In dieser Arbeit wird die Hypothese getestet, ob ein Teil der atmosphärischen Komponente interdekadischer Klimavariabilität durch die Anregung interner, atmosphärischer Moden, speziell von singulären Moden eines linearen, stationären Atmosphärenmodell, begründet werden kann. Die Analysen basieren auf einem linearen, baroklinen quasigeostrophischen Modell, wobei der Grundzustand aus Daten einer Langzeitintegration des ECHAMl/LSG abgeleitet wird. Sie beziehen sich auf eine detektierte Oszillationsmode mit einer Periode von 18 Jahren in dieser GCM Integration. Es zeigt sich, daß der führende rechte singuläre Vektor des linearen baroklinen Modells signifikant mit der interdekadischen Anomalie der atmosphärischen Zirkulation über der Nordhemisphäre korreliert. Damit kann ein Anteil von über 40% der räumlichen Varianz dieser interdekadischen Mode erklärt werden. / In the present paper we examine the hypothesis that a part of the atmospheric component of interdecadal variability is manifested in the exitation of internal, atmospheric modes, in particular in singular modes of a linearized, steady-state atmospheric model. This hypothesis is tested by using a baroclinic quasigeostrophic model, for which data from the dimate model ECHAMl/LSG are utilized to define the long-term mean basic state. The analysis refers to the interdecadal oscillation with a period of 18 years in this GCM-integration. A significant projection is found of the first singular vector and the interdecadal mode of atmospheric circulation. This singular vector is able to explain about 40% of the spatial variance of the interdecadal anomaly over the Northern Hemisphere.

The climate variability over the North Atlantic region is described in the 10-50 year band, using a 500-year integration of the Hamburg ECHAMl/LSG coupled general circulation model. In order to isolate nearly periodic components of the atmosphere and the ocean, the multichannel version of the singular spectrum analysis (MSSA) is applied to 11 components of the climate system. In doing so the main focus is on the turbulent exchange between the two subsystems. One interdecadal oscillation of the system ocean and atmosphere is detected with a period of about 18 years. The associated anomalies of sea level pressure (SLP) are situated east of Newfoundland while these of the geopotential height at 500 hPa are slightly shifted to the East. Both the fields undergo a primarily standing oscillation. The sea surface temperature (SST) and the sub surface temperature anomalies have a large extension along the 40° N latitude circle with most of their variability south of Newfoundland. lt is found that the SST anomaly is primarily generated by the temperature advection in the upper ocean layer which is coupled to the Subpolar Gyre strength and Ekman pumping vertical velocity. Both the processes are forced by the atmosphere. In a further analysis applied only to the SST and the Geopotential height at 500 hPa over the whole Northern Hemisphere this 18 year mode was also isolated. The modes obtained by the local and hemispheric analyses are well correlated both in time and space, suggesting a more active role of atmosphere than of the ocean, in addition a strong modulation of the amplitude of the oscillation due to local processes over the North Atlantic was detected. / Die Klimavariabilität über dem Nordatlantik wird anhand einer 500 Jahre Integration des Hamburger gekoppelten Klimamodels ECHAMl/LSG untersucht. Um periodische Komponenten in Ozean und Atmosphäre zu isolieren, wird die MSSA ("multichannel singular spectrum analysis") auf 11 Komponenten des Klimasystems angewandt. Bei den Analysen wird besonderes Augenmerk auf den turbulenten Austausch zwischen beiden Subsystemen gelegt. Es kann eine Oszillation des gekoppelten Systems mit einer Periode von etwa 18 Jahren detektiert werden. Die Anomalie des Bodendrucks weist ihre maximale Amplitude östlich von Neufundland auf, während die Anomalie des 500 hPa Geopotentials leicht östlich dazu verschoben ist. Die Anomalie der SST zeigt ihre größte Variabilität südlich von Neufundland. Diese wird im wesentlichen durch die Temperaturadvektion in der oberen Ozeanschicht generiert, welche im wesentlichen an den subpolaren ozeanischen Wirbel sowie an das "Ekman pumping" gekoppelt ist. Beide Prozesse werden durch die Atmosphäre angetrieben. In einer weiteren Analyse, in der die MSSA auf die SST und das 500 hPa Geopotential der gesamten nördlichen Hemisphäre angewandt wird, kann ebenfalls ein Oszillation von 18 Jahren detektiert werden. Diese Mode korreliert räumlich und zeitlich gut mit dem der lokalen Analyse über dem Nordatlantik, welches auf ein aktivere Rolle der Atmosphäre hinweist. Die starken Unterschiede der Amplitudenmodulation könnten durch lokale Prozesse bedingt sein.

