Papers by Frederik Schenk
Regional Climate Studies, 2015
This chapter describes observed changes in atmospheric conditions in the Baltic Sea drainage basi... more This chapter describes observed changes in atmospheric conditions in the Baltic Sea drainage basin over the past 200-300 years. The Baltic Sea area is relatively unique with a dense observational network covering an extended time period. Data analysis covers an early period with sparse and relatively uncertain measurements, a period with well-developed synoptic stations, and a final period with 30+ years of satellite data and sounding systems. The atmospheric circulation in the European/Atlantic sector has an important role in the regional climate of the Baltic Sea basin, especially the North Atlantic Oscillation. Warming has been observed, particularly in spring, and has been stronger in the northern regions. There has been a northward shift in storm tracks, as well as increased cyclonic activity in recent decades and an increased persistence of weather types. There are no long-term trends in annual wind statistics since the nineteenth century, but much variation at the (multi-)decadal timescale. There are also no long-term trends in precipitation, but an indication of longer precipitation periods and possibly an increased risk of extreme precipitation events.
ABSTRACT The North Atlantic Oscillation (NAO) is related to the near-surface climate of the Balti... more ABSTRACT The North Atlantic Oscillation (NAO) is related to the near-surface climate of the Baltic Sea area in winter, although the correlations undergo significant non-stationarities on longer time scales. For example the Maximum Ice extent of the Baltic Sea (MIB) from 1720 to 1999 correlates well (30%) with the reconstructed winter-NAO, but shows 30 year running correlations between 16% and 64%. These non-stationarities can be found for the NAO and monthly mean sea-level of Baltic gauges and historical station temperatures for shorter time scales in observations, and at longer time scales in different GCM simulations with and without external forcing. This poses an additional problem for reconstructions of circulation indices from near-surface proxies. For historical time scales, synoptical evidences derived from historical documents can reveal important additional information besides the sole statistical approaches. A detailed study of the long-term vs. short-term climate variability of the so called "Little Ice Age" (LIA, 1550- 1900 A.D.) reveals a very high year-to-year variability of the NAO and the near-surface climate of the Baltic with some pronounced clustering in time. As human affairs have been strongly influenced by short-term climate variability in these periods, we can show a strong relationship between societal developments and high short-term climate variability. The unpredictability and strong fluctuations of the seasonal conditions exert a high pressure on the agricultural society. Combining new evidences from historical documents with reconstructions of the NAO and its near-surface impacts, the high short-term variability is found to be a typical feature of the LIA, and seems to be mainly caused by the dynamics of a mostly negative mode of the NAO compared to the 20th century, which leads to more meridional circulation with more extreme fluctuations. This is also supported by other studies on Andalusian winter rainfall or drought severity-indices in Morocco since 1000 A.D., which have been significantly wetter than the 20th century due to a weak Azores High and negative values of the NAO index.
ABSTRACT Due to the shortness and/or the inhomogeneities contained in direct wind observations, e... more ABSTRACT Due to the shortness and/or the inhomogeneities contained in direct wind observations, evidence for changing storminess is mostly based on indirect data or reanalysis products. While reanalysis products generally show upward trends in high annual wind speeds for the last 40-60 years, direct local wind observations show different trends for Northern Europe dependent on the location and the considered time period. Independent from discrepancies among direct wind observations or reanalysis, the question whether storminess has indeed changed over longer timescales over the NE Atlantic and NW-Europe has been examined by multiple studies coming to different results. Reconstructions of annual storminess derived from pressure readings generally agree in showing no robust long-term trends for the last 150 to 200 years over NW Europe to central Sweden. In contrast, significant upward trends in annual storm activity since 1871 has been recently derived from the 20th century reanalysis (20CR) which only assimilates sea-level-pressure and monthly mean SST and sea-ice. As the inconsistency between 20CR storm activity and storminess derived from pressure readings increases further back in time, the decreasing station density assimilated in 20CR might be one reason to explain inconsistencies in the upward trend of storm activity in 20CR compared to other reconstructions. While the reason for this inconsistency cannot be evaluated based on reconstructions, we use a different statistical method to reconstruct historical storminess for the period 1850-2009. Daily station SLP is used to estimate the highest pattern similarity between days in the past since 1850 with days in the observational period 1958-2007. For this 50year period, corresponding atmospheric fields like SLP and wind are known from a reanalysis driven regional climate simulation. Based on the pattern similarity regarding station SLP, these fields are redistributed back to 1850 to complete the reconstruction of atmospheric fields. As this upscaling approach makes no other assumptions than that of similarity of SLP patterns, the reconstructed storminess is independent from other reconstructions. The validation of the reconstructed seasonal wind speeds with NCEP shows very high agreement regarding correlation and variability with exception of summer. The derived high annual percentiles of wind speed of our reconstruction shows large decadal variations with peaks in storminess in the 1880s and at the end of the 20th century with an unusually calm period in the 1960s. Hence, the reconstruction fully supports earlier reconstructions based on pressure readings and confirms the inconsistency in the long-term upward trend in 20CR regarding storm activity. Similar to other reconstructions, the inconsistency of 20CR regarding positive trends in storminess can be mainly attributed to opposite storminess conditions in the early period of 20CR i.e. around 1880. In this period, 20 CR shows very calm conditions with in contrast very stormy conditions in reconstructions based on station SLP. As our reconstruction uses an increasing station density over time like in 20CR, an additional reconstruction will be applied using only six stations being kept constant over the whole period in order to assess a possible sampling effect on the reconstructed long-term variability of storminess in the presented reconstruction.
