Search

The ability to predict changes in the right tail of daily precipitation distributions over short time periods, as well as the probability of clustering extreme values, is critical for current risk management. In the present study, we apply the well-established metric average value-at-risk (AVaR) for the first time within the field of climatology. We also investigate the evolution of the return level (RL) and the extremal index (EI), which we refer to as risk measures. In the case of precipitation processes, a rise in the first two and a reduction in the third may result in increased hazards. These methods were applied to the new data on the daily sum of precipitation from the region of Upper Vistula in Poland from 1951 to 2020 to analyze the dynamics of changes across time. We found that AVaR and RL have the smallest values for the middle of the analyzed period (years 1971–2010), while EI has a maximal value around 1963–92. Both the beginning and the end of the investigated period are characterized by more extreme and clustered precipitation events. Furthermore, this paper includes some recommendations for the tools used to compute the measures. [ABSTRACT FROM AUTHOR]

Long and homogeneous series are a necessary requirement for reliable climate analysis. Relocation of measuring equipment from one station to another, such as from the city center to a rural area or a nearby airport, is one of the causes of discontinuities in these long series that may affect trend estimates. In this paper, an updated procedure for the composition of long series, by combining data from nearby stations, is introduced. It couples an evolution of the blending procedure already implemented within the European Climate Assessment and Dataset (ECA&D, which combines data from stations no more than 12.5 km apart from each other) with a duplicate removal, alongside the quantile matching homogenization procedure. The ECA&D contains approximately 3000 homogenized series for each temperature variable prior to the blending procedure, and approximately 820 of these are longer than 60 years; the process of blending increases the number of long series to more than 900. Three case studies illustrate the effects of the homogenization on single blended series, showing the effectiveness of separate adjustments on extreme and mean values (Geneva, Switzerland), on cases in which blending is complex (Rheinstetten, Germany), and on series that are completed by adding relevant portions of Global Telecommunications System synoptic data (Siauliai, Lithuania). A trend assessment on the whole European continent reveals the removal of negative and very large trends, demonstrating a stronger spatial consistency. The new blended and homogenized dataset will allow a more reliable use of temperature series for indices calculation and for the calculation of gridded datasets and it will be available online for users (https://www.ecad.eu). [ABSTRACT FROM AUTHOR]

This paper proposes an optimal method for estimating time-dependent climate change signals from general circulation models. The basic idea is to identify vectors that maximize the mean-square detection statistic derived from optimal fingerprinting techniques. The method also provides an objective and systematic procedure for identifying the limit to which a signal can be restricted in space and time without losing detectability. As an illustration, the method is applied to the Coupled Model Intercomparison Project, phase 3 multimodel dataset to determine the continental seasonal-mean anomaly in surface air temperature and precipitation that is most detectable, on average, in these models. Anomalies in seasonal-mean surface air temperature are detectable in all seasons by almost all models on all continents but Europe; seasonal-mean anomalies over Europe are undetectable for some models, though this does not preclude other expressions of the signal, such as those that include longer time averages or time-lag information, from being detectable. Detectability in seasonal-mean temperature is found not only for multidecadal warming trends but also for cooling after major volcanic eruptions. In contrast, seasonal-mean precipitation anomalies are detectable in only a few models for averages over 5 yr or more, suggesting that the signal should include more spatiotemporal detail to be detectable across more models. Nevertheless, of the precipitation anomalies that are detectable, the signal appears to be of two characters: a systematic trend and enhanced frequency of extreme values. These results derived from twentieth-century simulations appear to be consistent with previous studies based on twenty-first-century simulations with larger signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

Satellite-derived high-resolution precipitation products (HRPP) have been developed to address the needs of the user community and are now available with 0.25° × 0.25° (or less) subdaily resolutions. This paper evaluates a number of commonly available satellite-derived HRPPs covering northwest Europe over a 6-yr period. Precipitation products include the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA), the Climate Prediction Center (CPC) morphing (CMORPH) technique, the CPC merged microwave technique, the Naval Research Laboratory (NRL) blended technique, and the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) technique. In addition, the Geosynchronous Operational Environmental Satellite (GOES) precipitation index (GPI) and the European Centre for Medium-Range Weather Forecasting (ECMWF) operational forecast model products are included for comparison. Surface reference data from the European radar network is used as ground truth, supported by the Global Precipitation Climatology Centre (GPCC) precipitation gauge analysis and gauge data over the United Kingdom. Measures of correlation, bias ratio, probability of detection, and false alarm ratio are used to evaluate the products. Results show that satellite products generally exhibit a seasonal cycle in correlation, bias ratio, probability of detection, and false alarm ratio, with poorer statistics during the winter. The ECMWF model also shows a seasonal cycle in the correlation, although the results are poorer during the summer, while the bias ratio, probability of detection, and false alarm ratio are consistent through all seasons. Importantly, all the satellite HRPPs underestimate precipitation over northwest Europe in all seasons. [ABSTRACT FROM AUTHOR]