The climate variability over the North Atlantic region is described in the 10-50 year band, using a 500-year integration of the Hamburg ECHAMl/LSG coupled general circulation model. In order to isolate nearly periodic components of the atmosphere and the ocean, the multichannel version of the singular spectrum analysis (MSSA) is applied to 11 components of the climate system. In doing so the main focus is on the turbulent exchange between the two subsystems. One interdecadal oscillation of the system ocean and atmosphere is detected with a period of about 18 years. The associated anomalies of sea level pressure (SLP) are situated east of Newfoundland while these of the geopotential height at 500 hPa are slightly shifted to the East. Both the fields undergo a primarily standing oscillation. The sea surface temperature (SST) and the sub surface temperature anomalies have a large extension along the 40° N latitude circle with most of their variability south of Newfoundland. lt is found that the SST anomaly is primarily generated by the temperature advection in the upper ocean layer which is coupled to the Subpolar Gyre strength and Ekman pumping vertical velocity. Both the processes are forced by the atmosphere. In a further analysis applied only to the SST and the Geopotential height at 500 hPa over the whole Northern Hemisphere this 18 year mode was also isolated. The modes obtained by the local and hemispheric analyses are well correlated both in time and space, suggesting a more active role of atmosphere than of the ocean, in addition a strong modulation of the amplitude of the oscillation due to local processes over the North Atlantic was detected. / Die Klimavariabilität über dem Nordatlantik wird anhand einer 500 Jahre Integration des Hamburger gekoppelten Klimamodels ECHAMl/LSG untersucht. Um periodische Komponenten in Ozean und Atmosphäre zu isolieren, wird die MSSA ('multichannel singular spectrum analysis') auf 11 Komponenten des Klimasystems angewandt. Bei den Analysen wird besonderes Augenmerk auf den turbulenten Austausch zwischen beiden Subsystemen gelegt. Es kann eine Oszillation des gekoppelten Systems mit einer Periode von etwa 18 Jahren detektiert werden. Die Anomalie des Bodendrucks weist ihre maximale Amplitude östlich von Neufundland auf, während die Anomalie des 500 hPa Geopotentials leicht östlich dazu verschoben ist. Die Anomalie der SST zeigt ihre größte Variabilität südlich von Neufundland. Diese wird im wesentlichen durch die Temperaturadvektion in der oberen Ozeanschicht generiert, welche im wesentlichen an den subpolaren ozeanischen Wirbel sowie an das 'Ekman pumping' gekoppelt ist. Beide Prozesse werden durch die Atmosphäre angetrieben. In einer weiteren Analyse, in der die MSSA auf die SST und das 500 hPa Geopotential der gesamten nördlichen Hemisphäre angewandt wird, kann ebenfalls ein Oszillation von 18 Jahren detektiert werden. Diese Mode korreliert räumlich und zeitlich gut mit dem der lokalen Analyse über dem Nordatlantik, welches auf ein aktivere Rolle der Atmosphäre hinweist. Die starken Unterschiede der Amplitudenmodulation könnten durch lokale Prozesse bedingt sein.

In dieser Arbeit wird die Hypothese getestet, ob ein Teil der atmosphärischen Komponente interdekadischer Klimavariabilität durch die Anregung interner, atmosphärischer Moden, speziell von singulären Moden eines linearen, stationären Atmosphärenmodell, begründet werden kann. Die Analysen basieren auf einem linearen, baroklinen quasigeostrophischen Modell, wobei der Grundzustand aus Daten einer Langzeitintegration des ECHAMl/LSG abgeleitet wird. Sie beziehen sich auf eine detektierte Oszillationsmode mit einer Periode von 18 Jahren in dieser GCM Integration. Es zeigt sich, daß der führende rechte singuläre Vektor des linearen baroklinen Modells signifikant mit der interdekadischen Anomalie der atmosphärischen Zirkulation über der Nordhemisphäre korreliert. Damit kann ein Anteil von über 40% der räumlichen Varianz dieser interdekadischen Mode erklärt werden. / In the present paper we examine the hypothesis that a part of the atmospheric component of interdecadal variability is manifested in the exitation of internal, atmospheric modes, in particular in singular modes of a linearized, steady-state atmospheric model. This hypothesis is tested by using a baroclinic quasigeostrophic model, for which data from the dimate model ECHAMl/LSG are utilized to define the long-term mean basic state. The analysis refers to the interdecadal oscillation with a period of 18 years in this GCM-integration. A significant projection is found of the first singular vector and the interdecadal mode of atmospheric circulation. This singular vector is able to explain about 40% of the spatial variance of the interdecadal anomaly over the Northern Hemisphere.

