional resolution of up to 7 km and the simulations cover the time period 1958-2009. We evaluate the capability of the model setup to reproduce a realistic deep water formation in the Labrador Sea. Two classes of modeled Labrador Sea Water (LSW), the lighter upper LSW (uLSW) and the denser deep LSW (dLSW), are analysed. Their layer thicknesses are compared to uLSW and dLSW layer thicknesses derived from observations in the formation region for the time interval 1988-2009. The results indicate a suitable agreement between the modeled and from observations derived uLSW and dLSW layer thicknesses except for the period 2003-2007 where deviations in the modeled and observational derived layer thickness could be linked to discrepancies in the atmospheric forcing of the model. It is shown that the model is able to reproduce four phases in the temporal evolution of the potential density, temperature and salinity, since the late 1980s, which are known in observational data. These four phases are characterized by a significantly different LSW formation. The first phase from 1988 to 1990 is characterized in the model by a fast increase in the convection depth of up to 2000 m, accompanied by an increased Spring production of deep Labrador Sea Water (dLSW). In the second phase (1991-1994), the dLSW layer thickness remains on a high level for several years, while the third phase (1995-1998) features a gradual decrease in the deep ventilation and the renewal of the deep ocean layers. The fourth X-5 ancy forcing to form either uLSW or dLSW. Many authors [Dickson et al., 1996; Pickart et al., 2003; Yashayaev et al., 2007] suggest that the buoyancy flux is mostly controlled by the strength of the North Atlantic Oscillation (NAO). The NAO index is defined as the normalized atmospheric pressure gradient between the Azores High and the Icelandic Low [e.g. Barnston and Livezey, 1978; Hurrell , 1995]. Other factors that can affect the formation of dLSW or uLSW are the density stratification that remains from preceding winters or large fresh water pools that propagate within the subpolar gyre like the Great Salinity Anomaly (GSA) of the 1970s described by Dickson et al. [1988], or the later salinity anomalies described by Belkin et al. [1998] and Belkin [2004]. Due to the harsh weather conditions, the temporal and spatial availability of ship data for the Labrador Sea, especially regarding the properties of the LSW, is limited especially to the summer season. Nowadays, profiling data from Argo drifters allow also a partial experimental insight into the winter deep convection of the Labrador Sea [Vage et al., 2009], although these data are still limited in their spatial and temporal availability. At this point, numerical ocean model approaches with high resolutions provide the possibility to analyze the spatial and temporal variability patterns. Such model simulations allow to investigate the processes and mechanisms responsible for setting the strength of the deep water formation, especially in regions that are usually difficult to access. Over the last decades different regional model studies regarding the ventilation and transformation of LSW have been carried out [e.g.,
The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by m... more The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by means of a three-dimensional global ocean general circulation model (OGCM). The influence of vertical diffusivity is examined using a wide range of mixing coefficients. For sufficiently large vertical diffusivity the model shows a pronounced hysteresis behavior, so that two equilibrium states of the thermohaline circulation are found: one mode with intense deep-water formation in the North Atlantic ("on" mode) and one mode with strongly reduced convective activity ("off" mode). By decreasing the diffusivity, the two branches of the hysteresis merge. In addition, the effect of numerical diffusion is analyzed by applying different advection schemes. A positive feedback between overturning strength and the numerical diffusivity of upstream advection is found, resulting in a reduced stability of the "on" mode. Comparing the results with those from zonally average...
The proxy δ13C as derived from benthic foraminifera shells is widely used by palaeoceanographers ... more The proxy δ13C as derived from benthic foraminifera shells is widely used by palaeoceanographers to reconstruct past water masses. A mechanistic description of the biogeochemical processes involved in forming the benthic foraminiferal δ13C signal, however, is still lacking. We are using a reaction-diffusion model for calcification in benthic foraminifera, coupled to a combined global ocean and a carbon cycle circulation model, in order to describe the formation of foraminiferal shell δ13C more mechanistically. The coupled models are then applied to a present-day control run and different glacial ocean circulation scenarios. Our results suggest that the effect of temperature on δ13C in benthic foraminiferal shells is more pronounced than previously thought: high (low) temperatures result in higher (lower) shell δ13C values when compared to the δ13C value of dissolved inorganic carbon (DIC) in the same location. Additionally, we find that the modelled respiration rate modulates benthi...
