The subseasonal variability of the extratropical large-scale atmospheric flow is characterized by recurrent or quasi-stationary circulation anomalies, termed weather regimes. Despite the usefulness of these regimes in numerous meteorological and socioeconomic applications, there is an ongoing debate as to whether they represent physical modes of the atmosphere, or are merely useful statistical categorizations. Here, we answer this question for wintertime Atlantic-European regimes. We argue that dynamical systems theory applied to a refined regime definition provides strong evidence in support of most weather regimes being physically meaningful. This finding underpins the broad relevance of weather regimes, for understanding the response of the atmosphere to external forcing, supporting subseasonal weather forecasting, and down scaling of climate projections.

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How to cite: Hochman, A., Messori, G., Quinitng, J., Pinto, J. G., and Grams, C.: Do Atlantic-European Weather Regimes Physically Exist?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4439, https://doi.org/10.5194/egusphere-egu22-4439, 2022.

Automated classifications of atmospheric circulations are a well-known tool to characterize large-scale patterns that predominantly determine day-to-day weather variations. Through its potential influence on the relative frequency of circulation patterns, global warming can also enhance or mitigate the occurrence of extreme weather events.

Here, we use a subset of 22 CMIP6  global climate models (GCMs) to assess their ability to capture these recurrent circulation patterns and their implication for the European climate and its projected changes.

We investigate links between synoptic circulations and short-term meteorological drought events that span one month. We employ the automated Jenkinson-Collison classification to determine daily atmospheric features based on mean sea-level pressure. We compute the conditional probability of dry days related to each circulation type. Furthermore, we confirm the influence of these patterns on the occurrence of dry months by computing the monthly relative frequency anomalies of the synoptic circulations given months where the Standardized Precipitation Index (SPI) was below minus 1. We evaluate the ability of the historical runs of global climate models (GCMs) to reproduce the observed features from the ERA5 reanalyses over the 1961-1990 reference period. Links between the mean directional flow characteristics of the circulation types and the dry days and months are well represented by most GCMs. The most robust relationships were found for the anticyclonic, easterly, and low flow types. These circulations are generally associated with a lack of precipitation and therefore show higher than average occurrences during dry months. 

How to cite: Herrera Lormendez, P., John, A., Douville, H., and Matschullat, J.: A CMIP6 evaluation of summer synoptic circulations linked to short-term droughts over Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4665, https://doi.org/10.5194/egusphere-egu22-4665, 2022.

The aim of this work is to produce a climate atmospheric circulation classification via Self-Organizing Maps (SOM) and to identify and evaluate the respective changes in the frequency of the identified regimes for GCM projections (Coupled Model Intercomparison Project phase 6, CMIP6). The main focus is to assess the ability to accurately represent the large-scale circulation over the Mediterranean and to generate atmospheric circulation regimes that can be used as an explanatory tool in multiple research fields. The classification framework uses an unsupervised learning algorithm for a low-dimensional representation of high-dimensional datasets (SOM) to identify non-linear relationships and patterns from complex spatiotemporal climatological fields. Upon the selection of the atmospheric variables and the corresponding spatial and temporal scales, the SOM framework will be applied initially to the historical period climate series of large-scale atmospheric circulation from reanalysis datasets and following to a multi-model ensemble of GCM simulations for multiple Socioeconomic Pathways (SSPs). The resulting atmospheric circulation regime catalogs will be compared and discussed in terms of the representativeness of the large-scale circulation by the GCM climate models for the Mediterranean domain. The catalogs will assess the corresponding changes in atmospheric circulation, focusing on the regimes’ frequency of occurrence, persistence, and transition probabilities

How to cite: Philippopoulos, K., Tzanis, C., Cartalis, C., Blougouras, G., and Agathangelidis, I.: SOM-based circulation types in the Mediterranean basin from reanalysis and CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12219, https://doi.org/10.5194/egusphere-egu22-12219, 2022.

Large scale circulation patterns are highly affected by midlatitude cyclones, their evolution, intensity, associated weather events (e.g., precipitation, consequent flood, etc.). The current study focuses on the western Mediterranean region where cyclones definitely play an important role in shaping the weather and climate of the area. Moreover, this region is considered as one of the most vulnerable areas to climate change, which implies the need for special attention. As the literature highlights, the geographical complexity of the Mediterranean region provides ideal conditions for cyclogenesis. Detailed analyses and a better understanding of the Mediterranean cyclones are crucial to develop proper adaptation strategies addressing future regional climate changes. For this purpose, we aim to analyze western Mediterranean cyclones on the basis of reanalysis data as reference, and then, compare the results of the historical runs of global climate models to the reference results. The analysis uses mean sea level pressure data from 1901 to 2020 with 6-hour temporal resolution, and daily precipitation in the western Mediterranean region and its vicinity. The results can serve as important input for impact modelers to further analysis and for decision makers to take into account when building long-term strategies.