Unter atmosphärischen Zirkulationsregimen versteht man bevorzugte quasi-stationäre Zustände der atmosphärischen Zirkulation auf der planetaren Skala, die für eine bis mehrere Wochen persistieren können. Klimaänderungen, ob natürlich entstanden oder anthropogen verursacht, äußern sich in erster Linie durch Änderungen der Auftrittswahrscheinlichkeiten der natürlichen Regime. <br><br> In der vorliegenden Arbeit wurden dynamische Mechanismen des Regimeverhaltens und der dekadischen Klimavariabilität der Atmosphäre bei Abwesenheit zeitlich veränderlicher externer Einflussfaktoren untersucht. Das Hauptwerkzeug dafür war ein quasi-geostrophisches Dreischichtenmodell der winterlichen atmosphärischen Zirkulation auf der Nordhemisphäre, das eine spektrale T21-Auflösung, einen orographischen und einen zeitlich konstanten thermischen Antrieb mit nicht-zonalen Anteilen besitzt. Ein solches Modell vermag großskalige atmosphärische Strömungsvorgänge außerhalb der Tropen mit einiger Genauigkeit zu simulieren. Nicht berücksichtigt werden Feuchteprozesse, die Wechselwirkung der Atmosphäre mit anderen Teilen des Klimasystems sowie anthropogene Einflüsse. <br><br> Für das Dreischichtenmodell wurde ein automatisiertes, iteratives Verfahren zur Anpassung des thermischen Modellantriebs neu entwickelt. Jede Iteration des Verfahrens besteht aus einer Testintegration des Modells, ihrer Auswertung, dem Vergleich der Ergebnisse mit den NCEP-NCAR-Reanalysedaten aus den Wintermonaten Dezember, Januar und Februar sowie einer auf diesem Vergleich basierenden Antriebskorrektur. Nach Konvergenz des Verfahrens stimmt das Modell sowohl bezüglich des zonal gemittelten Klimazustandes als auch bezüglich der zeitgemittelten nicht-zonalen außertropischen diabatischen Erwärmung nahezu perfekt mit den wintergemittelten Reanalysedaten überein. <br><br> In einer 1000-jährigen Simulation wurden die beobachtete mittlere Zirkulation im Winter sowie ihre Variabilität realitätsnah reproduziert, insbesondere die Arktische Oszillation (AO) und ihre vertikale Ausdehnung. Der AO-Index des Modells weist deutliche dekadische Schwankungen auf, die allein durch die interne Modelldynamik bedingt sind. Darüber hinaus zeigt das Modell ein Regimeverhalten, das gut mit den Beobachtungsdaten übereintimmt. Es besitzt ein Regime, das in etwa der negativen Phase der Nordatlantischen Oszillation (NAO) entspricht und eines, das der positiven Phase der AO ähnelt. <br><br> Eine weit verbreitete Hypothese ist die näherungsweise Übereinstimmung zwischen Regimen und stationären Lösungen der Bewegungsgleichungen. In der vorliegenden Arbeit wurde diese Hypothese für das Dreischichtenmodell überprüft, mit negativem Resultat. Es wurden mittels eines Funktionalminimierungsverfahrens sechs verschiedene stationäre Zustände gefunden. Diese sind allesamt durch eine äußerst unrealistische Zirkulation gekennzeichnet und sind daher weit vom Modellattraktor entfernt. Fünf der sechs Zustände zeichnen sich durch einen extrem starken subtropischen Jet in der mittleren und obereren Modellschicht aus. <br><br> Da die Ursache des Regimeverhaltens des Dreischichtenmodells nach wie vor unklar war, wurde auf ein einfacheres Modell, nämlich ein barotropes Modell mit T21-Auflösung zurückgegriffen. Für die Anpassung des Oberflächenantriebs wurde eine modifizierte Form der iterativen Prozedur verwendet. Die zeitgemittelte Zirkulation des barotropen Modells stimmt sehr gut mit der zeitlich und vertikal gemittelten Zirkulation des Dreischichtenmodells überein. Das dominierende räumliche Muster der Variabilität besitzt eine AO-ähnliche Struktur. Zudem besitzt das barotrope Modell zwei Regime, die näherungsweise der positiven und negativen Phase der AO entsprechen und somit auch den Regimen des Dreischichtenmodells ähneln. Im Verlauf der Justierung des Oberflächenantriebs konnte beobachtet werden, dass die zwei Regime des barotropen Modells durch die Vereinigung zweier koexistierender Attraktoren entstanden. Der wahrscheinliche Mechanismus der Attraktorvereinigung ist eine Randkrise eines der beiden Attraktoren, gefolgt von einer explosiven Bifurkation des anderen Attraktors. <br><br> Es wird die Hypothese aufgestellt, dass der beim barotropen Modell vorgefundene Mechanismus der Regimeentstehung für atmosphärische Zirkulationsmodelle mit realitätsnahem Regimeverhalten Allgemeingültigkeit besitzt. Gestützt wird die Hypothese durch vier Experimente mit dem Dreischichtenmodell, bei denen jeweils der Parameter der Bodenreibung verringert und die Antriebsanpassung wiederholt wurde. Bei diesen Experimenten erhöhte sich die Persistenz und die Separiertheit der Regime bei abnehmender Reibung