Approximately half of the world's population lives in the tropics, and future changes in the ... more Approximately half of the world's population lives in the tropics, and future changes in the hydrological cycle will impact not just the freshwater supplies but also energy production in areas dependent upon hydroelectric power. It is vital that we understand the mechanisms/processes that affect tropical precipitation and the eventual surface hydrological response to better assess projected future regional precipitation trends and variability. Paleo-climate proxies are well suited for this purpose as they provide long time series that pre-date and complement the present, often short instrumental observations. Here we present paleo-precipitation data from a speleothem located in Mesoamerica that reveal large multi-decadal declines in regional precipitation, whose onset coincides with clusters of large volcanic eruptions during the nineteenth and twentieth centuries. This reconstruction provides new independent evidence of long-lasting volcanic effects on climate and elucidates ke...
Recent geological studies revealed that the freshwater input to the Arctic Ocean was highly varia... more Recent geological studies revealed that the freshwater input to the Arctic Ocean was highly variable during the Holocene. In the present study we examine the influence of changing Arctic freshwater runoff and low-saline Bering Strait inflow on large-scale ocean-sea ice dynamics by means of a general circulation model of the Arctic Ocean, the Nordic Seas, and the Atlantic. Discharge distributions used are based on paleohydrological reconstructions for the early (approx. 10 ka) and middle (about 7 ka) Holocene. Keeping all other forcing ...
Decaying Northern Hemisphere ice sheets during deglaciation affect the high latitude hydrological... more Decaying Northern Hemisphere ice sheets during deglaciation affect the high latitude hydrological balance in the North Atlantic and therefore the ocean circulation after the Last Glacial Maximum. Surprisingly, geological data suggest that meltwater fluxes of about 14-20 m sea-level equivalent flushed into the North Atlantic without significantly influencing the Atlantic meridional overturning circulation. Using a threedimensional ocean circulation model coupled to an energy balance model of the atmosphere, we investigate the response of the ocean circulation to spatio-temporal variable deglacial freshwater discharges. Freshwater inputs are simulated by a threedimensional thermo-mechanical ice sheet model of the Northern Hemisphere. In our experiments, we find a strong sensitivity of the ocean circulation when the deglacial meltwater is injected into the surface layers yielding a quasi shutdown. On the other hand, the parameterization of huge sub-glacial outbursts as so-called hyperpycnal flows are mimicked through bottom injections in ocean models leading to a reduced sensitivity of the overturning circulation against freshwater perturbations and providing a consistent representation of the deglacial climate evolution.
We examine the role of the vegetation cover and the associated hydrological cycle on the deep oce... more We examine the role of the vegetation cover and the associated hydrological cycle on the deep ocean circulation during the Late Miocene (~10 million years ago). In our simulations, an open Central American gateway and exchange with fresh Pacific waters leads to a weak and shallow thermohaline circulation in the North Atlantic Ocean which is consistent with most other modeling studies for this time period. Here, we estimate the effect of a changed vegetation cover on the ocean general circulation using atmospheric circulation model simulations for the late Miocene climate with 353 ppmv CO2 level. The Late Miocene land surface cover reduces the albedo, the net evaporation in the North Atlantic catchment is affected and the North Atlantic water becomes more saline leading to a more vigorous North Atlantic Deep Water circulation. These effects reveal potentially important feedbacks between the ocean circulation, the hydrological cycle and the land surface cover for Cenozoic climate evolution.
climate changes: A review based on time series analysis of marine benthic 18O records. Reviews δ ... more climate changes: A review based on time series analysis of marine benthic 18O records. Reviews δ of Geophysics-Richmond Virginia then Washington-52 (3) , pp.
