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Dive into the research topics where Jürgen Jensen is active.

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Featured researches published by Jürgen Jensen.


Geophysical Research Letters | 2014

Evidence for long‐term memory in sea level

Sönke Dangendorf; Diego Rybski; Christoph Mudersbach; Alfred Müller; Edgar Kaufmann; Eduardo Zorita; Jürgen Jensen

Detection and attribution of anthropogenic climate change signals in sea level rise (SLR) has experienced considerable attention during the last decades. Here we provide evidence that superimposed on any possible anthropogenic trend there is a significant amount of natural decadal and multidecadal variability. Using a set of 60 centennial tide gauge records and an ocean reanalysis, we find that sea levels exhibit long-term correlations on time scales up to several decades that are independent of any systematic rise. A large fraction of this long-term variability is related to the steric component of sea level, but we also find long-term correlations in current estimates of mass loss from glaciers and ice caps. These findings suggest that (i) recent attempts to detect a significant acceleration in regional SLR might underestimate the impact of natural variability and (ii) any future regional SLR threshold might be exceeded earlier/later than from anthropogenic change alone.


Journal of Geophysical Research | 2014

Mean sea level variability in the North Sea: Processes and implications

Sönke Dangendorf; Francisco M. Calafat; Arne Arns; Thomas Wahl; Ivan D. Haigh; Jürgen Jensen

Mean sea level (MSL) variations across a range of time scales are examined for the North Sea under the consideration of different forcing factors since the late 19th century. We use multiple linear regression models, which are validated for the second half of the 20th century against the output of a tide+surge model, to determine the barotropic response of the ocean to fluctuations in atmospheric forcing. We find that local atmospheric forcing mainly initiates MSL variability on time scales up to a few years, with the inverted barometric effect dominating the variability along the UK and Norwegian coastlines and wind controlling the MSL variability in the south from Belgium up to Denmark. On decadal time scales, MSL variability mainly reflects steric changes, which are largely forced remotely. A spatial correlation analysis of altimetry observations and gridded steric heights suggests evidence for a coherent signal extending from the Norwegian shelf down to the Canary Islands. This fits with the theory of longshore wind forcing along the eastern boundary of the North Atlantic causing coastally trapped waves to propagate over thousands of kilometers along the continental slope. Implications of these findings are assessed with statistical Monte-Carlo experiments. It is demonstrated that the removal of known variability increases the signal to noise ratio with the result that: (i) linear trends can be estimated more accurately; (ii) possible accelerations (as expected, e.g., due to anthropogenic climate change) can be detected much earlier. Such information is of crucial importance for anticipatory coastal management, engineering, and planning.


Nature Communications | 2015

Detecting anthropogenic footprints in sea level rise

Sönke Dangendorf; Marta Marcos; Alfred Müller; Eduardo Zorita; Riccardo E. M. Riva; Kevin Berk; Jürgen Jensen

While there is scientific consensus that global and local mean sea level (GMSL and LMSL) has risen since the late nineteenth century, the relative contribution of natural and anthropogenic forcing remains unclear. Here we provide a probabilistic upper range of long-term persistent natural GMSL/LMSL variability (P=0.99), which in turn, determines the minimum/maximum anthropogenic contribution since 1900. To account for different spectral characteristics of various contributing processes, we separate LMSL into two components: a slowly varying volumetric component and a more rapidly changing atmospheric component. We find that the persistence of slow natural volumetric changes is underestimated in records where transient atmospheric processes dominate the spectrum. This leads to a local underestimation of possible natural trends of up to ∼1 mm per year erroneously enhancing the significance of anthropogenic footprints. The GMSL, however, remains unaffected by such biases. On the basis of a model assessment of the separate components, we conclude that it is virtually certain (P=0.99) that at least 45% of the observed increase in GMSL is of anthropogenic origin.


Ocean Dynamics | 2015

Determining return water levels at ungauged coastal sites: a case study for northern Germany

Arne Arns; Thomas Wahl; Ivan D. Haigh; Jürgen Jensen

We estimate return periods and levels of extreme still water levels for the highly vulnerable and historically and culturally important small marsh islands known as the Halligen, located in the Wadden Sea offshore of the coast of northern Germany. This is a challenging task as only few water level records are available for this region, and they are currently too short to apply traditional extreme value analysis methods. Therefore, we use the Regional Frequency Analysis (RFA) approach. This originates from hydrology but has been used before in several coastal studies and is also currently applied by the local federal administration responsible for coastal protection in the study area. The RFA enables us to indirectly estimate return levels by transferring hydrological information from gauged to related ungauged sites. Our analyses highlight that this methodology has some drawbacks and may over- or underestimate return levels compared to direct analyses using station data. To overcome these issues, we present an alternative approach, combining numerical and statistical models. First, we produced a numerical multidecadal model hindcast of water levels for the entire North Sea. Predicted water levels from the hindcast are bias corrected using the information from the available tide gauge records. Hence, the simulated water levels agree well with the measured water levels at gauged sites. The bias correction is then interpolated spatially to obtain correction functions for the simulated water levels at each coastal and island model grid point in the study area. Using a recommended procedure to conduct extreme value analyses from a companion study, return water levels suitable for coastal infrastructure design are estimated continuously along the entire coastline of the study area, including the offshore islands. A similar methodology can be applied in other regions of the world where tide gauge observations are sparse.