drastisch und damit auch der Anteil dekadischer Zeitskalen an der Variabilität. Die Zunahme der Persistenz der Regime ist charakteristisch für die Annäherung an eine inverse innere Krise, deren Existenz aber nicht nachgewiesen werden konnte. / Preferred quasi-stationary states of the planetary-scale atmospheric circulation, which may persist for one or several weeks, are referred to as atmospheric circulation regimes. Climate variations, either natural or anthropogenic, manifest themselves mainly in changes of the frequencies of occurrence of the natural regimes. <br><br> In the presented work, dynamical mechanisms of regime behavior and decadal climate variability of the atmosphere in absence of time-varying external forcing factors have been examined using a quasi-geostrophic three-level model of the wintertime atmospheric circulation over the northern hemisphere. This model has spectral T21 resolution, an orographic and a time-constant thermal forcing including non-zonal components. Such kind of a model is able to simulate large-scale extratropical atmospheric processes with reasonable accuracy. However, moisture processes, the interaction between the atmosphere and other parts of the climate system, and anthropogenic influences are not accounted for. <br><br> For the three-level model, a novel, automated, iterative procedure for the tuning of the thermal forcing has been developed. Every iteration of the procedure consists of a model test run, its evaluation, the comparison of the results with NCEP-NCAR reanalysis data for the winter months December, January, and February, and a forcing correction based on this comparison. After convergence of the procedure, the model matches the reanalysis data almost perfectly, as far as it concerns the zonal mean climate state and the time-mean non-zonal extratropical diabatic heating. <br><br> In a 1000-year simulation, the observed time-mean circulation in winter as well as its variability have been reproduced with considerable realism, in particular the Arctic Oscillation (AO) and its deep vertical extent. The modeled AO index exhibits pronounced decadal variations, exclusively caused by internal model dynamics. Furthermore, the model's regime behavior is in good agreement with observations. It possesses one regime resembling the negative phase of the North Atlantic Oscillation (NAO) and another resembling the positive phase of the AO. <br><br> A well-known hypothesis is the approximate correspondence between regimes and stationary solutions of the equations of motion. In the presented work, this hypothesis has been checked for the three-level model, but with negative result. Using a functional minimization method, six steady states have been found. All of them correspond to an extremely unrealistic circulation, and thus they are far away from the model's attractor. Five of the six steady states are characterized by a strongly exaggerated subtropical jet in the middle and upper model level. <br><br> As the origin of regime behavior was still unclear, a simpler model, namely a T21 barotropic model, has been reverted to. For the adaptation of the surface forcing, a modified version of the tuning procedure has been applied. The time-mean circulation of the barotropic model matches the temporally and vertically averaged circulation of the three-level model very well. The dominant spatial pattern of variability has an AO-like structure. Furthermore, the barotropic model possesses two regimes which approximately correspond to the positive and negative AO phase and therefore resemble the regimes of the three-level model. During the tuning of the surface forcing it has been observed that the two regimes of the barotropic model have emerged from the unification of two coexisting attractors. The mechanism responsible for this attractor merging is probably a boundary crisis of one of these attractors, followed by an explosive bifurcation of the other attractor. <br><br> It is hypothesized that the mechanism of regime genesis found in the barotropic model is universally valid for atmospheric circulation models with realistic regime behavior. This hypothesis is supported by four experiments with the three-level model, where the surface friction parameter has been decreased and the tuning procedure has been repeated, respectively. In these experiments, the persistence and separation of the regimes increases dramatically with decreasing friction, and thereby the fraction of decadal-scale variability. The increase of regime persistence is characteristic of approaching an inverse interior crisis, the existence of which, however, could not be proven.