We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and asso... more We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and associated changes in ocean circulation as forcing factor for the onset of Antarctic glaciation near the Eocene–Oligocene transition (~ 34 million years ago). In this paper this hypothesis is tested through sensitivity experiments, using numerical models for the global ocean and atmosphere and for the Antarctic Ice Sheet. The response of the Antarctic continent to the opening of the DP and to the establishment of the Antarctic ...
Components of the climate system, such as ice sheets and marine sediments serve as invaluable arc... more Components of the climate system, such as ice sheets and marine sediments serve as invaluable archives, which can be tapped into, to reconstruct paleoclimate conditions. The relative abundance of hydrogen and oxygen isotopes in ice cores is a proxy for past local temperature evolution. However the translation of these proxies into temperature is not straightforward. Complex interdependencies in the climate system can hide or override the local climate signal at which the ice core was drilled. Using 3D ice sheet modelling in concert with passive tracer advection one can simulate the isotopic distribution in ice sheets and compare them to ice core data. Combining this method in a coupled climate model environment, containing atmosphere and ocean components, one can theoretically simulate the isotopic cycle from the source to the actual ice record. Such an approach would greatly support the interpretation of proxy data whilst constraining the output of 3D ice sheet models (ISMs). We present the implementation of passive tracer advection in our 3D ISM RIMBAY (Thoma et al. in Geosci Model Dev 1:1–21, 2014, Goeller et al. in Cryosphere 7:1095–1106, 2014) and asses the potential of the method to reproduce chronologies of the polar ice sheets.
To describe the progress in Earth System Science, a conceptual framework is proposed which includ... more To describe the progress in Earth System Science, a conceptual framework is proposed which includes hypothesis testing, the formulation of models with different complexity as well as expressing discoveries in terms of metaphors. The later approach is demonstrated by the conveyor belt concept in oceanography which influenced the discussion about abrupt climate changes where the ocean circulation may be involved. It is argued that the combination of different methodologies/complexities and independent results is necessary to prevent over-simplistic views in each discipline of Earth System Science. Emphasis is given on typical steps to obtain new ideas for a new discovery. Examples for over-simplistic views are mentioned for past climate information from proxy data. The recorder system of the proxy has to be taken into account, otherwise the climate information can be misinterpreted. It is concluded that in the field of Earth System Science, basic knowledge and true collaborative problem solving is necessary to make scientists aware of the underlying principles, the limitations and open questions. This is furthermore necessary to develop and sharpen our ideas about the complex Earth System.
To achieve a better understanding of the hydrologic evolution of the North-West (NW) African mons... more To achieve a better understanding of the hydrologic evolution of the North-West (NW) African monsoon system during the Holocene, in particular during inferred abrupt climate changes at the end of the African Humid Period (AHP), we investigated terrigenous plant lipids deposited in marine sediments offshore NW Africa. Changes in rainfall amount were estimated by compound-specific hydrogen isotope (δD) analyses. The spatial gradient of rainfall isotopic compositions is reflected in marine surface sediments. δD changes in plant waxes covering the last 100 years confirm the observed decrease in rainfall during the late twentieth century Sahel drought, and thus can be used for a quantitative calibration of δD and precipitation. δD changes in sedimentary plant waxes show no abrupt change at the end of the AHP suggesting a gradual precipitation decline. These results are supported by Holocene climate simulations using a coupled atmosphere-land surface model, which includes an explicit modeling of isotopic fractionation within the hydrological cycle.
Abstract Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G cli... more Abstract Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G climate model which explicitly resolves the atmosphere-ocean-sea ice dynamics. It is shown that in contrast to boreal summer where the climate follows mainly the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation and the El Niño Southern Oscillation. We find that a positive phase of the Arctic Oscillation/North Atlantic ...