Coastal Engineering Journal | 2015

XtremRisK — integrated flood risk analysis for extreme storm surges at open coasts and in estuaries: methodology, key results and lessons learned

Hocine Oumeraci; Andreas Kortenhaus; Andreas Burzel; M. Naulin; Dilani R. Dassanayake; Jürgen Jensen; Thomas Wahl; Christoph Mudersbach; G. Gönnert; B. Gerkensmeier; P. Fröhle; G. Ujeyl

A brief overview of the joint research project XtremRisK is given. The project has been focusing on developing/improving/expanding the knowledge, methods and models with respect to (i) physically possible extreme storm surge for current conditions and scenarios for climate change, (ii) failure mechanisms of flood defenses, (iii) assessment of intangible losses (social and ecological) and their integration with direct/indirect economic losses, (iv) reliability analysis of flood defense systems and (v) source- pathway-receptor (SPR)-based integrated flood risk analysis involving both tangible and intangible losses and its implementation for two selected pilot sites (representative for an open coast and an urban estuarine area in Germany). The key results are briefly summarized and the lessons learned for future flood risk studies are finally drawn.


Archive | 2016

The Seasonal Mean Sea Level Cycle in the Southeastern North Sea

Sönke Dangendorf; Thomas Wahl; Christoph Mudersbach; Jürgen Jensen

ABSTRACT Dangendorf, S., Wahl, T., Mudersbach, C. and Jensen, J., 2013. The Seasonal Mean Sea Level Cycle in the Southeastern North Sea The seasonal cycle is a prominent feature in Mean Sea Level (MSL) time series with considerable influences on flood risk in coastal areas. When analyzing MSL it is often assumed that the seasonal cycle is a stationary process, independent from inter-annual variations, but there is no obvious reason for such an assumption. In this paper the seasonal cycle of MSL at 13 tide gauges in the German Bight is investigated for its average character as well as its time dependence over the past 166 years. A seasonal trend decomposition method is used to analyze the inter-annual fluctuations in amplitudes and phases of the seasonal cycle. In the German Bight the seasonal cycle accounts for up to 44 % of the observed monthly MSL variability. The mean seasonal cycle peaks during November at all stations. The mean amplitude varies between 14 and 20 cm and increases from the south-western to the north-eastern stations. Throughout the last 166 years it is found that the amplitudes as well as the phases of the seasonal cycle are marked by a considerable inter-annual variability. While most records, all starting in the 1930s or later, do not exhibit a significant trend the longest record at Cuxhaven displays a significant long-term trend of 0.2 ± 0.1 mm/yr. This trend is mainly caused by large values at the end of the 1970s and the beginning 1980s. Simultaneously, the annual peaks shift from the late autumn to winter months (December to February). These changes are caused by extraordinary large trends during the months from January to March, exceeding those in the remaining months by up to 4 mm/yr. These changes are in phase with an intensification of large-scale atmospheric circulation patterns over the North Atlantic bringing more frequent westerly winds over the North Sea.


Coastal Engineering Journal | 2015

Statistical Assessment of Storm Surge Scenarios Within Integrated Risk Analyses

Thomas Wahl; Christoph Mudersbach; Jürgen Jensen

This paper summarizes the results from calculating exceedance probabilities of different storm surge scenarios developed within the XtremRisK project, which were then used as boundary conditions for integrated risk analyses for the city of Hamburg in the Elbe Estuary and Sylt Island off the coastline of Schleswig-Holstein in northern Germany. A stochastic storm surge model is developed to simulate a large number of synthetic and physically consistent storm surge scenarios, before the resulting samples are used to calculate bivariate joint exceedance probabilities of the storm surge heights (total water levels with tides included) and intensities. The Copula theory is exploited and functions from the Archimedean family are used to build the statistical models. The latter are extended to the three-dimensional case to also take into account wave conditions where appropriate. The uncertainties associated with the results from the multivariate extreme value analyses are briefly discussed and (where possible) quantified and recommendations of how to exploit the presented methodologies in future applications are given.