Driven by many factors, the water supply services (streamflow and groundwater) of many rivers in the dryland area of China have declined significantly. This aggravates the inherent severe water shortages and results in increased severity in the water use conflicts that are threatening sustainable development in the region. Innovative strategies towards more water-efficient land management are vital for enhancing water quantity to ensure water supply security. A key step in the successful development and implementation of such measures is to understand the response of hydrological processes and related services to changes in land management and climate. To this end, it was decided to investigate these processes and responses in the upper reaches of the Jing River (Jinghe), an important meso-scale watershed in the middle reaches of the Yellow River on the Loess Plateau (NW China). It has been shown that vegetation restoration efforts (planting trees and grass) are effective in controlling soil erosion on the Loess Plateau. Shifts in land cover/use lead to modifications of soil physical properties. Yet, it remains unclear if the hydraulic properties have also been improved by vegetation restoration. A better understanding of how vegetation restoration alters soil structure and related soil hydraulic properties, such as water conductivity and soil water storage capacity, is necessary. Three adjacent sites, with comparable soil texture, soil type, and topography but contrasting land cover (Black locust forest, grassland, and cropland), were investigated in a small catchment in the upstream Jinghe watershed (near Jingchuan, Gansu province). Seasonal variations of soil hydraulic properties in topsoil and subsoil were examined. Results revealed that the type of land use had a significant impact on field-saturated, near-saturated hydraulic conductivity, and soil water characteristics. Specifically, conversion from cropland to grass or forests promotes infiltration capacity as a result of increased saturated hydraulic conductivity, air capacity, and macroporosity. Moreover, conversion from cropland to forest tends to promote the formation of mesopores that increase soil water storage capacity. Tillage in cropland temporarily created well-structured topsoil, but also compacted subsoil, as indicated by low subsoil saturated hydraulic conductivity, air capacity, and plant available water capacity. An impact of land cover conversion on unsaturated hydraulic conductivities was not identified, indicating that changes in land cover do not affect functional meso- and microporosity. Changes in soil hydraulic properties and associated hydrological processes and services due to soil conservation efforts need to be considered, should soil conservation measures be implemented in water-limited regions for sustaining adequate water supply. To differentiate between the impacts of land management and climate change on streamflow, the variation of annual streamflow, precipitation, potential evapotranspiration, and climatic water balance in a small catchment of the upstream Jinghe watershed (near Pingliang, Gansu province) was examined during the period of 1955 – 2004. During this time the relative contributions of changes in land management and climate to the reduction of streamflow were estimated. A statistically significant decreasing trend of -1.14 mm y-1 in annual streamflow was detected. Furthermore, an abrupt streamflow reduction due to afforestation and construction of terraces and check-dams was identified around 1980. Remarkably, 74% of the total reduction in mean annual streamflow can be attributed to the soil conservation measures. Among various conservation measures, streamflow could be considerably reduced by afforestation and terracing (including damland creation), due to their low contribution to water yield. In contrast, slope farmland and grassland can maintain a certain level of water supply services due to higher runoff coefficients. According to a meta-analysis of the published studies on the Loess Plateau, the impact of changes in land management on annual streamflow appears to diminish with increasing catchment size while the impact of climate change appears uniform across space. This means that there is a dependency between the catchment size and the response of hydrological processes to environmental change. At least at the local scale, it appears that well-considered land management may help to ensure the water supply services. Due to limited surface water