The idea of using large-scale information to predict local climate variability is widely exploited in climate change impact studies as an alternative to computationally expensive high-resolution models. This approach implies the hypothesis that the statistical relationship between large-scale climate states and local variables defined for the present-day climate remains valid in the altered climate. In this paper, the concept of weather regimes is used to deduce a relationship between large-scale circulation and European winter temperature. The change in temperature with increased greenhouse gases is, however, not homogeneous among the individual regimes. As a result, the impact of the weather regimes on local temperature changes varies in the future, limiting its usefulness for refining temperature changes to the small scale. [ABSTRACT FROM AUTHOR]

Groundwater and climate interact in a two-way manner. Precipitation ultimately controls groundwater recharge and, conversely, groundwater may influence climate through evapotranspiration. Yet very few global climate models or Earth system models actually simulate groundwater flows. And while the expected impacts of climate change on groundwater resources are the subject of a growing concern, global-scale groundwater–climate feedbacks have received very little attention so far. Here we show that the integration of unconfined aquifers in a global climate model can regionally affect the climate change signal on temperatures and precipitation. We assess the impact of groundwater under preindustrial and 4xCO2 conditions (after climate stabilization). In both cases, we find that groundwater has a cooling and a wetting effect in certain regions of the world. In eastern Europe, both these impacts are stronger in the warmer climate (4xCO2 forcing) where the presence of groundwater reduces the frequency of summer heatwaves by 40%, compared to a 15% reduction in the preindustrial world. This work constitutes one of the very first global assessments of the potential feedbacks of groundwater on climate change. Our results support the idea that groundwater should be represented in global climate models and Earth system models, as it does indeed play an active role in the climate system. [ABSTRACT FROM AUTHOR]

The extratropical storm tracks cause much of the high-impact weather in the midlatitudes, and thus it is of interest to examine the weather impacts of projected storm track change. Here, a framework for quantifying storm track impact on precipitation over Europe developed in Part I for winter is extended to all four seasons and applied to estimate the precipitation impacts of CMIP5 and CMIP6 model projected storm track change under the high-emission scenarios. For autumn and winter, a significant portion of the projected decrease in precipitation over southern Europe and the Mediterranean Sea region can be related to a projected decrease in storm track activity over these regions. In winter, the projected increase in storm track activity near the British Isles accentuates the precipitation increase over that region. In summer, a projected storm track decrease over northern Europe is expected to give rise to a decrease in precipitation that expands the projected drying region poleward and reduces the impact of the projected high-latitude precipitation increase related to the increase in atmospheric moisture. CMIP6 models project stronger drying than CMIP5 models over central and northern Europe in summer and autumn. Part of this increased drying can be related to an enhanced decrease in storm track activity projected by CMIP6 models. Apart from multimodel mean projections, different model projected storm track change patterns can give rise to very different regional precipitation impacts, and account for the model spread in precipitation projection in many regions. What causes such model diversity should be further examined. [ABSTRACT FROM AUTHOR]

