Prange, Matthias
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Prange, Matthias
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Prange, Matthias
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Item-typ:Veröffentlichung, North Atlantic climate variability in a coupled climate model : Multidecadal cold events and climate state transitions(2018-11-23); ; ; The main focus of this study is to investigate the climate variability in the North Atlantic region on timescales ranging from centuries to millennia to better understand the origin and dynamics of these fluctuations. A focus is set on analyzing multidecadal cold events and climate transitions from one climate state to another. These climate events and shifts can have severe consequences ranging from temperature anomalies, an expansion of sea-ice cover, and social and economical impacts. Although many studies deal with climate state transitions, the trigger of these transitions often remains unclear. Important factors are the stability of the background climate, external forcings, or internal forcings either by a natural source or due to anthropogenic impact. All these factors could lead the climate system to a lower stability and further to switch to another climate state. In the North Atlantic region, where deep-water is formed, a climate transition can lead to a shutdown of the Atlantic Meridional Overturning Circulation, which transports warm and relatively salty water northwards to the polar regions and relatively cold and fresh water southwards.Dissertation631 171 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Interglacial climate variability during MIS 15 to Holocene Insight from Coupled climate modelling(2016-09-15); ; ; Understanding the mechanisms and effects of natural long-term climate variability is essential for providing projections of possible climate change for the near future. This study examines the mechanisms of the climate variability over the time frame of the past 600 kyr using CCSM3-DGVM (Community Climate System Model version 3 with Dynamic Global Vegetation Model). A set of 13 interglacial time slice experiments was carried out to study global climate variability between and within the Quaternary interglacials of Marine Isotope Stages (MISs) 1, 5, 11, 13, and 15. Here, this study focuses on the effect of different roles of obliquity, precession and greenhouse gases (GHG) forcing on global surface temperature and precipitation patterns. Local insolation anomalies induced by the astronomical forcing play a role in most regions of seasonal surface temperature anomalies. Climate feedbacks, however, may modify the surface temperature response in specific regions, most pronounced in the monsoon domains and the polar oceans. Especially in high latitudes and early Brunhes interglacials (MIS 13 and 15) when GHG concentrations were much lower than during the later interglacials, GHG forcing may also play an important role for seasonal temperature anomalies. During boreal summer, high-versus-low obliquity climates are generally characterized by strong warming over the Northern Hemisphere extratropics and slight cooling in the tropics. A moderate cooling over large portions of the Northern Hemisphere continents and a strong warming at high southern latitudes during winter is found. Additionally, a significant role of obliquity in forcing the West African monsoon is identified. In this case, other regional monsoon systems are less sensitive or not sensitive at all to obliquity variations during interglacials. Based on two specific time slices (394 and 615 ka), the model results suggest that the West African and Indian monsoon systems do not always vary in concert, challenging the concept of a global monsoon system at orbital timescales. Furthermore, GHG forcing is positively correlated with surface temperature over most regions of the globe in the annual mean and GHG radiative forcing exhibits no clear response in annual and seasonal precipitation during the interglacials except for the high latitudes in both hemispheres during annual, for southern high latitudes during summer, and for northern high latitudes during winter where the hydrologic cycle accelerates with higher GHG concentrations. In order to disentangle the impact of dynamic vegetation on the early (9 ka) and mid- Holocene (6 ka) North African climate, experiments with the dynamic and fixed-vegetation were carried out. In this study, the coupled model simulates enhanced summer rainfall and a northward migration of the West African monsoon trough along with an expansion of the vegetation cover for the early and middle Holocene compared to pre-industrial. With dynamic vegetation, the orbitally triggered summer precipitation anomaly is enhanced by approximately 20% in the Sahara/Sahel region (10a 25AdegreeN,20AdegreeWa 30Adegree E) in both the early and mid-Holocene experiments compared to their fixed-vegetation counterparts. The primary vegetation-rainfall feedback identified here operates through surface latent heat flux anomalies by canopy evaporation and transpiration and their effect on the mid-tropospheric African Easterly Jet, whereas the effects of vegetation changes on surface albedo and local water recycling play a negligible role. Furthermore, this study constrains a three-dimensional thermomechanical-ice model Genie Land Ice Model with Multiple-Enabled Regions (GLIMMER) forced by CCSM3 climate model output for MIS 5 and MIS 11 time slices to simulate a sensitivity of Greenland ice sheet (GrIS). The GrIS is thought to have contributed substantially to high global sea levels during the interglacials of MIS 5 and MIS 11. Geological evidence suggests that the mass loss of the GrIS was similar or even greater during the interglacial of MIS 11 than MIS 5, despite a weaker insolation forcing. This study shows a stronger sensitivity of the GrIS to MIS 11 climate forcing than to MIS 5 forcing. The greater MIS 11 GrIS mass loss relative to MIS 5 is attributed to a larger heat transport towards high latitudes by a stronger Atlantic meridional ocean circulation in addition to a stronger GHG radiative forcing. The results, however, suggest a substantial modification of orbital insolation forcing by internal climate feedbacks, which add significant complexity to the traditional Milankovitch theory.Dissertation373 117 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Global climate and Indonesian Throughflow during the Middle Miocene Climate Transition : a modeling approach(2019-07-10); ; ; The current study focuses on the Middle Miocene Climate Transition (MMCT), a main global cooling step during the Cenozoic (66-0 Ma). This transition, likely triggered by changes in the Earth's orbital configuration and a decrease in atmospheric CO2 concentration, entailed major expansion of the Antarctic ice sheet, cooling of the surface and deep ocean, and global eustatic sea level fall during the interval 15-13 Ma. A central aim of this study