Geomatics, Natural Hazards and Risk | 2016

A simplified early-warning system for imminent landslide prediction based on failure index fragility curves developed through numerical analysis

Ugur Ozturk; Yalelet Alemnew Tarakegn; Laura Longoni; Davide Brambilla; Monica Papini; Jürgen Jensen

Early-warning systems (EWSs) are crucial to reduce the risk of landslide, especially where the structural measures are not fully capable of preventing the devastating impact of such an event. Furthermore, designing and successfully implementing a complete landslide EWS is a highly complex task. The main technical challenges are linked to the definition of heterogeneous material properties (geotechnical and geomechanical parameters) as well as a variety of the triggering factors. In addition, real-time data processing creates a significant complexity, since data collection and numerical models for risk assessment are time consuming tasks. Therefore, uncertainties in the physical properties of a landslide together with the data management represent the two crucial deficiencies in an efficient landslide EWS. Within this study the application is explored of the concept of fragility curves to landslides; fragility curves are widely used to simulate systems response to natural hazards, i.e. floods or earthquakes. The application of fragility curves to landslide risk assessment is believed to simplify emergency risk assessment; even though it cannot substitute detailed analysis during peace-time. A simplified risk assessment technique can remove some of the unclear features and decrease data processing time. The method is based on synthetic samples which are used to define the approximate failure thresholds for landslides, taking into account the materials and the piezometric levels. The results are presented in charts. The method presented in this paper, which is called failure index fragility curve (FIFC), allows assessment of the actual real-time risk in a case study that is based on the most appropriate FIFC. The application of an FIFC to a real case is presented as an example. This method to assess the landslide risk is another step towards a more integrated dynamic approach to a potential landslide prevention system. Even if it does not define absolute thresholds, the accuracy is satisfactory for a preliminary risk assessment, and it can provide more lead-time to understand the hazard level in order to make decisions as compared with a more sophisticated numerical approach. Hence, the method is promising to become an effective tool during landslide emergency.


Archive | 2016

Inter-annual and long-term mean sea level changes along the North Sea coastline

Thomas Wahl; Ivan D. Haigh; Sönke Dangendorf; Jürgen Jensen

ABSTRACT Wahl, T., Haigh, I.D., Dangendorf, S., and Jensen, J., 2013. Inter-annual and long-term mean sea level changes along the North Sea coastline. Globally, mean sea levels are rising and there is concern that the rate of rise will accelerate throughout the 21st century significantly impacting growing coastal communities. Currently, most coastal management assessments are based on global mean sea level projections from the Intergovernmental Panel on Climate Changes Fourth Assessment Report. However, temporal and spatial variability of mean sea level change needs to be identified in order to establish an accurate projection of coastal management needs due to erosion and flooding associated with global sea level rise. This paper assesses historic changes in mean sea level from the beginning of the 19th century to present using 30 long and high quality tide gauge records located around the coastline of the North Sea. The North Sea coast is one of the most densely populated coastlines in the world. It contains a significant proportion of Europes coastal flood risk as exemplified by London, Amsterdam and Hamburg, and the other extensive lowlands, and has a long history of significant coastal flooding. Previous analyses of mean sea level changes along Europes coastlines have tended to be conducted at a national level, using a variety of different methods and sea level records of different quality and length. This study has three main objectives: (1) to examine the inter-annual variations observed in mean sea level across the North Sea region; (2) to examine linear and non-linear longer-term trends in relative and absolute mean sea level from the beginning of the 19th century to present; and (3) to assess whether 19 years of altimetry data provide valuable information on the spatial patterns of sea level trends and inter-annual variability in the North Sea.


Hydrological Sciences Journal-journal Des Sciences Hydrologiques | 2016

A multivariate design framework for river confluences

Jens Bender; Thomas Wahl; Alfred Müller; Jürgen Jensen

ABSTRACT Throughout the last decade copula functions were widely used to assess a wide range of hydrological problems, often focusing on two distinct variables. In many of these studies it was ignored whether the two variables of interest actually occurred simultaneously (e.g. two annual maximum time series were analysed in a multivariate statistical framework). Here we introduce a novel approach to derive bivariate design events using copula functions allowing both simultaneous and non-simultaneous occurrence of the variables to be modelled. The methodology is exemplarily applied to assess the combined flood occurrence at the confluence of the rivers Rhine and Sieg (Germany). The results underline the validity of the methodology. Employing a hydrodynamic numerical model furthermore shows that commonly used statistical approaches to select a single design event out of a vast number of possible combinations can be critical for practical design purposes. Editor Z.W. Kundzewicz; Associate editor S. Grimaldi

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Thomas Wahl

University of South Florida St. Petersburg

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Andreas Burzel

Braunschweig University of Technology

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Hocine Oumeraci

Braunschweig University of Technology

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