This study uses small ensembles of convection-allowing, quasi-idealized simulations to examine the response ofNorth Atlantic tropical cyclones (TCs) undergoing extratropical transition (ET) to climate change. Using HURDAT2 and ERA5 data over a 40-yr period from 1979 to 2018, we developed storm-relative composite fields for past North Atlantic recurving, oceanic ET events. The quasi-idealized present-day simulations are initialized from these composites and run in an aquaplanet domain. A pseudo–global warming approach is used for future simulations: Thermodynamic changes between late twenty-first century and twentieth century, derived from an ensemble of 20 CMIP5GCMsunder the RCP8.5 scenario, are added to the present-day initial and lateral boundary conditions. The composite-initialized present-day simulations exhibit realistic ET characteristics. Future simulations show greater intensity, heavier precipitation, and stronger downstream midlatitude wave train development relative to the presentday case. Specifically, the future ET event is substantially stronger before ET completion, though the system undergoes less reintensification after ET completion. Reductions in lower-tropospheric baroclinicity associated with Arctic amplification could contribute to this result. The future simulation exhibits 3-hourly ensemble-mean precipitation rate increases ranging from ~23% to ~50%, depending on ET phase and averaging radius. In addition, larger eddy kinetic energy accompanies the future storm, partly created by increased baroclinic conversion, resulting in stronger amplification of downstream energy maxima via intensified ageostrophic geopotential flux convergence and divergence. These results suggest that future TCs undergoing ET could have greater potential to cause high-impact weather in western Europe through both direct and remote processes. [ABSTRACT FROM AUTHOR]

Changes in atmospheric circulation under increasing greenhouse gas concentrations are important because of their implications for weather extremes and associated societal risks. However, uncertainties in models and future projections are still large and drivers behind circulation changes are not well understood. Particularly for Europe, a potential weakening of the Atlantic meridional overturning circulation (AMOC) is considered important as it affects SST patterns and ocean–atmosphere heat fluxes and, subsequently, European climate. Here we detect and characterize changes in atmospheric circulation patterns over the North Atlantic under increasing CO2 concentrations in simulations of a very high-resolution, fully coupled climate model (CM2.6) with a realistic representation of the AMOC. We use an objective clustering technique (self-organizing maps) and validate the model's clusters against reanalysis data. We compare the frequency of those patterns in a CO2 doubling experiment, characterized by an AMOC decline, with those in a preindustrial run, and find statistically significant changes. The most robust findings are 1) a ~30% increase in zonal flow regimes in February, relevant for flood risk in northwestern Europe, and 2) a ~60% increase in anticyclonic (high pressure) circulation directly west of the United Kingdom in August, relevant for western and central European drought. A robust decrease in the frequency of Scandinavian blocking is also seen across most months and seasons. Despite the uncertainties regarding atmospheric circulation response to climate change, our findings contribute to the increasing evidence for the emergence of robust high-impact changes over Europe. [ABSTRACT FROM AUTHOR]

In this study, we aim to better understand the current and future projections of precipitation extremes in Europe in the context of climatic variability over a long-term period from the last millennium to the end of the twenty- first century. The daily gridded precipitation data from five global climate models (GCMs) of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are chosen to investigate natural variability and precipitation extremes during the last millennium (850-1849), the historical (1850-2005) period, and two representative concentration pathway (RCP2.6 and RCP8.5) scenarios (2006-99). First, the seasonal and annual precipitation and extreme statistics from GCMs are evaluated using reconstruction and observation. Second, the spatial and temporal patterns of extreme precipitation from GCMs are investigated from the last millennium to the end of the twenty-first century. Meanwhile, the characteristic changes of extreme precipitation for the five regions of Europe are further analyzed. The results revealed the following: GCMs underestimate extreme precipitation and overestimate mean precipitation compared with the observations from European Climate Assessment and Dataset (ECA&D) ; the majority of Europe except southern Europe will most likely have large magnitude increases in the extreme precipitation and mean precipitation in the future under both RCP scenarios; there is no systematic change of precipitation extremes from the last millennium to the historical period from all GCMs; and larger magnitude increases are shown in 100- and 200-yr than in 5- and 10-yr return period precipitation from both RCP scenarios. In addition, short-duration extreme precipitation willmost likely increase more than longer-duration extremes. [ABSTRACT FROM AUTHOR]

The science of event attribution has emerged to routinely answer the question whether and to what extent human-induced climate change altered the likelihood and intensity of recently observed extreme weather events. In Europe a pilot program to operationalize the method started in November 2019, highlighting the demand for timely information on the role of climate change when it is needed most: in the direct aftermath of an extreme event. Independent of whether studies are provided operationally or as academic studies, the necessity of good observational data and well-verified climate models imply most attributions are currently made for highly developed countries only. Current attribution assessments therefore provide very little information about those events and regions where the largest damages and socio-economic losses are incurred. Arguably, these larger damages signify a much greater need for information on how the likelihood and intensity of such high-impact events have been changing and are likely to change in a warmer world. In short, why do we not focus event attribution research efforts on the whole world, and particularly events in the developing world? The reasons are not just societal and political but also scientific. We simply cannot attribute these events in the same probabilistic framework employed in most studies today. We outline six focus areas to lessen these barriers, but we will not overcome them in the near future. [ABSTRACT FROM AUTHOR]

