How to cite: Vaittinada Ayar, P., Battisti, D., Li, C., King, M., Vrac, M., and Tjiputra, J.: A regime view of ENSO flavours through clustering in CMIP6 models, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-427, https://doi.org/10.5194/ems2023-427, 2023.

Circulation modes undergo substantial decadal variability, which also affects spatiotemporal impact of modes on climatic variables on long time scales. Previous studies focused either to one circulation mode or to limited geographical area. Here, the topic is substantially extended as we provide an overview of long-term variations of nine circulation modes in winter and their impact on relationships eith temperaure during the 20^th century over the Northern Extratropics. Circulation modes are identified by rotated Principal Component Analysis of 500 hPa geopotential heights in the ERA-20C reanalysis; gridded surface climatic data are gained from the CRUTS dataset. Temporal variations of relationships are evaluated by 15-year running correlations between circulation modes and climatic variables at all land gridpoints. Time series of running correlations with all nine circulation modes at all gridpoints are clustered using the partitioning around medoids (PAM) method into 18 clusters for temperature. Both composite maps and temperature anomalies in the 850 height during specific periods of strengthened, weakened, and normal relationships with climatic variables are used for determination of mechanisms responsible for the variation of relationships. The main mechanisms are changes in the location, shape, and intensity of centres, and formation or split of centres. These mechanisms affect mainly the intensity and direction of advection, which translate into the magnitude of relationships. Possible causes of changes in circulation modes are linked with El Niño‐Southern Oscillation (ENSO), shifts of the Pacific Decadal Oscillation (PDO), and long-term changes in the sea ice extent. The results of this study may be used for climate modelling.

How to cite: Hynčica, M. and Huth, R.: Temporal variation of relationships between circulation modes and temperature during the 20th century, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-491, https://doi.org/10.5194/ems2023-491, 2023.

General circulation models (GCMs) are effective tools to simulate the climate system and to assess its changings. Since large-scale atmospheric teleconnections are the imprints of the internal variability of the climate system, the evaluation of the GCMs concerning teleconnections became an important task in the field of climatology. Previously, a negative extrema-based method was applied on gridded 500 hPa geopotential height time series in the boreal wintertime. First, the Pearson correlations were calculated between all time series. Then, the strongest negative correlation was selected in each grid cell. Finally, the pairs of grid cells were selected which have the same associated negative correlation values. Based on the ECMWF’s ERA-20C and the NCEP/NCAR Reanalysis 1 datasets, it was found that, these pairs of grid cells are mostly located in a north‑south direction, surrounding the Northern Hemisphere in two belts. This means that the geopotential height in the two belts changes simultaneously in opposite directions. The two belts were represented by local polynomial regression (LOESS) curves. It was pointed out that the two belts can be detected in the first realizations of all available historical GCM outputs of the Coupled Model Intercomparison Project Phase 5 (CMIP5), but the courses of the LOESS curves were not captured well by some of the GCMs compared to the reanalyses, especially in case of the southerly located belt. However, different GCM realizations may produce significantly different LOESS curves. Therefore, the aim of this study is to quantify the differences of the LOESS curves obtained from multiple realizations of the GCMs disseminated by the Coupled Model Intercomparison Project Phase 6 (CMIP6). The future goal is to use LOESS curves to detect changes in the climate system, i.e. shifts of the oppositely varying belts.

How to cite: Kristóf, E.: Local polynomial regression-based evaluation of general circulation models regarding teleconnections in boreal wintertime, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-431, https://doi.org/10.5194/ems2023-431, 2023.

Weather conditions, including the threat of extreme weather, in the temperate zone are mainly related to the atmospheric circulation, including the presence of atmospheric fronts.

Despite many studies conducted on the distinction of atmospheric fronts in recent years, there is still no entirely satisfactory objective method of their discrimination with respect to traditional synoptic maps. Here we propose a method of distinguishing atmospheric fronts based on cluster analysis methodology treating border zones between air masses as fronts and testing the output against operational analyses

The basic data used for the calculations were meteorological fields taken from the ERA5 database for the period 1951-2020 in hourly time steps across a domain given by coordinates 30°W to 50°E and 30°N to 80°N. The following elements were taken into account: temperature, specific and relative humidity, geopotential height on pressure levels from 925 to 700 hPa as well as temperature and wind speed near the ground.