2) Swedish nuclear fuel and waste management company (SKB),
ABSTRACT Describing the low-frequency extratropical atmosphere in terms of space-stationary and t... more ABSTRACT Describing the low-frequency extratropical atmosphere in terms of space-stationary and time-fluctuating structures of different teleconnections (Wallace & Grutzler 1981), the North Atlantic Oscillation (NAO) is the dominant mode of SLP-variability over the northern North Atlantic and a key driver of the European climate. However, some studies also show a spatial evolution of teleconnection pattern like the NAO leading to a position's displacement of the centers of action over the northern North Atlantic and changed atmospheric flow. The latter is likely to change the relation between NAO and its regional climate impact leading to nonstationarity between the atmospheric circulation/NAO and the regional near-surface climate. The time evolution, magnitude and time scale of these nonstationarities are important for the understanding of regional climate variability in the past and has to be taken into account also for possible future climate change. In this study, nonstationarity is defined and detected as the time variation of decadal to multi-decadal running Pearson-correlation-coefficients between the NAO-index and variables of near-surface climate like e.g. air temperature or sea-ice extent. In a first step, the spatio-temporal nonstationarity of the NAO-climate-relation is evaluated using observational data from 1824-2008, reconstructions of the NAO-index and the sea-ice extend of the Baltic Sea from 1720-2008 and different model simulations of the fully coupled AOGCM ECHO-G for 1000-1990 AD. Four simulations with different external forcing and/or initial conditions were analysed and compared with two control-runs for the last centuries. For winter, the correlations between NAO and temperature/sea-ice extend show large multi-decadal nonstationarity with explained variances between 15-65% and 0-65% for model simulations, respectively. In a second step, a running EOF-analysis is applied on the different model simulations showing that the NAO-pattern is not always the leading mode (EOF1) of SLP variability over the North Atlantic in the last 1000 years. In a third step, changes in the SLP and temperature patterns in periods of high correlation are compared with periods of low or no correlation between NAO and near-surface temperature in Northern Europe in order to find explanations for the spatio-temporal nonstationarity. Here, no systematic explanation like displacements or changes of the NAO variability was found. Also, no significant differences can be found with respect to the magnitude and time evolution of nonstationarities in observational, reconstructed or modelled data. In particular, the four differently external forced simulations show no difference with respect to nonstationarity between NAO and regional climate over 1000 years compared with control runs. This indicates internal processes as reasonable explanation for nonstationarity albeit external reasons cannot be excluded for longer time scales.
Much attention is spent today to the reconstruction of past climates, increasing the amount and q... more Much attention is spent today to the reconstruction of past climates, increasing the amount and quality of (multi-)proxy-reconstructions from different regions. However, these reconstructions by means of statistical models usually assume a constant spatiotemporal relationship between the predictor variables and the predictands. Due to the complex nature of externally and internally forced climate variability on different time scales, reasoning arises to question and probably quantify the assumption about stationarity particularly for longer time scales. An advantage of the Baltic region is the high amount of quality-checked and long historical measurements (e.g. SLP, station temperature, gauge and sea-ice data) which make it possible to study the relationship between the large-scale forcing by the North Atlantic Oscillation (NAO) and the near-surface climate of the Baltic Sea catchment area in more detail. In a first step of our approach we relate the NAO to the near-surface climate...