Simulations of water fluxes at high spatial resolution that consistently cover historical observations, seasonal forecasts, and future climate projections are key to providing climate services aimed at supporting operational and strategic planning, and developing mitigation and adaptation policies. The End-to-end Demonstrator for improved decision-making in the water sector in Europe (EDgE) is a proof-of-concept project funded by the Copernicus Climate Change Service program that addresses these requirements by combining a multimodel ensemble of state-of-the-art climate model outputs and hydrological models to deliver sectoral climate impact indicators (SCIIs) codesigned with private and public water sector stakeholders from three contrasting European countries. The final product of EDgE is a water-oriented information system implemented through a web application. Here, we present the underlying structure of the EDgE modeling chain, which is composed of four phases: 1) climate data processing, 2) hydrological modeling, 3) stakeholder codesign and SCII estimation, and 4) uncertainty and skill assessments. Daily temperature and precipitation from observational datasets, four climate models for seasonal forecasts, and five climate models under two emission scenarios are consistently downscaled to 5-km spatial resolution to ensure locally relevant simulations based on four hydrological models. The consistency of the hydrological models is guaranteed by using identical input data for land surface parameterizations. The multimodel outputs are composed of 65 years of historical observations, a 19-yr ensemble of seasonal hindcasts, and a century-long ensemble of climate impact projections. These unique, high-resolution hydroclimatic simulations and SCIIs provide an unprecedented information system for decision-making over Europe and can serve as a template for water-related climate services in other regions. [ABSTRACT FROM AUTHOR]

Wintertime midlatitude cyclone activity and precipitation are projected to increase across northern Europe and decrease over southern Europe, particularly over the western Mediterranean. Greater confidence in these regional projections may be established by their replication in state-of-the-art, high-resolution global climate models that resolve synoptic-scale dynamics. We evaluated the representation of the wintertime eddy-driven and subtropical jet streams, extratropical cyclone activity, and precipitation across the North Atlantic and Europe under historical (1985–2011) and RCP8.5 sea surface temperature forcing in an ensemble of atmosphere-only HadGEM3-GA3.0 simulations, where horizontal atmospheric resolution is increased from 135 to 25 km. Under RCP8.5, increased (decreased) frequency of northern (southern) eddy-driven jet occurrences and a basinwide poleward shift in the upper-level westerly flow are simulated. Increasing atmospheric resolution significantly enhances these climate change responses. At 25-km resolution, these enhanced changes in large-scale circulation amplify increases (decreases) in extratropical cyclone track density and mean intensity across the northern (southern) Euro-Atlantic region under RCP8.5. These synoptic changes with resolution impact the overall climate change response of mean and heavy winter precipitation: wetter (drier) conditions in northern (southern) Europe are also amplified at 25-km resolution. For example, the reduction in heavy precipitation simulated over the Iberian Peninsula under RCP8.5 is ~15% at 135 km but ~30% at 25-km resolution. Conversely, a shift to more frequent high extratropical cyclone (ETC)-associated precipitation rates is simulated over Scandinavia under RCP8.5, which is enhanced at 25 km. This study provides evidence that global atmospheric resolution may be a crucial consideration in European winter climate change projections. [ABSTRACT FROM AUTHOR]

Future changes in geostrophic winds over Europe and the North Atlantic region were studied utilizing output data from 21 CMIP5 global climate models (GCMs). Changes in temporal means, extremes, and the joint distribution of speed and direction were considered. In concordance with previous research, the time mean and extreme scalar wind speeds do not change pronouncedly in response to the projected climate change; some degree of weakening occurs in the majority of the domain. Nevertheless, substantial changes in high wind speeds are identified when studying the geostrophic winds from different directions separately. In particular, in northern Europe in autumn and in parts of northwestern Europe in winter, the frequency of strong westerly winds is projected to increase by up to 50%. Concurrently, easterly winds become less common. In addition, we evaluated the potential of the GCMs to simulate changes in the near-surface true wind speeds. In ocean areas, changes in the true and geostrophic winds are mainly consistent and the emerging differences can be explained (e.g., by the retreat of Arctic sea ice). Conversely, in several GCMs the continental wind speed response proved to be predominantly determined by fairly arbitrary changes in the surface properties rather than by changes in the atmospheric circulation. Accordingly, true wind projections derived directly from the model output should be treated with caution since they do not necessarily reflect the actual atmospheric response to global warming. [ABSTRACT FROM AUTHOR]