Using methods such as k-means and dbscan, division into 3, 4 and 5 clusters was conducted over Europe. This analysis was conducted for the whole period, as well as for specific months and seasons, and included considerations of spatial diversity and temporal changes in different regions of the domain.

Validation of the results, including comparative studies with traditional synoptic maps from DWD and the Met Office, confirms the 3-classes distinction of air masses as the most reliable. The proposed method seems to be effective and self-dependent (contrary to, for example, supervised machine learning methods). This approach enables a method to be developed allowing calculations of severity of fronts and trends in the behaviour of fronts with time.

How to cite: Wypych, A., Bochenek, B., Ustrnul, Z., and Suri, D.: Atmospheric fronts detection over Europe – methodological approach, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-510, https://doi.org/10.5194/ems2023-510, 2023.

The focus of this contribution is on one particular aspect of short-term atmospheric variability, namely, the day-to-day temperature difference (DTD). Large DTDs negatively affect human health and impact also animals and plants, hence posing risk to society and asking for being thoroughly investigated.

Similar to other climate variables, risks are related to tails of DTD distributions, that is, to large day-to-day temperature drops and rises. It is known that the distribution of DTDT changes in central Europe is asymmetrical: large temperature increases prevail over large decreases in winter, and vice versa in summer. The opposite is true for small DTDT changes: in winter, small decreases prevail over small increases. However, unlike for temperature, mechanisms causing large DTDs and their asymmetry have only been hypothesized but not investigated in sufficient detail on scales larger than local.

In this contribution, we analyze two mechanisms that have been hypothesized to stay behind the asymmetry of DTD distributions. The first mechanism is passages of atmospheric fronts, which contribute to the asymmetry in large DTD changes: cold front passages are responsible for more frequent and stronger temperature drops than increases in summer, while warm front passages cause temperature increases to occur more often than drops in winter. To this end, we employ objective frontal analysis on the European scale. The second mechanism consists in circulation conditions conducive to radiative warming in summer or radiative cooling in winter, which govern the asymmetry in small DTD changes. The circulation conditions are described by classifications of atmospheric circulation patterns performed separately for individual gridpoints; anticyclonic types and types with warm advection in south and southwest direction contribute to the DTD asymmetry in summer, while anticyclonic types and types with cold northerly and northeasterly advection contribute to the DTD asymmetry in winter.

How to cite: Huth, R., Stryhal, J., Navrátilová, D., Kašpar, M., and Krauskopf, T.: Mechanisms governing asymmetry of day-to-day temperature changes, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-72, https://doi.org/10.5194/ems2023-72, 2023.

Days, when local heavy rainfall occur are often associated with characteristic large scale circulation patterns as a small part of the entire spectrum of the weather variability. To better understand the essence of the synoptical weather variability in observations and simulations, reanalyses, seasonal forecasts and climate projections have been explored in order to identify critical patterns, existing rules and long-term changes. As a study region serves Jordan located in the Eastern Mediterranean region, where heavy rainfall in the winter-half year can trigger flash floods in urban areas (Amman) or wadi systems (Petra). There is a urgent need to better estimate such weather related risks under climate change beyond the mean temperature rise. For this purpose we calculated composite patterns of the synoptical conditions for dates where the local amount of rainfall exceeded the 99th percentile.  The results of this study show probably only one side the medal. And this regards a rather intensification of extreme rainfall events associated with critical circulation patterns dependent on the global warming level across models. However, the recurring frequency of such patterns is widely regressive in the scenario simulations of the CMIP6 ensemble against our first expectation. According to the results, such extreme events at that location becoming rarer and stronger. Whether models overestimate the decline in frequency cannot be fully answered. However, retrospective analyses of ERA5 renalysis data indicate possible discrepancies and limitations of climate model scenarios beyond long-term trends in temperature indicators. A shift in the seasonality of extreme rainfall patterns are highly effected by the prevailing circulation patterns and thus need to be assessed with care.

How to cite: Hoffmann, P.: A contextualization of heavy rainfall events in climate forecasts: A case study for Amman in Jordan, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-554, https://doi.org/10.5194/ems2023-554, 2023.

