Wolfgang A. Müller’s research while affiliated with Max Planck Institute for Meteorology and other places

A wide range of important societal and economic applications on a national and international level strive to an integrated understanding and forecasting of weather and climate on all temporal and spatial scales. The global to regional model system ICON (Icosahedral Nonhydrostatic) has been applied to weather as well as to climate timescales with joint developments of the model infrastructure. However, ICON’s model configurations share the same dynamical core but differ substantially in their subgrid scale closures and physical parameterization packages, depending on whether they were designed for numerical weather prediction (NWP) or climate applications. Starting in 2020, a new modeling initiative has been launched as a joint project between climate modeling institutes and the Deutscher Wetterdienst, that “vertically” integrates NWP, climate predictions, climate projections and atmospheric composition modeling based on the ICON framework and targets a unified treatment of the respective subgrid-scale parametrizations. This initiative aims at the development of coupled model configurations of ICON to conduct operational weather and ocean forecasts for several days, climate predictions with timescales up to 10 years ahead as well as climate projections, and to provide a model baseline for joint research for NWP and climate. This paper illustrates the strategic direction of this modeling initiative, isolates key challenges and reports on first results.

We use hindcasts from a state-of-the-art decadal climate prediction system initialized between 1979 and 2017 to explore the predictability of the Antarctic dipole—that is, the seesaw between sea ice cover in the Weddell and Ross Seas, and discuss its implications for Antarctic sea ice predictability. Our results indicate low forecast skills for the Antarctic dipole in the first hindcast year, with a strong relaxation of March values toward the climatology contrasting with an overestimation of anomalies in September, which we interpret as being linked to a predominance of local drift processes over initialized large-scale dynamics. Forecast skills for the Antarctic dipole and total Antarctic sea ice extent are uncorrelated. Limited predictability of the Antarctic dipole is also found under preconditioning around strong warm and strong cold events of the El Niño-Southern Oscillation. Initialization timing and model drift are reported as potential explanations for the poor predictive skills identified.

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.

Extreme heat and drought typical of an end-of-century climate could soon occur over Europe, and repeatedly. Despite the European climate being potentially prone to multi-year successive extremes due to the influence of the North Atlantic variability, it remains unclear how the likelihood of successive extremes changes under warming, how early they could reach end-of-century levels, and how this is affected by internal climate variability. Using the Max Planck Institute Grand Ensemble, we find that even under moderate warming, end-of-century heat and drought levels virtually impossible 20 years ago reach 1-in-10 likelihoods as early as the 2030s. By 2050–2074, two successive years of single or compound end-of-century extremes, unprecedented to date, exceed 1-in-10 likelihoods; while Europe-wide 5-year megadroughts become plausible. Whole decades of end-of-century heat stress could start by 2040, by 2020 for drought, and with a warm North Atlantic, end-of-century decades starting as early as 2030 become twice as likely.

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.

Extreme heat and drought levels typical of an end-of-century climate could occur swiftly, and repeatedly. Despite the European climate being potentially prone to multi-year successive extremes due to the influence of the North Atlantic variability, it remains unclear how the likelihood of such successive extremes changes under warming, how early they could reach end-of-century levels, and how this is affected by internal climate variability. Using the MPI Grand Ensemble, we find that even under moderate warming, end-of-century heat and drought levels virtually impossible 20 years ago reach 1-in-10 likelihoods as early as the 2030s. By 2050-2075, two successive years of single or compound end-of-century extremes, unprecedented to date, exceed 1-in-10 likelihoods; while Europe-wide 5-year megadroughts become plausible. Whole decades of end-of-century heat stress could start by 2040, by 2020 for rain-deficit drought, and end-of-century decades starting as early as 2030 become twice as likely under a warm North Atlantic state.

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.

Plain Language Summary Wintertime temperature in the Northern Hemisphere is regulated by the variations of atmospheric pressure, represented by the so‐called North Atlantic Oscillation (NAO). The NAO's phase—negative or positive—is associated with the pathways of cold and warm air masses leading to cold or warm winters in Europe. While the NAO phase can be predicted well, predictions of the NAO‐dependent air temperature remain elusive. Specifically, it is challenging to predict the strength of the NAO, the most important requirement for the accurate prediction of wintertime temperature. Here, we improve wintertime temperature prediction by increasing the strength of the predicted NAO. We use observation based autumn Northern Hemisphere ocean and air temperature, as well as ice and snow cover for statistical estimation of the first guess NAO for the upcoming winter. Then, we sub‐select only those simulations from the multi‐model ensemble, which are consistent with our first guess NAO. As a result, based on these selected members, the wintertime temperature prediction is substantially improved over a large part of the Northern Hemisphere.

Multi-annual to decadal changes in climate are accompanied by changes in extreme events that cause major impacts on society and severe challenges for adaptation. Early warnings of such changes are now potentially possible through operational decadal predictions. However, improved understanding of the causes of regional changes in climate on these timescales is needed both to attribute recent events and to gain further confidence in forecasts. Here we document the Large Ensemble Single Forcing Model Intercomparison Project that will address this need through coordinated model experiments enabling the impacts of different external drivers to be isolated. We highlight the need to account for model errors and propose an attribution approach that exploits differences between models to diagnose the real-world situation and overcomes potential errors in atmospheric circulation changes. The experiments and analysis proposed here will provide substantial improvements to our ability to understand near-term changes in climate and will support the World Climate Research Program Lighthouse Activity on Explaining and Predicting Earth System Change.