Precipitation on the Tibetan Plateau (TP) showed different spatial changes during 1979–2016, with an increasing trend over the northern Tibetan Plateau (NTP) and a slightly negative trend over the southern Tibetan Plateau (STP). The changes in precipitation moisture sources over the NTP and STP are investigated using the improved Water Accounting Model with an atmospheric reanalysis as well as observational precipitation and evaporation data. The results show the region in the northwest (region NW), ranging from the TP to Europe dominated by the westerlies, provides 38.9% of precipitation moisture for the NTP, and the region in the southeast (region SE), ranging from the TP to the Indian Ocean and Indochina dominated by the Asian monsoons, provides 51.4% of precipitation moisture for the STP. For the precipitation increase over the NTP, the SE and TP are the main contributors, contributing around 35.8% and 51.7% of the increase, respectively. The contributions from the SE and TP to the STP are, however, minor and insignificant. Meanwhile, the NW shows a negative trend of −4.2 ± 2.9 mm yr−1 decade−1 (significant at the 0.01 level), which contributes to the negative precipitation trend over the STP. Results during the wet season indicate that moisture sources from the areas dominated by the Asian monsoons have contributed more precipitated moisture for the NTP, but not for the STP. Further analysis reveals that precipitated moisture originating from the Indian subcontinent has increased for the NTP while it has decreased for the STP during 1979–2016. [ABSTRACT FROM AUTHOR]

The influence of atmospheric circulation on winter temperature and precipitation trends over Europe in the period 2006-50 is investigated in a 21-member initial condition ensemble from a fully coupled global climate model and in a multimodel framework consisting of 40 different models. Five versions of a dynamical adjustment method based on empirical orthogonal function analysis of sea level pressure are introduced, and their performance in removing the effect of atmospheric circulation on temperature and precipitation is tested. The differences in atmospheric circulation as simulated by different models in their control runs and under the historical and representative concentration pathway 8.5 (RCP8.5) forcing scenarios are investigated. Dynamical adjustment is applied to the multimodel ensemble to demonstrate that a substantial fraction of the uncertainty in projected European temperature and precipitation trends is explained by atmospheric circulation variability. A statistically significant response of sea level pressure to anthropogenic forcing is identified in the multimodel ensemble under the RCP8.5 scenario. This forced response in atmospheric circulation is associated with a dynamical contribution to the long-term multimodel mean temperature and precipitation trends. The results highlight the importance of accounting for the impact of atmospheric circulation variability on trends in regional climate projections. [ABSTRACT FROM AUTHOR]

Heavy precipitation is a major hazard over Europe. It is well established that climate model projections indicate a tendency toward more extreme daily rainfall events. It is still uncertain, however, how this changing intensity translates at the subdaily time scales. The main goal of the present study is to examine possible differences in projected changes in intense precipitation events over Europe at the daily and subdaily (3-hourly) time scales using a state-of-the-science climate model. The focus will be on one representative concentration pathway (RCP8.5), considered as illustrative of a high rate of increase in greenhouse gas concentrations over this century. There are statistically significant differences in intense precipitation projections (up to 40%) when comparing the results at the daily and subdaily time scales. Over northeastern Europe, projected precipitation intensification at the 3-hourly scale is lower than at the daily scale. On the other hand, Spain and the western seaboard exhibit an opposite behavior, with stronger intensification at the 3-hourly scale rather than the daily scale. While the mean properties of the precipitation distributions are independent of the analyzed frequency, projected precipitation intensification exhibits regional differences. This finding has implications for the extrapolation of impacts of intense precipitation events, given the daily time scale at which the analyses are usually performed. [ABSTRACT FROM AUTHOR]

