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The internal variability of European summer temperatures has been linked to various mechanisms on seasonal to sub- and multi-decadal timescales. We find that sub-decadal timescales dominate summer temperature variability over large parts of the continent and determine mechanisms controlling extremely warm summers on sub-decadal timescales. We show that the sub-decadal warm phases of bandpass-filtered European summer temperatures, hereinafter referred to as extremely warm European summers, are related to a strengthening of the North Atlantic Ocean subtropical gyre, an increase in meridional heat transport, and an accumulation of ocean heat content in the North Atlantic several years prior to the extreme summer. This ocean warming affects the ocean–atmosphere heat fluxes, leading to a weakening and northward displacement of the jet stream and increased probability of occurrence of high-pressure systems over Scandinavia. Thus, our findings link the occurrence of extremely warm European summers to the accumulation of heat in the North Atlantic Ocean and provide the potential to improve the predictability of extremely warm summers several years ahead, which is of great societal interest. [ABSTRACT FROM AUTHOR]

The internal variability of European summer temperatures has been linked to various mechanisms on seasonal to sub- and multi-decadal timescales. We find that sub-decadal time scales dominate summer temperature variability over large parts of the continent, and the mechanisms controlling such sub-decadal variations remain unexplored. Extremely warm summers occurring in sub-decadal periods when abnormally warm summer temperatures conglomerate are controlled by a strengthening of the subtropical gyre, an increase of heat transport, and an accumulation of heat content several years prior to an extremely warm European summer, thereby affecting ocean-atmosphere heat fluxes during extreme summers. This leads to a weakening and northward displacement of the jet stream and increased probability of atmospheric blocking over Scandinavia. Our findings link the occurrence of extremely warm European summers to the inertia of the North Atlantic, whose potential to improve the predictability of extremely warm summers several years ahead is of great societal interest, especially in a warming climate. [ABSTRACT FROM AUTHOR]

Different spring and early summer North Atlantic sea surface temperature anomalies (SSTAs) have been shown to precede recent European summer heatwaves (EuSHWs). So far, the limited number of observed events associated with several physical mechanisms has prevented a robust identification of SSTAs as precursors. Here, we extend beyond previous studies by combining 100 historical simulations (1850–2005) of the MPI Grand‐Ensemble with an explainable neural‐network method. We find that the spring tripolar North Atlantic pattern with positive SSTAs in the Subtropical Gyre is a precursor of EuSHWs. In addition, positive SSTAs west of the Iberian Peninsula, and in the North Sea, the Baltic Sea and the Mediterranean Sea relate to distinct early summer soil moisture anomaly patterns and are precursors of western and southeastern EuSHWs, respectively. While the phase of the tripolar North Atlantic pattern indicates whether a EuSHW might emerge, regional SSTAs indicate the spatial characteristics of EuSHWs. Plain Language Summary: Past studies have investigated the influence of spring and early summer North Atlantic sea surface temperature anomalies (SSTAs) on recent European summer heatwaves (EuSHWs). These studies have proposed different SSTAs in the North Atlantic as the most important for the development of different EuSHWs. Yet, it has not been possible to generalize which spring North Atlantic SSTAs are the most important to anticipate EuSHWs because we have too few observed events and they showed different spatial and physical characteristics. Here, we analyze 100 historical simulations (1850–2005) of the MPI Grand‐Ensemble in which we identify a large number of EuSHWs. We use an explainable neural‐network method to find which spring North Atlantic SSTAs are the most important to anticipate EuSHWs. We find that warm SSTAs in the Subtropical Gyre surrounded by cold SSTAs in the north and in the south is an indicator of EuSHW occurrence. In addition, different regional SSTAs relate to drier than normal soil moisture in different parts of Europe that influence different EuSHWs. Warm SSTAs west of the Iberian Peninsula, and in the North Sea, the Baltic Sea and the Mediterranean Sea indicate the occurrence of western and southeastern EuSHWs, respectively. Key Points: A tripolar North Atlantic sea surface temperature (SST) pattern with an anomalously warm Subtropical Gyre is a precursor of European summer heatwavesDistinct regional spring SST anomalies relate to distinct early summer soil moisture (SM) anomaly patternsDistinct early summer SM anomaly patterns resemble the location of European summer heatwaves [ABSTRACT FROM AUTHOR]

