Description: The role of the large-scale atmospheric circulation in producing heavy rainfall events and floods in the eastern part of Europe, with a special focus on the Siret and Prut catchment areas (Romania), is analyzed in this study. Moreover, a detailed analysis of the socio-economic impacts of the most extreme flood events (e.g., July 2008, June–July 2010, and June 2020) is given. Analysis of the largest flood events indicates that the flood peaks have been preceded up to 6 days in advance by intrusions of high Potential Vorticity (PV) anomalies toward the southeastern part of Europe, persistent cut-off lows over the analyzed region, and increased water vapor transport over the catchment areas of Siret and Prut Rivers. The vertically integrated water vapor transport prior to the flood peak exceeds 300 kg m-1 s-1, leading to heavy rainfall events. We also show that the implementation of the Flood Management Plan in Romania had positive results during the 2020 flood event compared with the other flood events, when the authorities took several precaution measurements that mitigated in a better way the socio-economic impact and risks of the flood event. The results presented in this study offer new insights regarding the importance of large-scale atmospheric circulation and water vapor transport as drivers of extreme flooding in the eastern part of Europe and could lead to a better flood forecast and flood risk management.

Description: This paper aims to develop the first differentiated (earlywood—EW, latewood—LW, and total ring width—RW) dendrochronological series for ash (Fraxinus excelsior L.) and oak (Quercus robur L.) trees from the Republic of Moldova, and to analyze their climatic response and their spatio-temporal stability. For this, 18 ash and 26 oak trees were cored from the DobruÈ�a protected area, Republic of Moldova, Eastern Europe, and new EW, LW, and RW chronologies were developed for ash and oak covering the last century. The obtained results showed that the RW and LW have a similar climatic response for both species, while EW is capturing interannual climate variations and has a different reaction. The analyses performed with monthly climatic data revealed a significant and negative correlation with the mean air temperature and a significant and positive correlation with precipitation and the Standardized Precipitation-Evapotranspiration Index (SPEI) for both ash and oak. The temperature during the vegetation period has a strong influence on all tree-ring components of ash, while for oak the strong correlation was found only for LW. The positive and significant correlation between LW and RW with precipitation for both species, suggests that ash and oak are sensitive to the hydrological component and the precipitation is the main tree growth-limiting factor. Despite the significant correlation with precipitation and temperature for the whole analyzed period, the 25-year moving correlation analyses show that they are not stable in time and can switch from positive to negative or vice versa, while the correlation with SPEI3 drought index, which is a integration of both climatic parameters, is stable in time. By employing the stability map analysis, we show that oak and ash tree ring components, from the eastern part of the Republic of Moldova, have a stable and significant correlation with SPEI3 and scPDSI drought indices from February (January) until September, over the eastern part of Europe.

Description: The variability of stable oxygen isotope ratios (δ18O) from Greenland ice cores is commonly linked to changes in local climate and associated teleconnection patterns. In this respect, in this study we investigate ice core δ18O variability from a synoptic scale perspective to assess the potential of such records as proxies for extreme climate variability and associated weather patterns. We show that positive (negative) δ18O anomalies in three southern and central Greenland ice cores are associated with relatively high (low) Rossby Wave Breaking (RWB) activity in the North Atlantic region. Both cyclonic and anticyclonic RWB patterns associated with high δ18O show filaments of strong moisture transport from the Atlantic Ocean towards Greenland. During such events, warm and wet conditions are recorded over southern, western and central part of Greenland. In the same time the cyclonic and anticyclonic RWB patterns show enhanced southward advection of cold polar air masses on their eastern side, leading to extreme cold conditions over Europe. The association between high δ18O winters in Greenland ice cores and extremely cold winters over Europe is partly explained by the modulation of the RWB frequency by the tropical Atlantic sea surface temperature forcing, as shown in recent modeling studies. We argue that δ18O from Greenland ice cores can be used as a proxy for RWB activity in the Atlantic European region and associated extreme weather and climate anomalies.

Description: Clouds are notoriously difficult to simulate. Here, we separate and quantify the impact of Pacific climate modes on total cloud cover (TCC) variability, using reliable satellite observations together with state-of-the art reanalysis outputs, over the 1979–2020 period. The two most prominent modes of annual TCC variability show intense loadings over the Pacific basin and explain most of the variance in what could be considered the “signal” in satellite TCC data. Canonical correlation analysis (CCA) provides coupled TCC—sea surface temperature (SST) patterns that are linked to the Eastern Pacific (EP) ElNiño—Southern Oscillation (ENSO) and the Central Pacific (CP) ENSO in a physically consistent manner. The two ENSO modes dominate global coupled SST–TCC variability with the footprint of the CP ENSO explaining roughly half of the variance induced by the EP ENSO among these coupled fields. Both the EP and the CP ENSO exert an influence on Pacific decadal TCC variability. The impact of both ENSO modes on global total cloud cover variability is amplified by two positive feedbacks. These results could be used as a reference for model investigations on future projections of coupled TCC—SST variability responses to the CP and the EP ENSO.