Over recent decades Europe has experienced heavy floods, with major consequences for thousands of people and billions of euros worth of damage. In particular, the summer of 2013 flood in central Europe showed how vulnerable modern society is to hydrological extremes and emphasized once more the need for improved forecast methods of such extreme climatic events. Based on a multiple linear regression model, it is shown here that 55% of the June 2013 Elbe River extreme discharge could have been predicted using May precipitation, soil moisture, and sea level pressure. Moreover, the model was able to predict more than 75% of the total Elbe River discharge for June 2013 (in terms of magnitude) by also incorporating the amount of precipitation recorded during the days prior to the flood, but the predicted discharge for the June 2013 event was still underestimated by 25%. Given that all predictors used in the model are available at the end of each month, the forecast scheme can be used to predict extreme events and to provide early warnings for upcoming floods. The forecast methodology could be relevant for other rivers also, depending on their location and their climatic background. [ABSTRACT FROM AUTHOR]

The synoptic behavior of present-day heat waves (HW) over Europe is studied using the GFDL high-resolution atmospheric model (HiRAM) with 50-km grid spacing. Three regions of enhanced and coherent temperature variability are identified over western Russia, eastern Europe, and western Europe. The simulated HW characteristics are compared with those derived from Climate Forecast System Reanalysis products. Composite charts for outstanding HW episodes resemble well-known recurrent circulation types. The HW region is overlain by a prominent upper-level anticyclone, which blocks the passage of synoptic-scale transients. The altered eddy vorticity transports in turn reinforce the anticyclone. The anticyclone is part of a planetary-scale wave train. The successive downstream development of this wave train is indicative of Rossby wave dispersion. Additional runs of HiRAM are conducted for the 'time slices' of 2026-35 and 2086-95 in the climate scenario corresponding to representative concentration pathway 4.5 (RCP4.5). By the end of the twenty-first century, the average duration and frequency of HW in the three European sites are projected to increase by a factor of 1.4-2.0 and 2.2-4.5, respectively, from the present-day values. These changes can be reproduced by adding the mean shift between the present and future climatological temperatures to the daily fluctuations in the present-day simulation. The output from a continuous integration of a coupled general circulation model through the 1901-2100 period indicates a monotonic increase in severity, duration, and HW days during the twenty-first century. [ABSTRACT FROM AUTHOR]

Producing reliable estimates of changes in precipitation at local and regional scales remains an important challenge in climate science. Statistical downscaling methods are often utilized to bridge the gap between the coarse resolution of general circulation models (GCMs) and the higher resolutions at which information is required by end users. As the skill of GCM precipitation, particularly in simulating temporal variability, is not fully understood, statistical downscaling typically adopts a perfect prognosis (PP) approach in which high-resolution precipitation projections are based on real-world statistical relationships between large-scale atmospheric predictors and local-scale precipitation. Using a nudged simulation of the ECHAM5 GCM, in which the large-scale weather states are forced toward observations of large-scale circulation and temperature for the period 1958-2001, previous work has shown ECHAM5 skill in simulating temporal variability of precipitation to be high in many parts of the world. Here, the same nudged simulation is used in an alternative downscaling approach, based on model output statistics (MOS), in which statistical corrections are derived for simulated precipitation. Cross-validated MOS corrections based on maximum covariance analysis (MCA) and principal component regression (PCR), in addition to a simple local scaling, are shown to perform strongly throughout much of the extratropics. Correlation between downscaled and observed monthly-mean precipitation is as high as 0.8-0.9 in many parts of Europe, North America, and Australia. For these regions, MOS clearly outperforms PP methods that use temperature and circulation as predictors. The strong performance of MOS makes such an approach to downscaling attractive and potentially applicable to climate change simulations. [ABSTRACT FROM AUTHOR]

Daily rain gauge data over Europe for the period from 1950 to 2009 were used to analyze changes in the duration of wet and dry spells. The duration of wet spells exhibits a statistically significant growth over northern Europe and central European Russia, which is especially pronounced in winter when the mean duration of wet periods increased by 15%-20%. In summer wet spells become shorter over Scandinavia and northern Russia. The duration of dry spells decreases over Scandinavia and southern Europe in both winter and summer. For the discrimination between the roles of a changing number of wet days and of a regrouping of wet and dry days for the duration of the period, the authors suggest a fractional truncated geometric distribution. The changing numbers of wet days cannot explain the long-term variability in the duration of wet and dry periods. The observed changes are mainly due to the regrouping of wet and dry days. The tendencies in duration of wet and dry spells have been analyzed for a number of European areas. Over the Netherlands both wet and dry periods are extended in length during the cold and the warm season. A simultaneous shortening of wet and dry periods is found in southern Scandinavia in summer. Over France and central southern Europe during both winter and summer and over the Scandinavian Atlantic coast in summer, opposite tendencies in the duration of wet and dry spells were identified. Potential mechanisms that might be responsible for the changing durations of wet and dry periods and further perspectives are discussed. [ABSTRACT FROM AUTHOR]