We evaluate how hotspots of different types of extreme summertime heat change under global warming increase of up to 4 ∘ C ; and which level of global warming allows us to avert the risk of these hotspots considering the irreducible range of possibilities defined by well-sampled internal variability. We use large samples of low-probability extremes simulated by the 100-member Max Planck Institute Grand Ensemble (MPI-GE) for five metrics of extreme heat: maximum absolute temperatures, return periods of extreme temperatures, maximum temperature variability, sustained tropical nights, and wet bulb temperatures. At 2 ∘ C of warming, MPI-GE projects maximum summer temperatures below 50 ∘ C over most of the world. Beyond 2 ∘ C , this threshold is overshot in all continents, with the maximum projected temperatures in hotspots over the Arabic Peninsula. Extreme 1-in-100-years pre-industrial temperatures occur every 10–25 years already at 1.5 ∘ C of warming. At 4 ∘ C , these 1-in-100-years extremes are projected to occur every 1 to 2 years over most of the world. The range of maximum temperature variability increases by 10–50% at 2 ∘ C of warming, and by 50–100% at 4 ∘ C . Beyond 2 ∘ C , heat stress is aggravated substantially over non-adapted areas by hot and humid conditions that occur rarely in a pre-industrial climate; while extreme pre-industrial tropical night conditions become common-pace already at 1.5 ∘ C . At 4 ∘ C of warming, tropical night hotspots spread polewards globally, and are sustained during more than 99% of all summer months in the tropics; whilst extreme monthly mean wet bulb temperatures beyond 26 ∘ C spread both over large tropical as well as mid-latitude regions. [ABSTRACT FROM AUTHOR]

We use the 100-member Max Planck Institute Grand Ensemble (MPI-GE) to disentangle the contributions from colocated dynamic atmospheric conditions and local thermodynamic effects of moisture limitation as drivers of variability in European summer heat extremes. Using a novel extreme event definition, we find that heat extremes with respect to the evolving mean climate increase by 70% under a moderate warming scenario during the twenty-first century. With a multiple regression approach, we find that the dynamical mechanisms representing blocking and anticyclonic conditions are the main driver of variability in extreme European summer temperatures, both in past and future climates. By contrast, local thermodynamic drivers play a secondary role in explaining the total variability in extreme temperatures. We also find that considering both dynamical and thermodynamical sources of variability simultaneously is crucial. Assessing only one type of drivers leads to an overestimation of their effect on extreme temperatures, particularly when considering only thermodynamical drivers. Lastly, we find that although most past and future heat extremes occur under favorable dynamical atmospheric conditions; this occurs 10–40% less frequently over Central Europe in the twenty-first century. By contrast, heat extremes over Central Europe occur 40% more frequently under concurrent extreme moisture limitation in the twenty-first Century. Our findings highlight a new type of neutral-atmosphere, moisture-driven heat extremes, and confirm that the increase in European heat extremes and associated variability increase are dominated by the local thermodynamic effect of moisture limitation. [ABSTRACT FROM AUTHOR]

We improve seasonal hindcast skill of European summer climate in an ensemble based coupled seasonal prediction system by selecting individual ensemble members based on their respective consistent chain of processes that describe a physical mechanism. This mechanism is associated with the second mode of seasonal climate variability in the North-Atlantic-European sector and is contrary to the summer North Atlantic Oscillation. We initially analyse the mechanism in the ERA-Interim reanalysis and then test the influence of the mechanism on European hindcast skill in an initialised coupled seasonal climate model. We show that the mechanism originates in the tropical North Atlantic in spring, where either warm or cold sea surface temperature anomalies (SSTs) are connected with the European climate by an upper-level wave-train. This wave-train is accompanied by a zonal pressure gradient, that in turn influences the climate over central Europe in the following summer. We analyse the seasonal summer hindcast skill in a mixed resolution hindcast ensemble simulation generated by MPI-ESM, with 30 members starting every year in May. While the mean over the full ensemble shows no seasonal hindcast skill in summer, we achieve significant hindcast skill through forming a new mean over subselected ensemble members. For this selection, we test every ensemble member for the proposed consistent chain of connections between the wave-train, the zonal pressure gradient and their impact on European summer temperatures, and find that the processes that describe the mechanism are not represented in every ensemble member. Due to its influence on European summer climate, we use the condition of the persistent spring SSTs to anticipate the phase of the mechanism in each considered year. We thus use statistical relations to select ensemble members generated by a dynamical prediction system. With this approach, we significantly enhance the seasonal hindcast skill and the reliability of the hindcasts in the North-Atlantic-European sector, especially in the areas where the mechanism is showing a prominent signal. Since we only use knowledge that would be available in a real forecast set-up, this approach can potentially be applied in operational ensemble prediction systems. [ABSTRACT FROM AUTHOR]