Description: In this study, we made use of a regional oak tree-ring network from six stands that cover the northern Moldavian Plateau (eastern Europe) to analyze how different tree ring parameters (i.e., early wood tree-ring width, late wood tree-ring width, and total tree-ring width) of Quercus sp. are influenced by the occurrence of extreme climatic events (e.g., long-lasting drought events). In order to explore the influence of extreme hydroclimatic events on tree ring width, we have selected each of the six most extreme positive and negative years of tree growth and addressed the seasonal cycle of tree growth in comparison with the main climatic parameters, then evaluated both the current and lagged consequences of extreme hydroclimatic events on tree ring width and the capacity of trees to recover. Our results indicate that the variability of oak tree ring width from the Moldavian Plateau is mainly influenced by the availability of water resources, and that an important limiting growth factor for Quercus sp. is the occurrence of long-lasting drought events, e.g., at least two years in a row with severe drought conditions.

Description: In the context of forecasted climate change scenarios, the growth of forest tree species at their distribution margin is crucial to adapt current forest management strategies. Analyses of beech (Fagus sylvatica L.) growth have shown high plasticity, but easternmost beech populations have been rarely studied. To describe the response of the marginal beech population to the climate in the far east sites of its distribution, we first compiled new tree ring width chronologies. Then we analyzed climate–growth relationships for three marginal beech populations in the Republic of Moldova. We observed a relatively high growth rate in the marginal populations compared to core distribution sites. Our analyses further revealed a distinct and significant response of beech growth to all climatic variables, assessing for the first time the relationship between growth and vapor pressure deficit (VPD) which described how plant growth responds to drought. These results highlight that accumulated water deficit is an essential limiting factor of beech growth in this region. In conclusion, beech growth in the easternmost marginal population is drought-limited, and the sensitivity to VPD will need to be considered in future studies to update the forest management of other economic and ecologically important species.

Description: In this study, we show that the extreme Arctic winter 2015/16 can be partially explained by the superposition of different atmospheric teleconnection patterns, such as the Arctic Oscillation, the Pacific-North American teleconnection, and El Niño—Southern Oscillation, whereas winter 2016/17 had different trigger mechanisms. While the temperature anomalies for winter 2015/16 were mainly driven by the large-scale atmospheric circulation, the temperature anomalies throughout winter 2016/17 may possibly reflect a response to the extremely wet and warm autumn of 2016. The atmospheric circulation anomalies in winter 2016/17 were not as “spectacular” as the ones in the previous winter, but autumn 2016 was one of the most exceptional autumns in the observational record so far and it features some remarkable records: the lowest temperature gradient between the Arctic and the mid-latitudes over the last 70 years, the lowest autumn sea ice extent over the last 40 years, and the warmest and wettest autumn over the last 37 years over most of the Arctic basin. Moreover, we demonstrate that although the background conditions were similar for winters 2015/2016 and 2016/2017 (e.g., reduced sea ice cover, a reduced temperature gradient between the Arctic and the mid-latitudes, and a very warm Barents Sea and Kara Sea in the previous autumn), the response of the atmospheric circulation and the regions affected by extremes (e.g., cold spells and snow cover) were rather different during these two winters.

Description: The eastern part of Europe is very poorly represented in the Global Network for Isotopes in Precipitation (GNIP) database, mainly because the monitoring of the stable isotopes in precipitation started only recently compared with other regions. In this respect, the main objective of this article is to fill the gap in the GNIP database over the eastern part of Europe and show the temporal variability and potential drivers of an extended network of δ18O values in precipitation collected from 27 locations in Romania and the Republic of Moldova. We also present the first high-resolution map of the spatio-temporal distribution of δ18O values in precipitation in Romania and the Republic of Moldova, according to an observational dataset. According to our results, the stations from western and northern Romania tend to have LMWLS with higher values than those from southwestern Romania. The monthly variation of the δ18O and δ2H showed a clearly interannual variation, with distinct seasonal differences, following the seasonal temperatures. The analysis of the spatial distribution of stable isotopes in precipitation water was made on the basis of both observational data and modeled data. This allowed us to study the origin of the air moisture and the interaction with regional and local patterns and to analyze the link between the spatial δ18O variations and the large-scale circulation patterns on a seasonal scale.