ALBERT

Description: Persistent heat extremes can have severe impacts on ecosystems and societies, including excess mortality, wildfires, and harvest failures. Here we identify Europe as a heatwave hotspot, exhibiting upward trends that are three-to-four times faster compared to the rest of the northern midlatitudes over the past 42 years. This accelerated trend is linked to atmospheric dynamical changes via an increase in the frequency and persistence of double jet stream states over Eurasia. We find that double jet occurrences are particularly important for western European heatwaves, explaining up to 35% of temperature variability. The upward trend in the persistence of double jet events explains almost all of the accelerated heatwave trend in western Europe, and about 30% of it over the extended European region. Those findings provide evidence that in addition to thermodynamical drivers, atmospheric dynamical changes have contributed to the increased rate of European heatwaves, with implications for risk management and potential adaptation strategies.

Description: The summer of 2018 was an extraordinary season in climatological terms for northern and central Europe, bringing simultaneous, widespread, and concurrent heat and drought extremes in large parts of the continent with extensive impacts on agriculture, forests, water supply, and socio-economic sector. Here, we present a comprehensive, multi-faceted analysis of the 2018 extreme summer in terms of heat and drought in central and northern Europe, with a particular focus on Germany. The heatwave first affected Scandinavia by mid-July, shifted towards central Europe in late July, while Iberia was primarily affected in early August. The atmospheric circulation was characterized by strongly positive blocking anomalies over Europe, in combination with a positive summer North Atlantic Oscillation and a double jet stream configuration before the initiation of the heatwave. In terms of possible precursors common to previous European heatwaves, the Eurasian double jet structure and a tripolar sea-surface temperature anomaly over the North Atlantic were identified already in spring. While in the early stages over Scandinavia the air masses at mid- and upper-levels were often of remote, maritime origin, at later stages over Iberia the air masses had primarily a local-to-regional origin. The drought affected Germany the most, starting with warmer than average conditions in spring, associated with enhanced latent heat release that initiated a severe depletion of soil moisture. During summer, a continued precipitation deficit exacerbated the problem, leading to hydrological and agricultural drought. A probabilistic attribution assessment of the heatwave in Germany showed that such events of prolonged heat have become more likely due to anthropogenic global warming. Regarding future projections, an extreme summer such as this of 2018 is expected to occur every two out of three years in Europe under a 1.5 °C warmer world and virtually every single year under 2 °C of global warming. With such large-scale and impactful extreme events becoming more frequent and intense under anthropogenic climate change, comprehensive and multi-faceted studies like the one presented here quantify the multitude of effects and provide valuable information as basis for adaptation and mitigation strategies.

Description: Sources of uncertainty (i.e., internal variability, model and scenario) in Atlantic Niño variability projections were quantified in 49 models participating in the Coupled Model Intercomparison Phases 5 (CMIP5) and 6 (CMIP6). By the end of the twenty‐first century, the ensemble mean change in Atlantic Niño variability is −0.07 ± 0.10˚C, with 80% of CMIP models projecting a decrease, and representing a 16% reduction relative to the 1981–2005 ensemble mean. Models' projections depict a large spread, with variability changes ranging from 0.23˚C to −0.50˚C. Internal variability is the main source of uncertainty until 2045 but model uncertainty dominates thereafter, eventually explaining up to 80% of the total uncertainty. The scenario uncertainty remains low (〈1%) throughout the twenty‐first century. The total uncertainty on Atlantic Niño variability projections is not improved when considering only CMIP models with a realistic zonal equatorial Atlantic sea surface temperature gradient. Plain Language Summary Sources of uncertainty (i.e., internal variability, model and scenario) in future projections of the Atlantic Niño variability were evaluated in global coupled models participating in the Coupled Model Intercomparison Phases 5 (CMIP5) and 6 (CMIP6). Relative to 1981–2005, models' projections depict a large spread, ranging from increasing Atlantic Niño variability by up to 0.23˚C to decreasing by up to −0.50˚C. By the end of the twenty‐first century, the ensemble mean Atlantic Niño variability change is −0.07 ± 0.10˚C with 80% of the global coupled models simulating a decrease. This change in the ensemble mean Atlantic Niño variability, relative to the period 1981–2005, represents a 16% reduction. During the first four decades of projection, the internal variability is the main contributor to the total uncertainty; thereafter model uncertainty dominates and explains up to 80% of the total uncertainty at the end of the twenty‐first century. The scenario uncertainty remains low (〈1%) throughout the twenty‐first century. The total uncertainty on Atlantic Niño variability projections is not improved when considering only CMIP models with a realistic zonal equatorial Atlantic sea surface temperature gradient. Key Points 80% of the CMIP models simulate a decrease of the Atlantic Niño variability at the end of the 21st century The model uncertainty explains about 80% of the total uncertainty on Atlantic Niño variability projections at the end of the 21st century Global warming signal is not detectable throughout scenarios due to large internal variability and model uncertainties