The physical understanding and timely prediction of extreme weather events are of enormous importance to society due to their associated impacts. In this article, we highlight several types of weather extremes occurring in Europe in connection with a particular atmospheric flow pattern, known as atmospheric blocking. This flow pattern effectively blocks the prevailing westerly large-scale atmospheric flow, resulting in changing flow anomalies in the vicinity of the blocking system and persistent conditions in the immediate region of its occurrence. Blocking systems are long-lasting, quasi-stationary and self-sustaining systems that occur frequently over certain regions. Their presence and characteristics have an impact on the predictability of weather extremes and can thus be used as potential indicators. The phasing between the surface and the upper-level blocking anomalies is of major importance for the development of the extreme event. In summer, heat waves and droughts form below the blocking anticyclone primarily via large-scale subsidence that leads to cloud-free skies and, thus, persistent shortwave radiative warming of the ground. In winter, cold waves that occur during atmospheric blocking are normally observed downstream or south of these systems. Here, meridional advection of cold air masses from higher latitudes plays a decisive role. Depending on their location, blocking systems also may lead to a shift in the storm track, which influences the occurrence of wind and precipitation anomalies. Due to these multifaceted linkages, compound events are often observed in conjunction with blocking conditions. In addition to the aforementioned relations, the predictability of extreme events associated with blocking and links to climate change are assessed. Finally, current knowledge gaps and pertinent research perspectives for the future are discussed.

Average monthly precipitation, the original Palmer Drought Severity Index (PDSI) and a recent adaptation to Europe, the Self Calibrated PDSI (scPDSI) have been used here to analyse the spatial and temporal evolution of drought conditions in the Mediterranean during the 20th century. Monthly, seasonal and annual trends were computed for the period 1901–2000 and also for the first and second halves of this period. The statistical significance of trends was obtained with a modified version of the Mann-Kendall test that accounts for serial auto-correlation. The results show a clear trend towards drier conditions during the 20th century in most western and central Mediterranean regions, with the exceptions of northwestern Iberia and most of Turkey that reveal an increase of moisture availability. A Generalized Extreme Values (GEV) analysis was applied to the maximum and minimum regional values of scPDSI, with results pointing towards a significant decline of absolute extreme values in central areas (Italy and Balkans) and a less clear picture emerging in western (Iberia) and eastern (Turkey) realms. The inter-annual variability of the scPDSI index series is shown to be more realistic than the corresponding PDSI version, fitting better the drought episodes sequence and magnitude described in the literature for each sub-region. We assess the decadal and inter-annual variability of the scPDSI for each sub-domain and evaluate the role played by the major teleconnection patterns, and by several sea surface temperature (SST) anomalies. The main driver of scPDSI in western and central Mediterranean areas is the winter North Atlantic Oscillation (NAO) pattern that is also relevant during the following spring and summer seasons with anti-correlation values below −0.60. The second most important mode corresponds to the Scandinavian Pattern that is significantly associated to the scPDSI between winter and summer over central Mediterranean (correlation values around 0.50). Finally, the teleconnection and SST analysis has allowed us to calibrate a stepwise regression model, enabling the forecasting of summer drought conditions six months in advance. The final model obtained is capable of reproducing the observed scPDSI time series fairly well, with a correlation coefficient of 0.79 (0.77 after cross-validation) and a significant gain over climatology (SS_c=59%), while the corresponding result against persistence is more modest (SS_p6=11%).

An increase in the frequency of extremely hot and dry events has been experienced over the past few decades in South America, and particularly in Brazil. Regional climate change projections indicate a future aggravation of this trend. However, a comprehensive characterization of drought and heatwave compound events, as well as of the main land–atmosphere mechanisms involved, is still lacking for most of South America. This study aims to fill this gap, assessing for the first time the historical evolution of compound summer drought and heatwave events for the heavily populated region of Southeast Brazil and for the period of 1980–2018. The main goal is to undertake a detailed analysis of the surface and synoptic conditions, as well as of the land–atmosphere coupling processes that led to the occurrence of individual and compound dry and hot extremes. Our results confirm that the São Paulo, Rio de Janeiro and Minas Gerais states have recorded pronounced and statistically significant increases in the number of compound summer drought and heatwave episodes. In particular, the last decade was characterized by two austral summer seasons (2013/14 and 2014/15) with outstanding concurrent drought and heatwave conditions stemmed by severe precipitation deficits and a higher-than-average occurrence of blocking patterns. As result of these land and atmosphere conditions, a high coupling (water-limited) regime was imposed, promoting the re-amplification of hot spells that resulted in mega heatwave episodes. Our findings reveal a substantial contribution of persistent dry conditions to heatwave episodes, highlighting the vulnerability of the region to climate change.

Two separate heatwaves affected western Europe in June and July 2019, in particular France, Belgium, the Netherlands, western Germany and northeastern Spain. Here we compare the European 2019 summer temperatures to multi-proxy reconstructions of temperatures since 1500, and analyze the relative influence of synoptic conditions and soil-atmosphere feedbacks on both heatwave events. We find that a subtropical ridge was a common synoptic set-up to both heatwaves. However, whereas the June heatwave was mostly associated with warm advection of a Saharan air mass intrusion, land surface processes were relevant for the magnitude of the July heatwave. Enhanced radiative fluxes and precipitation reduction during early July added to the soil moisture deficit that had been initiated by the June heatwave. We show this deficit was larger than it would have been in the past decades, pointing to climate change imprint. We conclude that land-atmosphere feedbacks as well as remote influences through northward propagation of dryness contributed to the exceptional intensity of the July heatwave.