Pronounced decadal climate oscillations are detected in a multi-centennial record based on shell ... more Pronounced decadal climate oscillations are detected in a multi-centennial record based on shell growth rates of the marine bivalve mollusk, Arctica islandica, from Iceland. The corresponding analysis of patterns in sea level pressure and temperature exhibit large-scale teleconnections with North Atlantic climate quantities. We find that the record projects onto blocking situations in the northern North Atlantic. The associated circulation shows a low-pressure signature over Greenland and the Labrador Sea and a high-pressure system over Western Europe associated with northeasterly flow towards Iceland and weakening in the westerly zonal flow over Europe. It can be speculated that such circulation affects food availability controlling shell growth. On multidecadal time scales, the record shows a pronounced variability linked to North Atlantic temperature. In our record, we find enhanced variability of the shell growth rates on multidecadal time scales, and it appears that this oscillation has high amplitudes in the 16th to 18th century also consistent with marine alkenone data. It is conceivable that these climate oscillations, also linked to sea ice export and enhanced blocking, are a more pronounced feature during times when the climate was relatively cold.
In Supplement to Leduc Guillaume Schneider Ralph R Kim Jung Hyun Lohmann Gerrit Holocene and Eemian Sea Surface Temperature Trends As Revealed By Alkenone and Mg Ca Paleothermometry Quaternary Science Reviews 29 989 1004 Doi 10 1016 J Quascirev 2010 01 004, Apr 21, 2010
In this paper the relationship between the variability of the seasonal Diurnal Temperature Range ... more In this paper the relationship between the variability of the seasonal Diurnal Temperature Range (DTR) over Europe and the atmospheric circulation has been investigated. The spatiotemporal variability of the seasonal DTR over Europe and the influence of largescale atmospheric circulation on DTR have been examined by means of Empirical Orthogonal Function (EOF) analysis and composite map analysis. Based on the EOF analysis is found that the leading modes of seasonal DTR variability are seasondependent. The character of the links between the DTR variability and atmospheric circulation features also essential seasonality. The spatial structure of the leading EOFs shows some similarity from one season to another, but their temporal evolution differs significantly. It is found that during winter the first three EOFs of the DTR field are related to the North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO) and the Scandinavian Pattern (SCA). During summer the DTR variab...
The deep water formation in the Labrador Sea is simulated with the Finite-Element Sea-Ice Ocean M... more The deep water formation in the Labrador Sea is simulated with the Finite-Element Sea-Ice Ocean Model (FESOM) in a regionally focused, but globally covered model setup. The model has a regional resolution of up to 7 km and the simulations cover the time period 1958-2009. We evaluate the capability of the model setup to reproduce a realistic deep water formation in the Labrador Sea. Two classes of modeled Labrador Sea Water (LSW), the lighter upper LSW (uLSW) and the denser deep LSW (dLSW), are analysed. Their layer thicknesses are compared to uLSW and dLSW layer thicknesses derived from observations in the formation region for the time interval 1988-2009. The results indicate a suitable agreement between the modeled and from observations derived uLSW and dLSW layer thicknesses except for the period 2003-2007 where deviations in the modeled and observational derived layer thickness could be linked to discrepancies in the atmospheric forcing of the model. It is shown that the model is able to reproduce four phases in the temporal evolution of the potential density, temperature and salinity, since the late 1980s, which are known in observational data. These four phases are characterized by a significantly different LSW formation. The first phase from 1988 to 1990 is characterized in the model by a fast increase in the convection depth of up to 2000 m, accompanied by an increased Spring production of deep Labrador Sea Water (dLSW). In the second phase (1991-1994), the dLSW layer thickness remains on a high level for several years, while the third phase (1995-1998) features a gradual decrease in the deep ventilation and the renewal of the deep ocean layers. The fourth X-5 ancy forcing to form either uLSW or dLSW. Many authors [Dickson et al., 1996; Pickart et al., 2003; Yashayaev et al., 2007] suggest that the buoyancy flux is mostly controlled by