availability, groundwater is an essential water source for supporting ecosystem and socio-economic development in the dryland region. However, the groundwater process is susceptible and vulnerable to changes in climate and landscape (i.e., land cover and form) that in turn can result in profound adverse consequences on water supply services in water-limited regions. In addition, an improved understanding of the response of groundwater related processes to natural and artificial disturbances is likely to ensure more secure and more sustainable governance and management of such regions, as well as better options for adapting to climate change. Yet, this topic has seldom been researched, especially in areas that have already experienced large-scale alteration in landscape and are located in dryland regions, such as the Loess Plateau. Therefore, an investigation of the baseflow variation along the landscape change was conducted. The average annual baseflow has significantly decreased at catchment scale during the period of 1962 – 2002 without any obvious significant change in climate. At decadal scale, the reduction accounts for approximately 9% in the 1970s, 48% in the 1980s, and 92% in the 1990s, while the baseflow index declines averaging 5%, 16% and 67%, respectively. All of the monthly baseflow levels dropped at varying rates except in January, among which July was the most severe in terms of both magnitude (-4.17) and slope (-0.09 mm y-1). In perspective of landscape change, landform change (terrace and check-dam) tends to reduce baseflow by reallocation of surface fluxes and retention for crop growth causing limited deep drainage in other areas. Land cover change (i.e., afforestation) reduced the baseflow to a larger extent by enhanced evapotranspiration and thus hampered deep drainage as suggested by the soil moisture measurement underneath. The study indicates that knowledge about baseflow formation on catchment scale needs further improvement. Integrated soil conservation and water management for optimizing landscape structure and function in order to balance soil (erosion) and water (supply) related hydrological ecosystem services is vital. The governing processes to the changes of water-supply-services-related hydrological process (e.g., streamflow) are assumed to be different across space. To this end, the factors controlling streamflow were investigated on both a small and large scale. Streamflow in small catchments was found to be mainly controlled by precipitation and land cover type. On a larger scale, evaporative demand was found to be another additional major driving force. Hydrological modeling is a frequently used tool for the assessment of impacts of land use and climate change on water balance and water fluxes. However, application of the Soil and Water Assessment Tool (SWAT) model in the upstream Jinghe watershed was unsuccessful due to difficulties in calibration. The inability of the SWAT model to take the influence of terraces on steep slopes into consideration and the method how to calculate lateral flow were the main reasons for unsatisfactory calibration, at least for the current version of SWAT used in this study. Alternatively, Budyko’s frameworks were applied to predict the annual and long-term streamflow. However, the effect of changes in land management (e.g., afforestation) on streamflow could not be assessed due to a lack of vegetation factors. Therefore, an empirical analysis tool was derived based on an existing relationship for estimation. This method was found to be the most effective in reproducing the annual and long-term streamflow. The incorporation of temporal changes in land cover and form in the approach enables the estimation of the possible impact of soil conservation measures (e.g., afforestation or terracing). The importance of adaptive land management strategies for mitigating water shortage and securing the water supply services on the Loess Plateau was highlighted. A cross-sectoral view of the multiple services offered by managed ecosystems at different spatial scales under changing environments needs to be integrated to improve adaptive land management policy. In a water limited environment, such as the Loess Plateau, multiple ecosystem services including hydrological services need to be balanced with minimum trade-offs. This can only be achieved when management is based on a holistic understanding of the interdependencies among various ecosystem services and how they might change under alternative land management.