The prediction of multiyear to decadal climate variability is important for stakeholders and decision-makers who are engaged in near-term planning activities. The decadal climate prediction experiments (DPEs) by predicting near-term climate with initialized global climate models (GCMs) provide robust skill at predicting sea surface temperature variability in some ocean regions as the North Atlantic. However, the state-of-the-art DPEs, which reproduce the observed warming trend associated with forced climate change, fail at predicting land surface air temperature (SAT) interdecadal variability. Here, we develop an effective statistical-dynamical model to predict spatial and temporal evolutions in Northern-Hemisphere (NH) summer land SAT. We identify two dominant interdecadal variability modes of the NH summer land SAT, whose evolutions are synchronized with forced climate change and Atlantic multidecadal variability (AMV), respectively. Based on statistical relationships with physical interpretations, time series of the forced responses and the AMV skillfully predicted by GCMs, the land SAT over the past one hundred years is predicted retrospectively with significantly improved skill compared to that predicted by the DPEs. Our results indicate that the decadal variability of the NH land SAT is predictable, with predictability rooted in atmospheric interdecadal circumglobal teleconnection (CGT) forced by the AMV. More skillful NH climate prediction by DPEs, which would be more practical for stakeholders and decision-makers, can be achieved by improving interdecadal CGT simulations in GCMs. [ABSTRACT FROM AUTHOR]

The observed prominent multidecadal variations in the central to eastern (C-E) European summer temperature are closely related to the Atlantic multidecadal variability (AMV). Using the Twentieth Century Reanalysis project version 2 data for the period of 1930-2012, we present a mechanism by which the multidecadal variations in the C-E European summer temperature are associated to a linear baroclinic atmospheric response to the AMV-related surface heat flux. Our results suggest that over the north-western Atlantic, the positive heat flux anomaly triggers a surface baroclinic pressure response to diabatic heating with a negative surface pressure anomaly to the east of the heat source. Further downstream, this response induces an east-west wave-like pressure anomaly. The east-west wave-like response in the sea level pressure structure, to which we refer as North-Atlantic-European East West (NEW) mode, is independent of the summer North Atlantic Oscillation and is the principal mode of variations during summer over the Euro-Atlantic region at multidecadal time scales. The NEW mode causes warming of the C-E European region by creating an atmospheric blocking-like situation. Our findings also suggest that this NEW mode is responsible for the multidecadal variations in precipitation over the British Isles and north-western Europe. [ABSTRACT FROM AUTHOR]

We investigate European summer (July-August) precipitation variability and its global teleconnections using the NCEP/NCAR reanalysis data (1950-2010) and a historical Coupled Model Intercomparison Project climate simulation (1901-2005) carried out using the ECHAM6/MPIOM climate model. A wavelike pattern is found in the upper tropospheric levels (200 hPa) similar to the summer circumglobal wave train (CGT) extending from the North Pacific to the Eurasian region. The positive phase of the CGT is associated with upper level anomalous low (high) pressure over western (eastern) Europe. It is further associated with a dipole-like precipitation pattern over Europe entailing significantly enhanced (reduced) precipitation over the western (eastern) region. The anomalous circulation features and associated summer precipitation pattern over Europe inverts for the negative CGT phase. Accordingly, the global teleconnection pattern of a precipitation index summarizing summer precipitation over Western Europe entails an upper level signature which consists of a CGT-like wave pattern extending from the North Pacific to Eurasia. The imprint of the CGT on European summer precipitation is distinct from that of the summer North Atlantic Oscillation, despite the two modes of variability bear strong similarities in their upper level atmospheric pattern over Western Europe. The analysis of simulated CGT features and of its climatic implications for the European region substantiates the existence of the CGT-European summer precipitation connection. The summer CGT in the mid-latitude therefore adds to the list of the modes of large-scale atmospheric variability significantly influencing European summer precipitation variability. [ABSTRACT FROM AUTHOR]