This review assesses storm studies over the North Atlantic and Northwestern Europe regarding the ... more This review assesses storm studies over the North Atlantic and Northwestern Europe regarding the occurrence of potential long-term trends. Based on a systematic review of available articles, trends are classified according to different geographical regions, data sets, and time periods. Articles that used measurement and proxy data, reanalyses, regional and global climate model data on past and future trends are evaluated for changes in storm climate. The most important result is that trends in storm activity depend critically on the time period analysed. An increase in storm numbers is evident for the reanalyses period for the most recent decades, whereas most long-term studies show merely decadal variability for the last 100-150 years. Storm trends derived from reanalyses data and climate model data for the past are mostly limited to the last four to six decades. The majority of these studies find increasing storm activity north of about 55-60°N over the North Atlantic with a negative tendency southward. This increase from about the 1970s until the mid-1990s is also mirrored by long-term proxies and the North Atlantic Oscillation and constitutes a part of their decadal variability. Studies based on proxy and measurement data or model studies over the North Atlantic for the past which cover more than 100 years show large decadal variations and either no trend or a decrease in storm numbers. Future scenarios until about the year 2100 indicate mostly an increase in winter storm intensity over the North Atlantic and Western Europe. However, future trends in total storm numbers are quite heterogeneous and depend on the model generation used.
The detection of potential long-term changes in historical storm statistics and storm surges play... more The detection of potential long-term changes in historical storm statistics and storm surges plays a vitally important role for protecting coastal communities. In the absence of long homogeneous wind records, we present a novel, independent and homogeneous storm surge record based on water level observations in the North Sea since AD 1843. Storm surges are characterized by considerable inter-annual to decadal variability linked to large-scale atmospheric circulation patterns. Time periods of increased storm surge levels prevailed in the late 19th and 20th centuries without any evidence for significant long-term trends. This contradicts with recent findings based on reanalysis data, which suggest increasing storminess in the region since the late 19th century. We compare the wind and pressure fields from the 20th century reanalysis (20CRv2) with the storm surge record by applying state of the art empirical wind surge formulas. The comparison reveals that the reanalysis is a valuable tool which leads to good results over the past 100 years; previously the statistical relationship fails, leaving significantly lower values in the upper percentiles of the predicted surge time series. These low values lead to significant upward trends over the entire investigation period, which are in turn neither supported by the storm surge record nor by an independent circulation index based on homogeneous pressure readings. We therefore suggest that these differences are related to higher uncertainties in the earlier years of the 20CRv2 over the North Sea region.
The Baltic Sea is located in Northern Europe and exhibits significant climate variability, with i... more The Baltic Sea is located in Northern Europe and exhibits significant climate variability, with influence of air masses from arctic to subtropical origin. By updating and discussing results described in the framework of the BACC project (BALTEX Assessment of Climate Change for the Baltic Sea Basin), this study presents observed changes in atmospheric parameters during the last 200 yr. Circulation patterns show large decadal variability with a northward shift of storm tracks and increased cyclonic activity in recent decades with increased persistence of weather types. However, the wind climate shows no robust long-term trends, and is dominated by pronounced (multi-)decadal variability. Near-surface temperatures show continued warming, in particular during spring and winter; this is stronger over northern regions. Up to this point, no long-term trends are detectable for precipitation, although some regional indications exist for an increased length of precipitation periods, and possibly an increased risk of extreme precipitation events.
We present a multi-model ensemble study for the Baltic Sea, and investigate the combined impact o... more We present a multi-model ensemble study for the Baltic Sea, and investigate the combined impact of changing climate, external nutrient supply, and fisheries on the marine ecosystem. The applied regional climate system model contains state-of-the-art component models for the atmosphere, sea ice, ocean, land surface, terrestrial and marine biogeochemistry, and marine food-web. Time-dependent scenario simulations for the period 1960–2100 are performed and uncertainties of future projections are estimated. In addition, reconstructions since 1850 are carried out to evaluate the models sensitivity to external stressors on long time scales. Information from scenario simulations are used to support decision-makers and stakeholders and to raise awareness of climate change, environmental problems, and possible abatement strategies among the general public using geovisualization. It is concluded that the study results are relevant for the Baltic Sea Action Plan of the Helsinki Commission.
"The analog method (AM) has found application to reconstruct gridded climate fields from the info... more "The analog method (AM) has found application to reconstruct gridded climate fields from the information provided by proxy data and climate model simulations. Here, we test the skill of different setups of the AM, in a controlled but realistic situation, by analysing several statistical properties of reconstructed daily high-resolution atmospheric fields for Northern Europe for a 50-yr period. In this application, station observations of sea-level pressure and air temperature are combined with atmospheric fields from a 50-yr high-resolution regional climate simulation. This reconstruction aims at providing homogeneous and physically consistent atmospheric fields with daily resolution suitable to drive high resolution ocean and ecosystem models.