Weather and/or circulation type classification has engaged meteorologists and climatologists for a long time. Classification usually means to group something that can appear as complex and chaotic into something discrete, structured and easy to understand based on characteristic features of the process to be grouped. In atmospheric sciences classifications have a long tradition, and got a renaissance in the 1990’ies as helpful tools to understand the impacts of a changing climate. This required a transition from the traditional manual and subjective classifications such as the German Grosswetterlagen and the British Lamb Weather Types to objective and automatic methods that could run on computers, and a large number of classifications were developed using the opportunities given by computers and tools offered by statistical software. The high number of different methods applied for classification of circulation types implied challenges to the meteorological-climatological communities since most circulation type classifications were adapted to a specific region, and not necessarily easily transferable to another region and they were focused on a specific (environmental) problem, and the spatial and temporal scale are adapted to this purpose. Comparison and evaluation of  the various weather types classifications per se as well as the applications using these classifications was therefore difficult. 

COST Action 733 was initiated aiming to meet the need for a coordinated European effort to produce one or a few standard methods designed to facilitate such comparisons, and to establish a unified, transferable approach to synoptic classifications. COST733 was active between 2005 and 2010, collecting, evaluating and comparing a wide range of synoptic classification methods for various applications. It was a large network of more than 60 scientists. During the course of the actions more than 80 scientific papers were published. 

The main outcome and contribution to the scientific community of the action was however the jointly developed cost733class-software that includes the circulation classification methods selected for evaluation and comparison. This software has been widely used by a large number of scientists world wide over the last 10 years, and has set the standard for  addressing classification of weather types.  During the last 5 years COST733 has been mentioned by more than 300 scientific contributions. In this presentation we will take a short  retrospect of COST733 and look at its role as an inspiration and tool for continued research involving circulation type classifications.

How to cite: Tveito, O. E.: The footprint of COST733, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-588, https://doi.org/10.5194/ems2023-588, 2023.

Self-organizing maps (SOMs) represent an increasingly popular method in climatology. They are able to reduce very complex atmospheric circulation data into a two-dimensional array of patterns, or provide a topologically-ordered nonlinear classification of high-dimensional data. As such, their output combines weather/circulation types with exploratory projection / dimensionality reduction, that is, two approaches to feature extraction that are usually conceptualized and interpreted in very different ways and studied by sets of rather distinct methods. As a consequence to this complexity of SOMs, they have been interpreted not only as weather/circulation types, but, alternatively and less intuitively, the array has also been suggested to represent a continuum of teleconnections. The distinct characteristics of SOMs, in addition to the absence of a widely accepted framework for defining teleconnections and a standard set of terminologies, can potentially cause confusion and misinterpretations when comparing works of authors with different perspectives. This is particularly true when reading the conclusions of studies without a thorough understanding of the context. Our contribution briefly introduces SOMs and their relation to circulation types, circulation regimes, modes of variability, and teleconnections. We use synthetic datasets generated from idealized modes of variability and from main modes of Euro-Atlantic atmospheric circulation variability extracted from daily SLP fields by a rotated principal component analysis to demonstrate how SOM arrays capture the spatial patterns of modes as well as changes in the strength and phase of modes. Additionally, we explore the possibility of using linear feature-extraction tools such as PCA to train high-quality SOMs more efficiently.

How to cite: Stryhal, J., Beranová, R., and Huth, R.: On using self-organizing maps in analyses of circulation types and circulation modes, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-515, https://doi.org/10.5194/ems2023-515, 2023.

The 29 circulation types by Hess & Brezowsky, called “Großwetterlagen”, are one of the most established classification schemes of the large-scale atmospheric circulation influencing Europe. They are widely used in order to assess linkages between atmospheric forcing and surface conditions e.g. extreme events like floods or heat waves. Because of the connection between driving circulation type and extreme event, it is of high interest to understand future changes in the occurrence of circulation types in the context of climate change. Even though the “Großwetterlagen” have been commonly used in conjunction with historical data, only very few studies examine future trends in the frequency distribution of these circulation types using climate models. Potential reasons for this, are the lack of an open-source classification method of the “Großwetterlagen” and their high range of internal variability. Due to the dynamic nature of the large-scale atmospheric circulation in the mid-latitudes, it is highly relevant to consider the range of internal variability when studying future changes in circulation patterns and to separate the climate change signal from noise.