the strength of the North Atlantic Oscillation (NAO). The NAO index is defined as the normalized atmospheric pressure gradient between the Azores High and the Icelandic Low [e.g. Barnston and Livezey, 1978; Hurrell , 1995]. Other factors that can affect the formation of dLSW or uLSW are the density stratification that remains from preceding winters or large fresh water pools that propagate within the subpolar gyre like the Great Salinity Anomaly (GSA) of the 1970s described by Dickson et al. [1988], or the later salinity anomalies described by Belkin et al. [1998] and Belkin [2004]. Due to the harsh weather conditions, the temporal and spatial availability of ship data for the Labrador Sea, especially regarding the properties of the LSW, is limited especially to the summer season. Nowadays, profiling data from Argo drifters allow also a partial experimental insight into the winter deep convection of the Labrador Sea [Vage et al., 2009], although these data are still limited in their spatial and temporal availability. At this point, numerical ocean model approaches with high resolutions provide the possibility to analyze the spatial and temporal variability patterns. Such model simulations allow to investigate the processes and mechanisms responsible for setting the strength of the deep water formation, especially in regions that are usually difficult to access. Over the last decades different regional model studies regarding the ventilation and transformation of LSW have been carried out [e.g.,
The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by m... more The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by means of a three-dimensional global ocean general circulation model (OGCM). The influence of vertical diffusivity is examined using a wide range of mixing coefficients. For sufficiently large vertical diffusivity the model shows a pronounced hysteresis behavior, so that two equilibrium states of the thermohaline circulation are found: one mode with intense deep-water formation in the North Atlantic ("on" mode) and one mode with strongly reduced convective activity ("off" mode). By decreasing the diffusivity, the two branches of the hysteresis merge. In addition, the effect of numerical diffusion is analyzed by applying different advection schemes. A positive feedback between overturning strength and the numerical diffusivity of upstream advection is found, resulting in a reduced stability of the "on" mode. Comparing the results with those from zonally average...
The proxy δ13C as derived from benthic foraminifera shells is widely used by palaeoceanographers ... more The proxy δ13C as derived from benthic foraminifera shells is widely used by palaeoceanographers to reconstruct past water masses. A mechanistic description of the biogeochemical processes involved in forming the benthic foraminiferal δ13C signal, however, is still lacking. We are using a reaction-diffusion model for calcification in benthic foraminifera, coupled to a combined global ocean and a carbon cycle circulation model, in order to describe the formation of foraminiferal shell δ13C more mechanistically. The coupled models are then applied to a present-day control run and different glacial ocean circulation scenarios. Our results suggest that the effect of temperature on δ13C in benthic foraminiferal shells is more pronounced than previously thought: high (low) temperatures result in higher (lower) shell δ13C values when compared to the δ13C value of dissolved inorganic carbon (DIC) in the same location. Additionally, we find that the modelled respiration rate modulates benthi...
Approximately half of the world's population lives in the tropics, and future changes in the ... more Approximately half of the world's population lives in the tropics, and future changes in the hydrological cycle will impact not just the freshwater supplies but also energy production in areas dependent upon hydroelectric power. It is vital that we understand the mechanisms/processes that affect tropical precipitation and the eventual surface hydrological response to better assess projected future regional precipitation trends and variability. Paleo-climate proxies are well suited for this purpose as they provide long time series that pre-date and complement the present, often short instrumental observations. Here we present paleo-precipitation data from a speleothem located in Mesoamerica that reveal large multi-decadal declines in regional precipitation, whose onset coincides with clusters of large volcanic eruptions during the nineteenth and twentieth centuries. This reconstruction provides new independent evidence of long-lasting volcanic effects on climate and elucidates ke...