How to cite: Dolgos, E., Pongrácz, R., and Bartholy, J.: Analysis of the western Mediterranean cyclone characteristics for the past decades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8162, https://doi.org/10.5194/egusphere-egu22-8162, 2022.

Based on a centenary meteorological observatory, the mean temperature and frequency of heat waves at the city of Barcelona is increasing due to global warming. The number of heat wave episodes by climatic periods of 30 years, with a specific percentile 95 (P95) of summer maximum temperature, has been increased from 23 (1951-1980) with a P95 of 31˚C to 59 days (1991-2020) with a P95 of 32.8˚C.  Therefore, it is necessary to analyse weather circulation to understand the reason and predict the possible future impacts. In this study, heat wave events for 1951-2020 have been analysed with ERA5 data, in order to classify the different synoptic patterns and each evolution. It is possible to get climate trends from past reanalysis data, but it is also interesting to predict future possibilities. Most of the previous research is focused on temperature and precipitation while the target of this study is the mesoscale and synoptic circulation. For this reason, Coordinated Regional Climate Downscaling Experiment (CORDEX) data at 0.11˚ grid mesh has been used to analyse the future scenarios compared to the historical simulations of the same source. More specifically, for this study, it was necessary to download mean sea level pressure (MSLP) and 500hPa geopotential height (GHP) data. Due to the big amount of information, it has been applied a statistical method called Principal Component Analysis (PCA) to simplify and reduce the dimensions of the sample. Afterwards, working on a cluster analysis has been necessary to classify the simplified data. The synoptic patterns remain relatively constant throughout the last climatic periods according to ERA5, being the synoptic type “Shallow Cyclone or Undetermined pressure gradient” the recurrent type to cause heat wave periods in summer. CORDEX data shows more dynamism in comparison to ERA5 reanalysis and expects to remain similar until 2100. Northwest and north advections are expected to increase by 7.3% and 6.6% respectively and lows or cyclones are expected to decrease by 8.3% according to RCP4.5 scenarios.

How to cite: Ventura, S., Villalba, G., Miró, J. R., and Peña, J. C.: Past and future climate trends focused on synoptic patterns in the northeast of the Iberian Peninsula, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8214, https://doi.org/10.5194/egusphere-egu22-8214, 2022.

Ice storms, caused by severe freezing rains and freezing drizzles are among the major weather hazards in winter. They influence seriously the air and ground traffic, disturb and even interrupt the distribution of electricity by snapping power lines or standstill the production from wind farms.

The conducted research had two main goals: conducting of an automatic classification of the weather types connected with freezing rains and freezing drizzles and assessment of the possible impact of the expected climate change tendencies on these events for the region of Bulgaria.

Data about 180 freezing precipitation events in the period 1958/59-2014/15 has been used and information for the classification has been taken from the six hourly reanalysis upper-air data for the same variables at five standard air pressure levels (1000, 925, 850, 700 and 500 hPa) from NCEP/NCAR reanalysis, which covers the same period.

Due to the large amount of the classification outputs, it was decided to focus on the classification approaches which already had given good results during a former investigation in COST 733 – these are the methods based on cluster analysis such as KMN, CKM and DKM k-means clustering.

Classifications have been made mainly for five standard air pressure levels and additional meteorological variables have been included as well. Following variables have been combined as the most important for the determination of freezing precipitation: see level pressure, air temperature at 850 hPa, the precipitation amount and the persistence of the event itself. Firstly, we have made classifications with 9, 18 and 27 classes but without including the event. We received 750 classifications, but with very low percentages of freezing precipitations among the classes.

After that we continued only with the DKM and KMN classifications. We have added the freezing precipitation events and have given different weights (from 0 till 1) of the variables and examining the behavior of the classifications. Increasing stepwise the percentage of freezing rains and freezing drizzles in the classes, we have established an upper. limit of 17 % with 206 classifications left.

After that χ2 -test has been applied to each class of these classifications in order to emphasis those classes with freezing cases which are not uniform distributed.

All used classifications have been evaluated by the means of different evaluation metrics such as frequency of the freezing precipitation cases in the classification classes, and correlation matrix of the classes. All of the classifications have been made using the software developed in COST Action 733.