Die Gletscherschmelze in den südlichen Anden trägt maßgeblich zum Anstieg des Meeresspiegels der letzten Jahrzehnte bei und beeinflusst regional die saisonale Wasserverfügbarkeit. In jüngster Zeit wurde eine rapide Zunahme der Massenverluste insbesondere einzelner großer Auslassgletscher des Südlichen Patagonisches Eisfeldes beobachtet. Im Rahmen der Dissertation wurden die rezente Variabilität des Klimas und der klimatischen Massenbilanz für ausgewählte vergletscherte Gebiete in Patagonien und Feuerland untersucht. Die Verbesserung unseres Verständnisses über räumliche und zeitliche Muster der klimatischen Massenbilanz, ihrer atmosphärischen Antriebsfaktoren und ihres Einflusses auf das in jüngster Vergangenheit beobachtete individuelle Gletscherverhalten, sind weitere wichtige Ziele. Da die Klimavariabilität die Hauptursache für lokale Veränderungen in der Kryosphäre der südlichen Anden ist, wurden langjährige meteorologische Beobachtungen im Gebiet der Gran Campo Nevado-Eiskappe im südlichsten Patagonien im Hinblick auf räumliche und zeitliche Variabilität untersucht und der Einfluss mesoskaliger Wettermuster und Modi atmosphärischer Oszillationen auf die Ausprägung des Klimas analysiert. Darüber hinaus wurde die rezente Variabilität der klimatischen Massenbilanz für ausgewählte Gletscher in Südpatagonien und Feuerland durch die Implementierung des Energie- und Massenbilanzmodells COSIMA simuliert. Eine unterschiedliche Ausprägung der Oberflächenmassenbilanz und geodätischer Massenbilanz unterstreicht wie wichtig ein besseres Verständnis über die Prozesse der klimatischen Massenbilanz und Eisdynamik ist. Des Weiteren wurden Simulationen der klimatischen Massenbilanz eingesetzt, um eine ausgeglichene Massenbilanz für rezente und vergangene Ausdehnungen des Gletschers Schiaparelli abzuleiten. Ziel war es, eine modellgestützte Annäherung an die klimatischen Bedingungen während der Kleinen Eiszeit zu simulieren. / Glacier mass loss of the Southern Andes contributes largely to sea-level rise during recent decades and also affects the regional water availability. Despite the overall glacier retreat of most glaciers in Patagonia and Tierra del Fuego, a recent increase in mass loss of individual glaciers has been observed. The recent variability of climate and climatic mass balance for selected glaciated study sites in Patagonia and Tierra del Fuego are investigated in this thesis. Improving our understanding on the spatial and temporal variations of climatic mass balance processes, its atmospheric drivers, and their impact on the recently observed individual glacier behavior are further important aims. Since climate variability is the key driver of local changes in the cryosphere in the Southern Andes, a unique record of meteorological observations across the Gran Campo Nevado Ice Cap in Southernmost Patagonia was analyzed with regard to main climate features and the relationship between the in-situ observations, large-scale climate modes and mesoscale weather patterns. Furthermore, recent climatic mass balance variability was simulated for selected glaciers in Southern Patagonia and Tierra del Fuego by implementing the ’COupled Snow and Ice energy and MAss balance model’ COSIMA. Contrasting patterns of positive simulated annual climatic mass balance and clearly negative geodetic mass balance were found for two neighboring glaciers of the Southern Patagonia Icefield between 2000 and 2014. This highlights the importance of understanding of both, the climatic mass balance, and the ice-dynamical processes. Climatic mass balance simulations were further used to derive glacier steady-state conditions for recent and past glacier extents of Schiaparelli Glacier, aiming for a model-based approximation of climate conditions during the Little Ice Age.

Seit der industriellen Revolution haben Menschen durch Verbrennung von fossilen Energieträgern die Treibhausgaskonzentration in der Atmosphäre erhöht. Die daraus folgende Erderwärmung hat weitreichende Folgen für das Klima, unter anderem häufigere und intensivere Wetterextreme. Wegen ihrer gravierenden Auswirkungen auf die Gesellschaft, ist es von allgemeinem Interesse zu verstehen, wie der menschengemachte Klimawandel diese Wetterextreme beeinflusst. In dieser kumulativen Dissertation analysiere ich erst zwei komplexe Wettereignisse, die die Nahrungsmittelproduktion in Europa beeinträchtigen: Frosttage nach dem Beginn der Apfelblüte und Feuchte Frühsommerperioden nach warmen Wintern. In einer dritten Studie untersuche ich wie dynamische Klimaveränderungen in den mittleren Breiten der Nordhalbkugel zu beständigerem Sommerwetter beitragen. Schließlich beschäftige ich mich mit tropischen Stürmen im Nordatlantik und damit, wie sie von der globalen Erwärmung beeinflusst werden. Eine zentrale methodische Herausforderung in diesem Forschungsfeld ist, dass Wetterextreme per Definition selten sind und dass es aufgrund der starken internen Klimavariabilität schwierig ist, die Veränderungen zu quantifizieren, die auf den menschgemachten Klimawandel zurück zu führen sind. In dieser Arbeit verfolge ich zweigegenläufige Ansätze um mit dieser Herausforderung um zu gehen: 1) Ich verwende große Klimasimulationsensembles um den Effekt der internen Klimavariabilität aus zu glätten und dadurch die erzwungenen Veränderungen beim Apfelfrost und in der Persistenz zu ergründen. 