An extreme-value analysis of projected changes in heavy precipitation is carried out for an ensemble of eight high-resolution regional climate model simulations over the European domain. The consideration of several regional climate models that are forced by different global models allows for an assessment of the robustness of the results in terms of intersimulation agreement. The extreme-value statistical method is based on a model that includes time-dependent parameters. Summer and winter are examined separately. This allows for identifying and sharpening the understanding of physical processes inducing the changes in precipitation characteristics. Thermodynamic aspects of changes in heavy precipitation are discussed. Variables that are related to the process of precipitation formation, such as precipitable water and cloud liquid water, are examined. In this context, the scaling of changes in heavy precipitation and other thermodynamic quantities with changes in temperature is explored. The validity of a Clausius-Clapeyron scaling of heavy precipitation is assessed on regional scales. Significant regional and seasonal differences in trends of heavy precipitation and only a limited validity of the Clausius-Clapeyron scaling are found. In winter, enhanced moisture transport and storm-track intensity lead to an increase in heavy precipitation, especially over the northern parts of the European continent. In summer, the increase of precipitable water is less than that required to maintain the same probability for saturation over southern Europe, which results in negative trends of heavy precipitation in these regions. [ABSTRACT FROM AUTHOR]

The authors estimate the change in extreme winter weather events over Europe that is due to a long-term change in the North Atlantic Oscillation (NAO) such as that observed between the 1960s and 1990s. Using ensembles of simulations from a general circulation model, large changes in the frequency of 10th percentile temperature and 90th percentile precipitation events over Europe are found from changes in the NAO. In some cases, these changes are comparable to the expected change in the frequency of events due to anthropogenic forcing over the twenty-first century. Although the results presented here do not affect anthropogenic interpretation of global and annual mean changes in observed extremes, they do show that great care is needed to assess changes due to modes of climate variability when interpreting extreme events on regional and seasonal scales. How changes in natural modes of variability, such as the NAO, could radically alter current climate model predictions of changes in extreme weather events on multidecadal time scales is also discussed. [ABSTRACT FROM AUTHOR]

The influence of the natural multidecadal variability of the Atlantic meridional overturning circulation (MOC) on European climate is investigated using a simulation with the coupled atmosphere–ocean general circulation model ECHAM5/Max Planck Institute Ocean Model (MPI-OM). The results show that Atlantic MOC fluctuations, which go along with changes in the northward heat transport, in turn affect European climate. Additionally, ensemble predictability experiments with ECHAM5/MPI-OM show that the probability density functions of surface air temperatures in the North Atlantic/European region are affected by the multidecadal variability of the large-scale oceanic circulation. Thus, some useful decadal predictability may exist in the Atlantic/European sector. [ABSTRACT FROM AUTHOR]

Operational probabilistic (ensemble) forecasts made at ECMWF during the European summer heat wave of 2003 indicate significant skill on medium (3–10 day) and monthly (10–30 day) time scales. A more general “unified” analysis of many medium-range, monthly, and seasonal forecasts confirms a high degree of probabilistic forecast skill for European temperatures over the first month. The unified analysis also identifies seasonal predictability for Europe, which is not yet realized in seasonal forecasts. Interestingly, the initial atmospheric state appears to be important even for month 2 of a coupled forecast. Seasonal coupled model forecasts capture the general level of observed European deterministic predictability associated with the persistence of anomalies. A review is made of the possibilities to improve seasonal forecasts. This includes multimodel and probabilistic techniques and the potential for “windows of opportunity” where better representation of the effects of boundary conditions (e.g., sea surface temperature and soil moisture) may improve forecasts. “Perfect coupled model” potential predictability estimates are sensitive to the coupled model used and so it is not yet possible to estimate ultimate levels of seasonal predictability. The impact of forecast information on different users with different mitigation strategies (i.e., ways of coping with a weather or climate event) is investigated. The importance of using forecast information to reduce volatility as well as reducing the expected expense is highlighted. The possibility that weather forecasts can affect the cost of mitigating actions is considered. The simplified analysis leads to different conclusions about the usefulness of forecasts that could guide decisions about the development of “end-to-end” (forecast-to-user decision) systems. [ABSTRACT FROM AUTHOR]