The project was supported by the German Academic Exchange Service (DAAD).

How to cite: Nikolov, D. and Beck, C.: Classification of Circulation Types Associated with Freezing Precipitations in Bulgaria , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9269, https://doi.org/10.5194/egusphere-egu22-9269, 2022.

Investigation of the relationship between climatological variables and circulation patterns expressed by various classifications of circulation types is a key procedure in synoptic climatology. This study provides new insights into circulation drivers of hydroclimate based on an analysis of precipitation and soil drought in multiple parts of the Czech Republic with respect to the subjective classification of the Czech Hydrometeorological Institute and objective classification based on the flow strength, flow direction, and vorticity during the 1961–2020 period. Circulation types are investigated in regard to their contributions to the total precipitation, mean daily precipitation totals, and precipitation probability. Types with a westerly airflow and a trough over Central Europe exhibit the highest proportions in precipitation totals. Types with a cyclone over Central Europe, especially combined with a northwestern or northeastern airflow, result in the highest daily mean totals and precipitation probability. Types with a southwestern airflow transport more precipitation to western parts of the Czech Republic, while those with a northeastern airflow transport more precipitation to the eastern parts, with a slight seasonal shift in the gradient axis between winter and summer. Circulation drivers of drought episodes are investigated based on soil-drought values calculated from the SoilClim model. Increased frequencies of north-eastern anticyclonic types, and decreased frequencies of directional and cyclonic are important for the origin and course of soil-drought episodes. In addition, the suitability of both classifications for long-term trends analysis is investigated.

How to cite: Řehoř, J., Brázdil, R., Trnka, M., Lhotka, O., Balek, J., Štěpánek, P., and Zahradníček, P.: Hydroclimate variability in the Czech Republic studied through subjective and objective classifications of atmospheric circulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7522, https://doi.org/10.5194/egusphere-egu22-7522, 2022.

Seasonal cycle including day-to-day fluctuations shows great variety from region to region even among the middle/higher latitudes. Thus to know such detailed seasonal cycle for each region is the common basis for deeper understanding of (1) seasonal backgrounds of extreme meteorological or climatological events and (2) that of cultural generation leading to the cultural understanding education through the “seasonal feeling”. For example, the “seasonal feeling” of the severe winter relating to the traditional event for driving the winter away (“Fasnacht”) around Germany might be greatly reflected by the intermittent appearance of the extremely low temperature events, although the winter mean temperature there is lower only by about 3~5℃than in the southern part of the Japan Islands area.

Hamaki et al.(2018, although written in Japanese) suggested, based on the case study for 2000/01 winter, that the appearance of such events with large intraseasonal variation are greatly controlled by that of the Icelandic low. Furthermore, a case study for the same winter by Miyake and Kato (EGU2020 Assembly) pointed out that the rapid seasonal increase in the appearance frequency of extremely low temperature events there as the intraseasonal variation around mid-December, although the seasonal mean the Icelandic low already appeared in mid-October.

However, the climatological appearance features of the extremely low temperature events for longer periods should be examined further. Thus we will describe the detailed synoptic climatological features for 1971/72 to 2010/11 winters, based on the NCEP/NCAR reanalysis data. In this study, the days with daily mean surface air temperature less than or equal to -7℃ are referred to as the "extremely low temperature days".

Amplitude of the intraseasonal variation of the surface air temperature around Germany increased seasonally in association with the beginning of the nearly minimum seasonal mean temperature period (around December to February), resulting in the seasonal increase in the appearance frequency of the extremely low temperature days from around December. Such features are clearly found for the 10 winters (referred to as the "typical years") of the total 40 winters.

In the typical years, amplitude of the intrasesaonal variation increased rapidly around December (although this timing is somewhat different among the typical winters) and the persistent extremely low temperature days for about a week appeared two or three cycles in a winter. According to the case study for a typical winter (e.g., 1984/85 winter), while the temperature around Germany was relatively higher at the eastward shift phase of the Icelandic low due to the strong warm air advection by the SW-ly wind, the temperature was extremely low there when the Icelandic low was weakened and retreated westward as pointed out for the case study for 2000/2001 by Hamaki et al. (2018). It is also noted that the low corresponding to a part of the equivalent barotropic wave train lined up zonally was located around Germany in the extremely low temperature phase in 1984/85 winter.

How to cite: Kato, K., Otani, K., Miyake, C., Nagayasu, K., and Kato, H.: Synoptic climatological analysis on the appearance features of extremely low temperature days around Germany for 1971/72 to 2010/11 winters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2186, https://doi.org/10.5194/egusphere-egu22-2186, 2022.