We have therefore developed an open-source, automated method for classifying the “Großwetterlagen” using deep learning. We apply this method to the SMHI-LE, an initial-condition single-model large ensemble of the CMIP6 generation with 50 members on a daily resolution. A convolutional neural network has been trained to classify the circulation patterns using the atmospheric variables sea level pressure and geopotential height at 500 hPa at 5° resolution. The convolutional neural network is trained for this supervised classification task with a long-term historical record of the “Großwetterlagen” covering the 20^th century. It is derived from a subjective catalog of the German Weather Service with daily class affiliations and atmospheric variables from ECMWFs’ reanalysis dataset of the 20th century, ERA-20C.

We present the challenges of the deep learning-based classification of subjectively defined circulation types and quantify the uncertainty range intrinsic to deep neural networks using deep ensembles. We furthermore demonstrate the benefits of this automated classification of “Großwetterlagen” with respect to the application to large datasets of climate model ensembles. Our results show the ensemble-averaged future trends in the occurrence of “Großwetterlagen” and the range of internal variability, including the signal-to-noise ratio, for the CMIP6 SMHI-LE under the SSP37.0 scenario.

How to cite: Mittermeier, M., Weigert, M., Rügamer, D., Küchenhoff, H., and Ludwig, R.: Deep learning-based classification of atmospheric circulation types over Europe in a CMIP6 Large Ensemble, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-327, https://doi.org/10.5194/ems2023-327, 2023.

Weather type classification in Croatia has been based on subjective Poje’s classification since 1965. In the first stage of the analysis the results of the COST733 action “Harmonization and Applications of Weather Types Classifications for European Regions” were applied to the Croatian area to improve previous knowledge and practice, and to make the whole classification process automatized. Based on the obtained results and conclusions from the first stage, analysis in the second stage is done by running COST733class software for different settings. This analysis aims to determine the most suitable combination of main classification parameters such as domain size, number of types, input variables and classification methods to develop a classification that is intended to be the most appropriate to capture precipitation conditions in Croatian regions. The assessment of evaluation results takes into account the effect of varying numbers of types and specifies temporal (among seasons) and as well spatial (among regions) variations. For this purpose, Croatia is divided into five regions to acknowledge the orography and climate diversity of the Croatian area which lead to different weather conditions within the same atmospheric circulation. Two regions belong to the continental part of Croatia (Eastern and Central Croatia), while the rest, including mountainous parts, are placed along with the Adriatic sea (North, Middle and South Adriatic). The efficacy of different objective classifications developed using COST733class is examined utilizing Croatian meteorological station data. For evaluating the discriminative power (synoptic skill) of the classifications and the relevance of specific settings several statistical metrics are used and the statistical significance of the differences in statistic metrics between classifications is tested. The results from the first stage have shown better classification performance for optimization and threshold based methods among other methods, hence we proceed with GWT (GrossWetterTypes) and DKM (dkmeans) methods in the second stage. Results for all different settings have shown better classification performance along the Adriatic coast and in the mountainous parts than in the more continental parts. Further, some preliminary results show that classifications based on DKM methods perform better when using input data with spatial-mean-removed and that domain size and position should be tailored for each input variable. Also, the improvement of classification is found when including some of the additional variables together with the main variable (usually mean sea level pressure). However, it is not clear the inclusion of which additional variable is the most relevant and statistically significant, and how many of them should be included at the same time.

How to cite: Marinovic, I. and Beck, C.: Development and evaluation of new weather types classification for Croatia, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-420, https://doi.org/10.5194/ems2023-420, 2023.

Everyday weather conditions and meteorological situations at a given point or area on the earth's surface are primarily a function of the atmospheric circulation conditions above it. On monthly, seasonal or annual basis these daily conditions form atmospheric circulation in climatological aspect. Many climatological studies are dedicated to the baric formations and their positions relative to a given area. Thus, cyclones and anticyclones strongly affect the weather conditions through the values of the variety of the meteorological elements as the wind speed and direction, temperature and precipitation quantities. The multi-year fluctuations of the circulation also shape it from a climatic point of view. The classification of circulation types over Bulgaria in present work is made on daily basis using subjective (manual) approach, covers 150-year period (1871-2020) and is based on the data of two atmospheric levels – mean sea level pressure (SLP) and 500 hPa in the middle troposphere, derived from “20th century reanalysis” visualization maps. The circulation types are composed as the position of the center of the baric formations relative to the territory of Bulgaria. The methods used in composing the atmospheric classification and the circulation types are explained in detail in a previous study. 13 atmospheric circulation types are separated at 500 hPa level (5 anti-cyclonic and 8 cyclonic) and 16 at the surface (6 anti-cyclonic, 8 cyclonic and 2 low gradient). Annual and seasonal frequencies as well as seasonal distribution of all circulation types in the examined period at both levels are presented and analyzed. Some of the types are united in order to emphasize their climatological impact. Trend analyzes are performed on all 29 circulation types and on some type combinations, as for this aim the methods of Man-Kendall and Sen Slope estimator are used. A special attention is paid to the high throws and cyclones at 500 hPa and their major influence on surface cyclogenesis and respectively to the precipitation regime in different parts of the country. Some other suggestions are proposed and conclusions are made, as the increased influence, at least concerning the region of Balkan Peninsula of the Azores anticyclone in the last four decades.