On seasonal time scales, ENSO prediction has become feasible in an operational framework in recent years. On decadal to multidecadal time scales, the variability of the oceanic circulation is assumed to provide a potential for climate prediction. To investigate the decadal predictability of the coupled atmosphere–ocean general circulation model (AOGCM) European Centre-Hamburg model version 5/Max Planck Institute Ocean Model (ECHAM5/MPI-OM), a 500-yr-long control integration and “perfect model” predictability experiments are analyzed. The results show that the sea surface temperatures (SSTs) of the North Atlantic, Nordic Seas, and Southern Ocean exhibit predictability on multidecadal time scales. Over the ocean, the predictability of surface air temperature (SAT) is very similar to that of SST. Over land, there is little evidence of decadal predictability of SAT except for some small maritime-influenced regions of Europe. The AOGCM produces predictable signals in lower-tropospheric temperature and precipitation over the North Atlantic, but not in sea level pressure. [ABSTRACT FROM AUTHOR]

Month-long integrations with a regional climate model covering Europe and the Northern Atlantic are utilized to study the sensitivity of the summertime European precipitation climate with respect to the continental-scale soil moisture content. Experiments are conducted for July 1990 and 1993. For each of the two months, the control experiment with the initial soil water distribution derived from the operational ECMWF analysis is compared against two sensitivity experiments with dry and wet initial soil moisture distributions. The results demonstrate that summertime European precipitation climate in a belt ;1000 km wide between the wet Atlantic and the dry Mediterranean climate heavily depends upon the soil moisture content. In this belt, changes in monthly mean precipitation amount to about half of the changes in mean evapotranspiration. Budget analysis of water substance over selected subdomains demonstrate that the simulated sensitivity cannot be interpreted with the classical recycling mechanism, that is, the surplus of precipitation that falls over wet (as compared to dry) soils does not directly derive from evapotranspiration. Rather, the surplus of precipitation primarily originates from water vapor extracted from the ambient atmospheric flow. Thus, the soil--precipitation feedback must rely on some indirect mechanism, whereby wet soils increase the efficiency of convective precipitation processes. In order to isolate the physical mechanisms underlying the soil--precipitation feedback, a detailed analysis including an investigation of the mean diurnal cycle throughout the integration period is performed. The key elements of the feedback are the following. First, wet soils (small Bowen ratios) imply the buildup of a comparatively shallow boundary layer. The surface fluxes of heat and moisture are thus concentrated into a comparatively small volume of air, leading to the buildup of high values of low-level moist entropy, thereby providing a... [ABSTRACT FROM AUTHOR]

Extratropical weather systems are an essential feature of the midlatitude climate and global circulation. At the last glacial maximum (LGM), the formation of regions of high transient activity, referred to as "storm tracks," is strongly affected by the presence of large ice sheets over northern America and Scandinavia and by differences in sea surface temperature (SST) distributions. In the framework of the Palaeoclimate Modelling Intercomparison Project, simulations of the LGM climate have been run with a wide range of atmospheric general circulation models (AGCMs) using the same set of boundary conditions, allowing a valuable comparison between simulations of a climate very different from the present one. In this study, the authors focus on the storm track representation in the models and its relationship with the surface temperatures, the mean flow, and the precipitation. Storm tracks are described using transient eddy diagnostics such as mean sea level pressure variance and three-dimensional E vectors, computed from daily output. It is found that the general response to the changes in boundary conditions from present day to LGM is consistent for all models: they nearly all give an eastward shift for both storm tracks, with a larger shift for the Atlantic one. This is intrinsically linked to changes in stationary waves, which is also studied using the E vector diagnostic. Differences between the models reside in the value of the shift of the storm tracks and the change in their amplitude, which the authors analyze in terms of differences in resolution and parameterizations in the models. The sensitivity of the storm tracks to the sea surface temperatures and sea-ice extent are also examined by comparing the differences between prescribed and computed SST simulations. All in all, it is the eastern part of the storm tracks that is found to be most model-dependent, which relates to differences in the simulated climates over America's west... [ABSTRACT FROM AUTHOR]