2) Mit Methoden, die auf Beobachtungsdaten beruhen, quantifiziere ich den Einfluss der internen Klimavariabilität auf tropische Zyklone um dann einschätzen zu können, in welchem Maß der beobachtete Anstieg der tropischen Zyklonaktivität im Atlantik der internen Klimavariabilität oder erzwungenen Veränderungen zugeschrieben werden kann. / Since the industrial revolution, humans have increased the greenhouse gas concentration of the atmosphere by burning fossil fuels. The resulting global warming has far reaching impacts on the climate system including increasingly frequent and intense weather extremes. Due to the severe impacts these weather extremes cause to societies, there is a strong interest in understanding how anthropogenic climate change affects weather extremes. In this cumulative thesis I first study two compound weather extremes that affect food production in Europe: frost days after apple blossom and wet early summers after warm winters. In a third study I quantify how dynamic changes in the climate system contribute to more persistent summer weather extremes in the northern hemispheric mid-latitudes. Finally, I analyze tropical cyclones in the Atlantic basin and changes in tropical cyclone activity as a result of global warming. One central methodological challenge in the research field is that weather extremes are rare by definition and that due to the strong internal climate variability it is difficult to quantify changes that are forced by anthropogenic climate change. In this thesis I explore two divergent approaches to this challenge: 1) Using large ensemble climate simulations I smooth out the effect of internal variability thereby exposing the forced change in apple frost and weather persistence. 2) Using observation based approaches, I quantify the contributions of internal climate variability on tropical cyclones in order to subsequently estimate to which extent the observed increase in tropical cyclone activity in the Atlantic can be attributed to internal climate variability or forced changes.

Study region Nyangores River watershed, headwater catchment of Mara River basin in Kenya. Study focus Climate variability and human activities are the main drivers of change of watershed hydrology. The contribution of climate variability and land use change to change in streamflow of Nyangores River, was investigated. Mann Kendall and sequential Mann Kendall tests were used to investigate the presence and breakpoint of a trend in discharge data (1965–2007) respectively. The Budyko framework was used to separate the respective contribution of drivers to change in discharge. Future response of the watershed to climate change was predicted using the runoff sensitivity equation developed. New hydrological insights for the region There was a significant increasing trend in the discharge with a breakpoint in 1977. Land use change was found to be the main driver of change in discharge accounting for 97.5% of the change. Climate variability only caused a net increase of the remaining 2.5% of the change; which was caused by counter impacts on discharge of increase in rainfall (increased discharge by 24%) and increase in potential evapotranspiration (decreased discharge by 21.5%). Climate change was predicted to cause a moderate 16% and 15% increase in streamflow in the next 20 and 50 years respectively. Change in discharge was specifically attributed to deforestation at the headwaters of the watershed.

Study region Nyangores River watershed, headwater catchment of Mara River basin in Kenya. Study focus Climate variability and human activities are the main drivers of change of watershed hydrology. The contribution of climate variability and land use change to change in streamflow of Nyangores River, was investigated. Mann Kendall and sequential Mann Kendall tests were used to investigate the presence and breakpoint of a trend in discharge data (1965–2007) respectively. The Budyko framework was used to separate the respective contribution of drivers to change in discharge. Future response of the watershed to climate change was predicted using the runoff sensitivity equation developed. New hydrological insights for the region There was a significant increasing trend in the discharge with a breakpoint in 1977. Land use change was found to be the main driver of change in discharge accounting for 97.5% of the change. Climate variability only caused a net increase of the remaining 2.5% of the change; which was caused by counter impacts on discharge of increase in rainfall (increased discharge by 24%) and increase in potential evapotranspiration (decreased discharge by 21.5%). Climate change was predicted to cause a moderate 16% and 15% increase in streamflow in the next 20 and 50 years respectively. Change in discharge was specifically attributed to deforestation at the headwaters of the watershed.