Mischa Croci-Maspoli

Extreme Cold Winter Temperatures in Europe under the Influence of North Atlantic Atmospheric Blocking

North Atlantic atmospheric blocking conditions explain part of the winter climate variability in Europe, being associated with anomalous cold winter temperatures. In this study, the generalized extreme value (GEV) distribution isfitted tomonthlyminimaof Europeanwinter6-hourlyminimumtemperatures fromthe ECHAM5/MPI-OM global climate model simulations and the ECMWF reanalysis product known as ERA40, with an indicator for atmospheric blocking conditions being used as covariate. It is demonstrated that relating the location and scale parameter of the GEV distribution to atmospheric blocking improves the fit to extreme minimum temperatures in large areas of Europe. The climate model simulations agree reasonably with ERA-40 in the present climate (1961‐2000). Under the influence of atmospheric blocking, a decrease in the 0.95th quantiles of extreme minimum temperatures can be distinguished. This cooling effect of atmospheric blocking is, however, diminished in future climate simulations because of a shift in blocking location, and thus reduces the chances of very cold winters in northeastern parts of Europe.

A Multifaceted Climatology of Atmospheric Blocking and Its Recent Linear Trend

Abstract A dynamically based climatology is derived for Northern Hemisphere atmospheric blocking events. Blocks are viewed as large amplitude, long-lasting, and negative potential vorticity (PV) anomalies located beneath the dynamical tropopause. The derived climatology [based on the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40)] provides a concise, coherent, and illuminating description of the main physical characteristics of blocks and the accompanying linear trends. The latitude–longitude distribution of blocking frequency captures the standard bimodal geographical distribution with major peaks over the North Atlantic and eastern North Pacific in all four seasons. The accompanying pattern for the age distribution, the genesis–lysis regions, and the track of blocks reveals that 1) younger blocks (1–4 days) are more prevalent at lower latitudes whereas significantly older blocks (up to 12 days) are located at higher latitudes; 2) genesis is confined predominantly t...

Key Dynamical Features of the 2005/06 European Winter

Abstract A three-part study of the anomalously cold European winter of 2005/06 is undertaken. Climatological analysis indicates that the dominant pattern of climate variability in the Euro–Atlantic sector during this winter was not a negative phase of the North Atlantic Oscillation (NAO), but a pattern with a “blocklike” center located immediately upstream of the continent. Synoptic-dynamical diagnosis of the winter indicates the frequent occurrence of long-lasting blocks in this region, and a Lagrangian trajectory analysis points to the significant role of cloud-diabatic effects in the dynamics of block inception. A series of heuristic numerical simulations lend credence to the hypothesis that the occurrence of the blocks was sensitive to, and significantly influenced by, the warm surface temperature anomalies upstream over the western Atlantic Ocean and North America. Brief comments are made on the significance of the foregoing results for seasonal numerical weather prediction and also their relevance t...

Linkage of atmospheric blocks and synoptic-scale Rossby waves : a climatological analysis

The link between atmospheric blocking and propagating and breaking synoptic-scale Rossby waves (termed PV streamers) are explored for the climatological period 1958–2002, using the ERA-40 re-analysis data. To this end, potential vorticity (PV) based climatologies of blocking and breaking waves are used, and features of the propagating waves is extracted from Hovmöller diagrams. The analyses cover the Northern Hemisphere during winter, and they are carried out for the Atlantic and Pacific basins separately. The results show statistically significant wave precursor signals, up to 5 d prior to the blocking onset. In the Atlantic, the precursor signal takes the form of a coherent wave train, emanating approximately 110◦ upstream of the blocking location. In the Pacific, a single long-lived (10 d) northerly velocity signal preludes the blocking onset. A spatial analysis is conducted of the location, frequency and form of breaking synoptic-scale Rossby waves, prior to the onset, during the lifetime and after the blocking decay. It reveals that cyclonic streamers are present to the southwest and anticyclonic streamers to the south and southeast, approximately 43% (36%) of the time in the Atlantic (Pacific) basin, and this is significantly above a climatological distribution.