Different settings of the AM are evaluated in this study for the period 1958–2007 to estimate the robustness of the reconstruction and its ability to replicate high and low-frequency variability, realistic probability distributions and extremes of different meteorological variables. It is shown that the AM can realistically reconstruct variables with a strong physical link to daily sea-level pressure on both a daily and monthly scale. However, to reconstruct low-frequency decadal and longer temperature variations, additional monthly mean station temperature as predictor is required. Our results suggest that the AM is a suitable upscaling tool to predict daily fields taken from regional climate simulations based on sparse historical station data."
A comprehensive reconstruction of the Baltic Sea state from 1850 to 2006 is presented: driving fo... more A comprehensive reconstruction of the Baltic Sea state from 1850 to 2006 is presented: driving forces are reconstructed and the evolution of the hydrography and biogeochemical cycles is simulated using the model BALTSEM. Driven by high resolution atmospheric forcing fields (HiResAFF), BALTSEM reproduces dynamics of salinity, temperature, and maximum ice extent. Nutrient loads have been increasing with a noteworthy acceleration from the 1950s until peak values around 1980 followed by a decrease continuing up to present.
BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.
Arxiv preprint arXiv:1205.5295, Jan 1, 2012
Global atmospheric reanalyses have become a common tool for both the validation of climate models... more Global atmospheric reanalyses have become a common tool for both the validation of climate models and diagnostic studies, such as assessing climate variability and long-term trends. Presently, the 20th Century Reanalysis (20CR), which assimilates only surface pressure reports, sea-ice, and sea surface temperature distributions, represents the longest global reanalysis dataset available covering the period from 1871 to the present. Currently, the 20CR dataset is extensively used for the assessment of climate variability and trends. Here, we compare the variability and long-term trends in Northeast Atlantic storminess derived from 20CR and from observations. A well established storm index derived from pressure observations over a relatively densely monitored marine area is used. It is found that both, variability and long-term trends derived from 20CR and from observations, are inconsistent. In particular, both time series show opposing trends during the first half of the 20th century. Only for the more recent periods both storm indices share a similar behavior. While the variability and long-term trend derived from the observations are supported by a number of independent data and analyses, the behavior shown by 20CR is quite different, indicating substantial inhomogeneities in the reanalysis most likely caused by the increasing number of observations assimilated into 20CR over time. The latter makes 20CR likely unsuitable for the identification of trends in storminess in the earlier part of the record at least over the Northeast Atlantic. Our results imply and reconfirm previous findings that care is needed in general, when global reanalyses are used to assess long-term changes.
Multi-model ensemble simulations for the marine biogeochemistry and food web of the Baltic Sea we... more Multi-model ensemble simulations for the marine biogeochemistry and food web of the Baltic Sea were performed for the period 1850–2098, and projected changes in the future climate were compared with the past climate environment. For the past period 1850–2006, atmospheric, hydrological and nutrient forcings were reconstructed, based on historical measurements. For the future period 1961–2098, scenario simulations were driven by regionalized global general circulation model (GCM) data and forced by various future greenhouse gas emission and air- and riverborne nutrient load scenarios (ranging from a pessimistic 'business-as-usual' to the most optimistic case). To estimate uncertainties, different models for the various parts of the Earth system were applied. Assuming the IPCC greenhouse gas emission scenarios A1B or A2, we found that water temperatures at the end of this century may be higher and salinities and oxygen concentrations may be lower than ever measured since 1850. There is also a tendency of increased eutrophication in the future, depending on the nutrient load scenario. Although cod biomass is mainly controlled by fishing mortality, climate change together with eutrophication may result in a biomass decline during the latter part of this century, even when combined with lower fishing pressure. Despite considerable shortcomings of state-of-the-art models, this study suggests that the future Baltic Sea ecosystem may unprecedentedly change compared to the past 150 yr. As stakeholders today pay only little attention to adaptation and mitigation strategies, more information is needed to raise public awareness of the possible impacts of climate change on marine ecosystems.
coast.gkss.de
We would like to introduce and shortly discuss a new dataset of spatio-temporal HIghly RESolved A... more We would like to introduce and shortly discuss a new dataset of spatio-temporal HIghly RESolved Atmospheric Forcing Fields (HIRESAFF) for Northern Europe since 1850. As an outcome of the BONUS project ECOSUPPORT, the reconstructed fields provide a new basis for ecosystem (or similar) models to run longer simulations also prior to a large human impact on the Baltic Sea. The dataset also allows a better validation and estimation of model uncertainties under different climatic or nutrient load conditions.