is to assess, by means of the global coupled model Community Climate System Model version 3 (CCSM3), the ocean response to atmospheric CO2 dropdown and Antarctic ice sheet expansion during the MMCT. In particular, it is investigated whether the combined effects of the CO2 decrease and Antarctic ice sheet expansion could explain the cooling of surface and deep waters across the MMCT inferred from proxy data, the separate effects of these two forcings on surface and deep water temperatures, and the mechanisms these forcings were triggering that explain their modeled effects on ocean temperatures. Ocean gateways are relatively narrow channels of water separating two main ocean basins. Changes in the bathymetry of ocean gateways alter the water properties of the basins they connect and this can have regional to global scale climate effects. An example of ocean gateway is the Indonesian gateway, the tropical passage connecting the Pacific and Indian oceans, which has a significant influence on the climatic states of those oceans. The origin of the West Pacific Warm Pool, for example, the most extensive warm surface water mass on Earth, has been suggested to be linked to narrowing of the Indonesian gateway. A further aim of this study is that of modeling the characteristics of the Indonesian Throughflow during the MMCT by means of CCSM3, providing estimates of volume transport, analyzing the vertical structure of the waterflow, the direction of waterpaths, the relative contributions of North and South Pacific water to the Indonesian Throughflow, the control mechanisms of its seasonal variability, and examining whether a similar structure to the present-day West Pacific Warm Pool existed during the MMCT in the Indonesian Throughflow area. The model boundary conditions employed in our experiments include Middle Miocene global vegetation, topography, and bathymetry - comprising a geographic reconstruction of Southeast Asia for 15 Ma - as well as Antarctic ice sheet configurations, sea levels, and atmospheric CO2 concentrations characterizing the periods before (Middle Miocene Climatic Optimum) and after the transition (Middle Miocene Glaciation).Dissertation741 269 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Hydroclimate variations in the Caspian Sea region from the late Quaternary to the future : a model perspective(2019-09-10); ; ; The northern hemispheric glacial-interglacial climate states during the late Quaternary period drove Caspian Sea level (CSL) changes of up to 150 m and can be used as an analogue for assessing present and future climate impacts. Geologic reconstructions of these paleo-lake levels and potential links with different climate events remain complex while future declines in modelled lake levels vary widely and are uncertain. The key drivers for such CSL include variations in the water budget balance between precipitation and evaporation (P-E). This thesis employs a climate modeling approach to investigate long-term changes in the Caspian Sea (CS) hydroclimate during different climate states from the late Quaternary to the end of the 21st century. The new results from the Community Earth System Model (CESM1.2.2), contribute to an improved interpretation of reconstructed paleo-lake levels with respect to changing P-E patterns and identify key drivers for future CSL changes. This study produced new modeling results for the late Quaternary period, that constrain the timings and identify the climate conditions favourable for major CS transgressions and regressions, in comparison with selected geological reconstructions; under three glacial (Marine Isotope 3 (MIS3), Last Glacial Maximum (LGM), Heinrich event 1 (H1)) and two interglacial (last interglacial (LIG) and early Holocene (EH)) climate states. The two interglacial climate states suggest favourable climate conditions (higher temperature and precipitation) for the CS that result in a positive water budget (LIG-P-E anomalies of 14.6 meter/1000 years and the EH-P-E anomalies 5 meter/1000yr). The results propose a transgression was initiated by the summer large-scale and convective precipitation, triggered by enhanced summer insolation and the associated wind anomalies. The warmer and wetter MIS3 interstadial climate is identified as being responsible for a transgression with P-E anomalies of 16 meter/1000yr. The colder and drier LGM climate favours a regression with P-E of -12 meter/ 1000yr. These P-E anomalies and climate conditions agree with the reconstructions. However, our simulated P-E anomalies (for the H1); do not capture the magnitude of the reconstructed highstands during the deglaciation, and it is clear that meltwater into the CS is responsible for this highstand; and as our model does not include a sophisticated meltwater routing into the CS; hence comparisons with selected reconstructions remain complicated. This study also assessed different CESM horizontal resolutions and model setups to identify the best version that can represent the CS climate and climate modes of variability such as North Atlantic Oscillation (NAO) for the period 1850-2000 CE, as well as presenting new CSL under two new emission scenarios by the end of the 21st century. CESM1.2.2 with 1AAAdegree CAM5 is identified as the best skill in simulating the NAO and its effects on CS catchment hydrology. Projections under the Representative Concentration Pathways RCP4.5 and RCP8.5 confirm the winter NAO remains the major winter variability with a significant impact on the Caspian catchment hydroclimate. However, under global warming, the evaporation over the sea is the key driver for a CSL decrease of about 9 m and 18 m between 2020 and 2100 for the RCP4.5 and RCP8.5 scenarios, respectively. The new CSL values are larger than previous projections of CSL, and include an overestimated total evaporation due to a larger CS surface area in CESM. Despite the clear potential for this, current global climate models neglect to include accurate representations of CS area. This study generated new results to notably aid in evaluating the impacts of different CS surface areas on the regional and large-scale climate. Regionally, the presence of a larger CS area has a clear impact as higher evaporation over the sea and higher precipitation over the south-west catchment, while reducing (summer) and increasing (winter) surface air temperatures and vice versa for smaller CS area. Most importantly, this summer temperature disturbs the upper atmospheric circulation (at the 200 hPa and the 500 hPa level) with a southward shift and increase in speed of the summer jet stream. This leads to enhanced summer precipitation over central Asia and increased winter warming over the north-western Pacific. An accurate Caspian Sea area representation is thus vital in global climate models for paleo and future scenarios and share serious implications for expanding coastal communities, agricultural activities, fisheries and the ecosystem.Dissertation684 182