Emerging trends in heavy precipitation and hot temperature extremes in Switzerland

Changes in intensity and frequency of daily heavy precipitation and hot temperature extremes are analyzed in Swiss observations for the years 1901–2014/2015. A spatial pooling of temperature and precipitation stations is applied to analyze the emergence of trends. Over 90% of the series show increases in heavy precipitation intensity, expressed as annual maximum daily precipitation (mean change: +10.4% 100 years−1; 31% significant, p < 0.05) and in heavy precipitation frequency, expressed as the number of events greater than the 99th percentile of daily precipitation (mean change: +26.5% 100 years−1; 35% significant, p < 0.05). The intensity of heavy precipitation increases on average by 7.7% K−1 smoothed Swiss annual mean temperature, a value close to the Clausius-Clapeyron scaling. The hottest day and week of the year have warmed by 1.6 K to 2.3 K depending on the region, while the Swiss annual mean temperature increased by 1.9 K. The frequency of very hot days exceeding the 99th percentile of daily maximum temperature has more than tripled. Despite considerable local internal variability, increasing trends in heavy precipitation and hot temperature extremes are now found at most Swiss stations. The identified trends are unlikely to be random and are consistent with climate model projections, with theoretical understanding of a human-induced change in the energy budget and water cycle and with detection and attribution studies of extremes on larger scales.

Key climate indices in Switzerland; expected changes in a future climate

Climate indices facilitate the interpretation of expected climate change impacts for many sectors in society, economy, and ecology. The new localized data set of climatic change signals for temperature and precipitation presented by Zubler et al. (Clim Change, 2013) is applied for an analysis of frequently used climate indices in Switzerland. The indices considered are: number of summer days and tropical nights, growing season length, number of frost days and ice days, heating and cooling degree days, and the number of days with fresh snow. For the future periods 2020-49, 2045-74 and 2070–2099 the indices are computed using a delta-change approach based on the reference period 1980–2009 for the emission scenarios A1B, A2, and RCP3PD. The scenario data suggest the following relevant findings: (1) a doubling of the number of summer days by the end of the century under the scenarios A1B and A2, (2) an appearance of tropical nights even above 1500 m asl, (3) a possible reduction of the number of frost days by more than 3 months at altitudes higher than 2500 m asl, (4) a decline of heating degree days by about 30 % until the end of the century, and (5) the near disappearance of days with fresh snow at low altitudes. It is also shown that the end-of-the-century projections of all indices strongly depend on the chosen emission scenario.

Climate change and circulation types in the Alpine region

The frequency of circulation types over the Alpine region is explored using 20 different global and regional climate model chains. The projected changes in these circulation types are investigated for the 21st century using the SRES A1B scenario. The multi-model approach relies on the climate models from the ENSEMBLES project and circulation type classifications provided by the COST Action 733. For the latter, the two circulation type classifications GWT (Grosswetter-types) and CAP (Cluster Analysis of Principal components) are selected. GWT is applied to sea level pressure and geopotential height at 500 hPa whereas CAP is applied to sea level pressure. Overall, the ensemble of climate models captures the frequency of individual circulation types well, as shown by the comparison of circulation types from climate models and re-analysis data between 1980 and 2009. Discrepancies occur during winter (DJF) when westerlies are overestimated both at the sea level and at the 500 hPa geopotential height level. The model spread is largest during summer. The frequency of circulation types is simulated best during spring and autumn irrespective of the applied circulation type classification. The analysis of circulation types in the climate projections indicates that in winter easterlies are expected to decrease mostly at the benefit of westerlies until the end of the 21st century. In summer projected changes depend on the height level considered. At sea level westerlies are projected to decrease while easterlies increase markedly in their frequency. This change is not occurring on the 500 hPa geopotential height level.

Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim

AbstractThis paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zurich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first applicati...