The new dataset of HIRESAFF provides daily atmospheric fields for Northern Europe since 1850 for the variables sea-surface pressure (SLP), zonal (U) and meridional (V) wind components, relative humidity (RH), total cloud cover (TCC), near-surface temperature (T2m) and precipitation (PREC). The atmospheric forcing fields span from 71° N to 48° N and from 5° W to 37° E (see fig. 4) with a horizontal resolution of 0.25° x 0.25°.
… , held Sept. 13-17, 2010 in …, Jan 1, 2010
Two simulations with the atmosphere-ocean general circulation model ECHO-G have been analyzed for... more Two simulations with the atmosphere-ocean general circulation model ECHO-G have been analyzed for temperature and precipitation changes over the last 7.000 years over the Mediterranean area (11 • E -41 • W and 29 • N -42.5 • N). The first simulation has been driven with changes in orbital forcing (ORB), the second with additional changes in solar activity and changes in greenhouse gas concentrations (ORBSG).
in the southern Baltic region
An important factor influencing the climate of the catchment area of the Baltic Sea is the North ... more An important factor influencing the climate of the catchment area of the Baltic Sea is the North Atlantic Oscillation (NAO). The knowledge about this influence over the last centuries is of great importance reconstructing past terrestrial and oceanic climatic conditions. Bottom up approaches are widely used to reconstruct the NAO from proxies reflecting the local near surface climate. However, as we demonstrate in this study, localizations of proxies are partly inadequate for the large-scale influence distinct from the North Atlantic Ocean due to non stationarities in the NAO-temperature relation.
In a first step of our approach we relate the NAO-index to the near-surface climate (i.e. temperature and sea-ice extent) of the Baltic region on monthly and seasonal basis. In a second and third step, we also use multi-proxy reconstructions and climate model simulations to investigate the NAO-climate relationship on longer time scales.
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Papers by Frederik Schenk
Different settings of the AM are evaluated in this study for the period 1958–2007 to estimate the robustness of the reconstruction and its ability to replicate high and low-frequency variability, realistic probability distributions and extremes of different meteorological variables. It is shown that the AM can realistically reconstruct variables with a strong physical link to daily sea-level pressure on both a daily and monthly scale. However, to reconstruct low-frequency decadal and longer temperature variations, additional monthly mean station temperature as predictor is required. Our results suggest that the AM is a suitable upscaling tool to predict daily fields taken from regional climate simulations based on sparse historical station data."
BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.
The new dataset of HIRESAFF provides daily atmospheric fields for Northern Europe since 1850 for the variables sea-surface pressure (SLP), zonal (U) and meridional (V) wind components, relative humidity (RH), total cloud cover (TCC), near-surface temperature (T2m) and precipitation (PREC). The atmospheric forcing fields span from 71° N to 48° N and from 5° W to 37° E (see fig. 4) with a horizontal resolution of 0.25° x 0.25°.
In a first step of our approach we relate the NAO-index to the near-surface climate (i.e. temperature and sea-ice extent) of the Baltic region on monthly and seasonal basis. In a second and third step, we also use multi-proxy reconstructions and climate model simulations to investigate the NAO-climate relationship on longer time scales.
Different settings of the AM are evaluated in this study for the period 1958–2007 to estimate the robustness of the reconstruction and its ability to replicate high and low-frequency variability, realistic probability distributions and extremes of different meteorological variables. It is shown that the AM can realistically reconstruct variables with a strong physical link to daily sea-level pressure on both a daily and monthly scale. However, to reconstruct low-frequency decadal and longer temperature variations, additional monthly mean station temperature as predictor is required. Our results suggest that the AM is a suitable upscaling tool to predict daily fields taken from regional climate simulations based on sparse historical station data."
BALTSEM shows a delayed response to the massive load increase with most eutrophic conditions occurring only at the end of the simulation. This is accompanied by an intensification of the pelagic cycling driven by a shift from spring to summer primary production. The simulation indicates that no improvement in water quality of the Baltic Sea compared to its present state can be expected from the decrease in nutrient loads in recent decades.
The new dataset of HIRESAFF provides daily atmospheric fields for Northern Europe since 1850 for the variables sea-surface pressure (SLP), zonal (U) and meridional (V) wind components, relative humidity (RH), total cloud cover (TCC), near-surface temperature (T2m) and precipitation (PREC). The atmospheric forcing fields span from 71° N to 48° N and from 5° W to 37° E (see fig. 4) with a horizontal resolution of 0.25° x 0.25°.
In a first step of our approach we relate the NAO-index to the near-surface climate (i.e. temperature and sea-ice extent) of the Baltic region on monthly and seasonal basis. In a second and third step, we also use multi-proxy reconstructions and climate model simulations to investigate the NAO-climate relationship on longer time scales.