Heat waves (HWs) can cause large socioeconomic and environmental impacts and they have become more intense and frequent across most land regions in the world since 1950s. Additionally, they will further increase in the future even if global warming stabilized at 1.5ºC. Therefore, HWs are one of the climate extremes of major concern. In this study we obtain for the first time a regional catalogue of warm season (WS: October – March). HW events for southern South America (approximately 17–60ºS, 35–80ºW) during 1979 – 2018 using maximum temperature data from 137 conventional weather stations. Moreover, we assess the synoptic patterns associated with regional HWs from daily data of the ERA5 reanalysis.

By applying hierarchical clustering of local HWs at the weather stations, we identify five regions based on stations with high co-occurrence of simultaneous HWs. Regional HW events are defined as WS periods of at least three consecutive days with maximum temperature exceeding the 90^th percentile of the local T_Max distribution on more than 40% of the stations of each region. We also calculate the accumulative maximum temperature anomaly to rank the magnitude of regional HW events.

For all regions, HW events are associated with significant and co-located positive 500 hPa geopotential height (z500) anomalies and with the intensification of the Northwestern Argentinean Low (NAL, The exception is the southernmost region, where HW events are linked to barotropic extratropical systems blocking the westerly flow. HW events affecting northern and central-eastern transitional regions are also associated with positive outgoing longwave radiation (OLR) anomalies, pointing to enhanced subsidence by intensification of the South Atlantic convergence zone (SACZ), as well as an intensification of the northerly low-level flow associated with South American Low-Level Jet (SALLJ) events. The results suggest that small changes in the magnitude and / or location of the aforementioned signatures can shape HWs over different regions of the domain. Finally, the frequency of regional HW days shows a significant increase over central and northern Chile, central Argentina, and northern Argentinian Patagonia. On the whole, except for the two southernmost regions, the most severe regional events have occurred during the 21st century.

How to cite: Suli, S., Barripedro, D., García-Herrera, R., and Rusticucci, M.: Characterization of regional heatwaves in southern South America, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2598, https://doi.org/10.5194/egusphere-egu22-2598, 2022.

The aridity of the Southern Hemisphere subtropical latitudes is frequently punctuated by near-planetary scale bands of cloud that diagonally link the tropics to the mid-latitudes. These cloud bands preferentially occur in the subtropical convergence zones situated in each of the three ocean basins. However, much of the research focused on these regions has considered seasonal averages, which are not suitable for fully capturing the synoptic-scale variability of these events. The MetBot, first developed to identify cloud bands over southern Africa and now successfully adapted to the South American area, overcomes this problem by identifying these tropical-extratropical cloud bands in daily data. In this study, we use the near-real-time version of the MetBot, currently implemented to monitor the progress of cloudband seasons over the Southern Hemisphere. Cloud bands are identified in the daily outgoing longwave radiation data from the High-Resolution Infrared Radiation Sounder (HIRS), developed by the National Oceanic and Atmospheric Administration (NOAA), with a spatial resolution of 1° and latency time of a couple of days (Lee, 2011). The results of the near-real-time monitoring area are available at <https://hart-ncg.github.io/real-time/about.html>. Here, we considered the cloud bands dataset with events identified across the Southern Hemisphere between 1979 and 2020 to evaluate their variability and recent changes. The climatological analysis highlights four hotspot regions: South Pacific and South America, which are the strongest; southern Africa, with events only forming through austral summer; and Australia, with summer (winter) events over the eastern (western) coast. The hemispheric scale of the results provides a unified view of the interannual and intraseasonal variability of the cloud bands, including the preferential location of the convective activity during different ENSO phases. By isolating the systems responsible for most of the rainy season precipitation over the subtropics, we also identify changes in the cloud band activity in recent years and their impact on the total precipitation. Future applications include the use in tandem with S2S forecast data, as currently being implemented over South America, as well as a tool to evaluate historical and future climate model simulations of various resolutions.  

Lee, Hai-Tien and NOAA CDR Program (2011): NOAA Climate Data Record (CDR) of Daily Outgoing Longwave Radiation (OLR), Version 1.2. NOAA National Climatic Data Center. doi:10.7289/V5SJ1HH2 

How to cite: Zilli, M. and Hart, N.: Variability and changes of tropical-extratropical cloudband events over the Southern Hemisphere using a synoptic climatology approach , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12800, https://doi.org/10.5194/egusphere-egu22-12800, 2022.