Recent geological studies revealed that the freshwater input to the Arctic Ocean was highly varia... more Recent geological studies revealed that the freshwater input to the Arctic Ocean was highly variable during the Holocene. In the present study we examine the influence of changing Arctic freshwater runoff and low-saline Bering Strait inflow on large-scale ocean-sea ice dynamics by means of a general circulation model of the Arctic Ocean, the Nordic Seas, and the Atlantic. Discharge distributions used are based on paleohydrological reconstructions for the early (approx. 10 ka) and middle (about 7 ka) Holocene. Keeping all other forcing ...
Decaying Northern Hemisphere ice sheets during deglaciation affect the high latitude hydrological... more Decaying Northern Hemisphere ice sheets during deglaciation affect the high latitude hydrological balance in the North Atlantic and therefore the ocean circulation after the Last Glacial Maximum. Surprisingly, geological data suggest that meltwater fluxes of about 14-20 m sea-level equivalent flushed into the North Atlantic without significantly influencing the Atlantic meridional overturning circulation. Using a threedimensional ocean circulation model coupled to an energy balance model of the atmosphere, we investigate the response of the ocean circulation to spatio-temporal variable deglacial freshwater discharges. Freshwater inputs are simulated by a threedimensional thermo-mechanical ice sheet model of the Northern Hemisphere. In our experiments, we find a strong sensitivity of the ocean circulation when the deglacial meltwater is injected into the surface layers yielding a quasi shutdown. On the other hand, the parameterization of huge sub-glacial outbursts as so-called hyperpycnal flows are mimicked through bottom injections in ocean models leading to a reduced sensitivity of the overturning circulation against freshwater perturbations and providing a consistent representation of the deglacial climate evolution.
We examine the role of the vegetation cover and the associated hydrological cycle on the deep oce... more We examine the role of the vegetation cover and the associated hydrological cycle on the deep ocean circulation during the Late Miocene (~10 million years ago). In our simulations, an open Central American gateway and exchange with fresh Pacific waters leads to a weak and shallow thermohaline circulation in the North Atlantic Ocean which is consistent with most other modeling studies for this time period. Here, we estimate the effect of a changed vegetation cover on the ocean general circulation using atmospheric circulation model simulations for the late Miocene climate with 353 ppmv CO2 level. The Late Miocene land surface cover reduces the albedo, the net evaporation in the North Atlantic catchment is affected and the North Atlantic water becomes more saline leading to a more vigorous North Atlantic Deep Water circulation. These effects reveal potentially important feedbacks between the ocean circulation, the hydrological cycle and the land surface cover for Cenozoic climate evolution.
climate changes: A review based on time series analysis of marine benthic 18O records. Reviews δ ... more climate changes: A review based on time series analysis of marine benthic 18O records. Reviews δ of Geophysics-Richmond Virginia then Washington-52 (3) , pp.
We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and asso... more We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and associated changes in ocean circulation as forcing factor for the onset of Antarctic glaciation near the Eocene–Oligocene transition (~ 34 million years ago). In this paper this hypothesis is tested through sensitivity experiments, using numerical models for the global ocean and atmosphere and for the Antarctic Ice Sheet. The response of the Antarctic continent to the opening of the DP and to the establishment of the Antarctic ...
Components of the climate system, such as ice sheets and marine sediments serve as invaluable arc... more Components of the climate system, such as ice sheets and marine sediments serve as invaluable archives, which can be tapped into, to reconstruct paleoclimate conditions. The relative abundance of hydrogen and oxygen isotopes in ice cores is a proxy for past local temperature evolution. However the translation of these proxies into temperature is not straightforward. Complex interdependencies in the climate system can hide or override the local climate signal at which the ice core was drilled. Using 3D ice sheet modelling in concert with passive tracer advection one can simulate the isotopic distribution in ice sheets and compare them to ice core data. Combining this method in a coupled climate model environment, containing atmosphere and ocean components, one can theoretically simulate the isotopic cycle from the source to the actual ice record. Such an approach would greatly support the interpretation of proxy data whilst constraining the output of 3D ice sheet models (ISMs). We present the implementation of passive tracer advection in our 3D ISM RIMBAY (Thoma et al. in Geosci Model Dev 1:1–21, 2014, Goeller et al. in Cryosphere 7:1095–1106, 2014) and asses the potential of the method to reproduce chronologies of the polar ice sheets.