How to cite: Pophristov, V.: Frequencies, Variations and Trends in Atmospheric Circulation Types over Bulgaria and their climatological impacts in the last 150 years, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-161, https://doi.org/10.5194/ems2023-161, 2023.

High-latitude cyclones in the Southern Ocean can reach and shape the Antarctic sea-ice. Rare in-situ measurements from a winter cyclone in the marginal ice zone evidence how it caused both significant wave activity and large atmospheric temperature and moisture anomalies. We investigate here with the help of reanalysis data, whether cyclones may routinely lead to the presence of unusually warm, moist air masses over ice-covered regions in the Southern Ocean. We find that extreme atmospheric anomalies over sea ice often occur in the absence of cyclones, and that intense cyclones have a stronger association with extreme temperature anomalies than extreme moisture anomalies. This points to a nuanced link between high-latitude Southern Ocean cyclones and atmospheric anomalies over Antarctic sea ice.  We next consider whether cyclones may also lead to variability in winter Antarctic sea-ice concentration. We characterise sea-ice variability and cyclone activity in different Southern Ocean sectors using atmospheric reanalyses and a cyclone-tracking algorithm, and then quantify the proportion of extreme sea-ice variability engendered by cyclones of different intensity. We conclude that there is a significant link between variability in winter sea-ice concentration and: (1) all cyclones in the Ross/Amundsen sector; (2) all but the weakest cyclones in the King Haakon VII, East Antarctic, and Bellinghausen sectors; and (3) all but the most intense cyclones in the Weddell sector. More generally, up to 40% of episodes of extreme sea-ice variability are caused by extratropical cyclones within all regions apart from the Weddell sector, (where extreme sea-ice variability is less connected to the fewer intense cyclones and more to the weaker cyclones).

How to cite: Messori, G., Hepworth, E., and Vichi, M.: Southern Ocean polar cyclones drive extreme atmospheric anomalies over Antarctic sea ice, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-632, https://doi.org/10.5194/ems2023-632, 2023.

The weather and climate conditions of the Mediterranean region is greatly influenced by mid-latitude cyclones. The region is considered as one of the most vulnerable areas to climate change, and Mediterranean cyclones and their characteristics might change in the future. We selected the western part of the Mediterranean region as our study area. In this area we aim to identify cyclones and get an understanding of their characteristics on the basis of ECMWF reanalysis data and the historical simulations of global climate models from the CMIP6 project. In the study we identify the potential areas of low pressure systems based on mean sea level pressure, with temporal resolution of 6 hours, and then determine their trajectories. Mediterranean cyclones affect other parts of Europe, including Hungary and its vicinity. We aim to analyze what portion of the annual precipitation is connected to Mediterranean cyclones and their frontal systems, and how it will change in the future. For this purpose, we use precipitation data from the above-mentioned databases. This study is focused on the present and historical conditions. Our findings are validated by comparing the results of reanalysis and CMIP6-simulation data. The study continues with the evaluation of future trends based on different climate scenarios (RCP and/or SSP scenarios upon availability), which represent different anthropogenic impacts and mitigation efforts. The results can serve as essential input for further impact studies, and then for decision makers and stakeholders when building the long-term strategies of their corresponding sectors.

Acknowledgements: The study contributes to the COST CA19109 action (MEDCYCLONES). Research leading to this study has been supported by the following sources: the Hungarian National Research, Development and Innovation Fund (under grants K-120605 and K-129162), and the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014).

How to cite: Dolgos, E., Pongrácz, R., and Bartholy, J.: Analysis of Western Mediterranean cyclones and their precipitation zones, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-516, https://doi.org/10.5194/ems2023-516, 2023.