This study examines the influence of the mid-latitude circulation on the surface heat low (HL) and associated monsoon rainfall over northwestern India and Pakistan using the ERA40 data and high resolution (T106L31) climate model ECHAM5 simulation. Special emphasis is given to the surface HL which forms over Pakistan and adjoining areas of India, Iran and Afghanistan during the summer season. A heat low index (HLI) is defined to depict the surface HL. The HLI displays significant correlations with the upper level mid-latitude circulation over western central Asia and low level monsoon circulation over Arabian Sea and acts as a bridge connecting the mid-latitude wave train to the Indian summer monsoon. A time-lagged singular value decomposition analysis reveals that the eastward propagation of the mid-latitude circumglobal wave train (CGT) influences the surface pressure anomalies over the Indian domain. The largest low (negative) pressure anomalies over the western parts of the HL region (i.e., Iran and Afghanistan) occur in conjunction with the upper level anomalous high that develops over western-central Asia during the positive phase of the CGT. The composite analysis also reveals a significant increase in the low pressure anomalies over Iran and Afghanistan during the positive phase of CGT. The westward increasing low pressure anomalies with its north-south orientation provokes enormous north-south pressure gradient (lower pressure over land than over sea). This in turn enables the moist southerly flow from the Arabian Sea to penetrate farther northward over northwestern India and Pakistan. A monsoon trough like conditions develops over northwestern India and Pakistan where the moist southwesterly flow from the Arabian Sea and the Persian Gulf converge. The convergence in association with the orographic uplifting expedites convection and associated precipitation over northwestern India and Pakistan. The high resolution climate model ECHAM5 simulation also underlines the proposed findings and mechanism. [ABSTRACT FROM AUTHOR]

Hindcast skill for seasonal predictions of European climate is still very limited in current state-of-the-art prediction systems, especially for the summer season, since various different mechanisms are influencing the seasonal variability of European summer climate. Here, we focus on the first two modes of seasonal climate variability in the North-Atlantic-European sector and analyse their variability throughout the entire 20th century. With a pattern adopting cluster analysis that allows for the pattern to vary over time, we identify the North Atlantic Oscillation, a meridional pressure difference, and the East Atlantic pattern, a zonal pressure difference. We investigate their positive and negative phases in the ERA-20C reanalysis for 1900-2010 and assign which phase of either mechanism is dominating a specific summer. With this method we find that the different phases influence different regions over the North-Atlantic-European sector. We use this analysis to show in which region which domination mechanism influences hindcast skill. For this, we analyse the hindcast skill for 1900-2010 using 10 ensemble members generated by MPI-ESM-MR, starting every year in May. By identifying the different phases of the mechanisms in the individual ensemble members, we further find that the hindcast skill in the influenced regions varies strongly over time. [ABSTRACT FROM AUTHOR]

It remains an unsolved challenge to predict summer temperature extremes on the seasonal-to-decadal time scale. One possible way to understand the emergence of summer temperature extremes, and therefore their prediction, is to identify connections between large-scale features of climate variability and such temperature extremes. Here, we show evidence that the phase of the Atlantic Multidecadal Variability (AMV) influences the occurrence and decadal prediction skill of summer temperature extremes on the Northern Hemisphere.We use a 10-member ensemble of yearly initialized decadal hindcasts with the CMIP6 version of the MPI-ESM-HR model covering the period 1960-2017. Predictions of summer (JJA) temperature variability are skillful for up to 8 years ahead in regions in Scandinavia, North-East Asia and the Central-Midwest USA. Moreover, prediction skill for summer temperature in these regions strongly depends on the phase of the AMV: summer temperature in Scandinavia and the USA shows high predictive skill in positive AMV phases, while North-East Asia shows high predictive skill in negative AMV phases. Due to a large number of other mechanisms influencing summer temperature variability over Central Europe, we find no decadal prediction skill for Central European summer temperature in this model simulation. The likelihood with which extreme summer temperatures are predicted by the hindcast ensemble depends on the phase of the AMV. The amount of summer temperature extremes predicted by the model is highly correlated between the four regions, because the AMV and the occurrence of such extremes between these regions are strongly connected via a circumglobal atmospheric Rossby Wave, the circumglobal wave train. On average, summer temperature extremes are predicted to occur every 6 years in a negative AMV phase and every 4 years in a positive AMV phase. This shows the strong link between the AMV and the prediction of surface temperature extremes on the Northern Hemisphere. [ABSTRACT FROM AUTHOR]