To describe the progress in Earth System Science, a conceptual framework is proposed which includ... more To describe the progress in Earth System Science, a conceptual framework is proposed which includes hypothesis testing, the formulation of models with different complexity as well as expressing discoveries in terms of metaphors. The later approach is demonstrated by the conveyor belt concept in oceanography which influenced the discussion about abrupt climate changes where the ocean circulation may be involved. It is argued that the combination of different methodologies/complexities and independent results is necessary to prevent over-simplistic views in each discipline of Earth System Science. Emphasis is given on typical steps to obtain new ideas for a new discovery. Examples for over-simplistic views are mentioned for past climate information from proxy data. The recorder system of the proxy has to be taken into account, otherwise the climate information can be misinterpreted. It is concluded that in the field of Earth System Science, basic knowledge and true collaborative problem solving is necessary to make scientists aware of the underlying principles, the limitations and open questions. This is furthermore necessary to develop and sharpen our ideas about the complex Earth System.
To achieve a better understanding of the hydrologic evolution of the North-West (NW) African mons... more To achieve a better understanding of the hydrologic evolution of the North-West (NW) African monsoon system during the Holocene, in particular during inferred abrupt climate changes at the end of the African Humid Period (AHP), we investigated terrigenous plant lipids deposited in marine sediments offshore NW Africa. Changes in rainfall amount were estimated by compound-specific hydrogen isotope (δD) analyses. The spatial gradient of rainfall isotopic compositions is reflected in marine surface sediments. δD changes in plant waxes covering the last 100 years confirm the observed decrease in rainfall during the late twentieth century Sahel drought, and thus can be used for a quantitative calibration of δD and precipitation. δD changes in sedimentary plant waxes show no abrupt change at the end of the AHP suggesting a gradual precipitation decline. These results are supported by Holocene climate simulations using a coupled atmosphere-land surface model, which includes an explicit modeling of isotopic fractionation within the hydrological cycle.
Abstract Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G cli... more Abstract Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G climate model which explicitly resolves the atmosphere-ocean-sea ice dynamics. It is shown that in contrast to boreal summer where the climate follows mainly the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation and the El Niño Southern Oscillation. We find that a positive phase of the Arctic Oscillation/North Atlantic ...
Pronounced decadal climate oscillations are detected in a multi-centennial record based on shell ... more Pronounced decadal climate oscillations are detected in a multi-centennial record based on shell growth rates of the marine bivalve mollusk, Arctica islandica, from Iceland. The corresponding analysis of patterns in sea level pressure and temperature exhibit large-scale teleconnections with North Atlantic climate quantities. We find that the record projects onto blocking situations in the northern North Atlantic. The associated circulation shows a low-pressure signature over Greenland and the Labrador Sea and a high-pressure system over Western Europe associated with northeasterly flow towards Iceland and weakening in the westerly zonal flow over Europe. It can be speculated that such circulation affects food availability controlling shell growth. On multidecadal time scales, the record shows a pronounced variability linked to North Atlantic temperature. In our record, we find enhanced variability of the shell growth rates on multidecadal time scales, and it appears that this oscillation has high amplitudes in the 16th to 18th century also consistent with marine alkenone data. It is conceivable that these climate oscillations, also linked to sea ice export and enhanced blocking, are a more pronounced feature during times when the climate was relatively cold.
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Papers by Gerrit Lohmann