Enduring and extensive heavy precipitation events associated with widespread river floods are among the main natural hazards affecting central Europe. Since such events are characterized by long return periods, it is difficult to adequately quantify their frequency and intensity solely based on the available observations of precipitation. Furthermore, long-term observations are rare, not homogeneous in space and time, and thus not suitable to running hydrological models (HMs) with respect to extremes. To overcome this issue, we make use of the recently introduced LAERTES-EU (LArge Ensemble of Regional climaTe modEl Simulations for EUrope) data set, which is an ensemble of regional climate model simulations providing over 12 000 simulated years. LAERTES-EU is adapted for use in an HM to calculate discharges for large river basins by applying quantile mapping with a parameterized gamma distribution to correct the mainly positive bias in model precipitation. The Rhine basin serves as a pilot area for calibration and validation. The results show clear improvements in the representation of both precipitation (e.g., annual cycle and intensity distributions) and simulated discharges by the HM after the bias correction. Furthermore, the large size of LAERTES-EU also improves the statistical representativeness for high return values above 100 years of discharges. We conclude that the bias-corrected LAERTES-EU data set is generally suitable for hydrological applications and posterior risk analyses. The results of this pilot study will soon be applied to several large river basins in central Europe.

A twentieth century-long coupled atmosphere-ocean regional climate simulation with COSMO-CLM (Consortium for Small-Scale Modeling, Climate Limited-area Model) and NEMO (Nucleus for European Modelling of the Ocean) is studied here to evaluate the added value of coupled marginal seas over continental regions. The interactive coupling of the marginal seas, namely the Mediterranean, the North and the Baltic Seas, to the atmosphere in the European region gives a comprehensive modelling system. It is expected to be able to describe the climatological features of this geographically complex area even more precisely than an atmosphere-only climate model. The investigated variables are precipitation and 2 m temperature. Sensitivity studies are used to assess the impact of SST (sea surface temperature) changes over land areas. The different SST values affect the continental precipitation more than the 2 m temperature. The simulated variables are compared to the CRU (Climatic Research Unit) observational data, and also to the HOAPS/GPCC (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data, Global Precipitation Climatology Centre) data. In the coupled simulation, added skill is found primarily during winter over the eastern part of Europe. Our analysis shows that, over this region, the coupled system is dryer than the uncoupled system, both in terms of precipitation and soil moisture, which means a decrease in the bias of the system. Thus, the coupling improves the simulation of precipitation over the eastern part of Europe, due to cooler SST values and in consequence, drier soil.

The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO₂ concentrations, analog to the upper range of end‐of‐century CO₂ projections. Preindustrial and early Eocene simulations, at a range of CO₂ levels are used to quantify the MHT changes in response to both CO₂ and non‐CO₂ related forcings. We found that atmospheric poleward heat transport increases with CO₂, while oceanic poleward heat transport decreases. The non‐CO₂ boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO₂ concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO₂ warming, which is in line with the currently observed precipitation increase of present day monsoon systems.

The configuration of continents and oceans has a major influence on Earth's climate by shaping large-scale atmospheric circulation patterns. In this study, we investigate the effect of early Eocene paleogeography, specifically from the Ypresian stage, on extratropical eddies. In our analysis we highlight the influence of the epicontinental West Siberian Sea as well as the impact of the lack of the Antarctic Circumpolar Current on mid-latitude cyclones and blocking events. Previous work from the Deep-Time Model Intercomparison Project (DeepMIP) has shown changes in atmospheric eddy heat transport under early Eocene boundary conditions. This motivates our analyse of mid-latitude cyclones and blocking events under early Eocene boundary conditions. For the analysis we use six-hourly output from an atmosphere-only CESM1.2 simulation, which is an extension of the corresponding DeepMIP 1 × CO₂ experiment. In our simulation, cyclonic activity strengthens at the northern mid-latitudes and weakens at the southern mid-latitudes under early Eocene boundary conditions compared to pre-industrial. Moreover, the blocking signal, which is dominated by the northern hemisphere under pre-industrial conditions, becomes less pronounced. Our results show that, through air–sea interactions the paleogeographic features of the early Eocene produce a more balanced heat transport between the hemispheres and atmospheric processes compared to pre-industrial conditions.

The configuration of continents and oceans has a major influence on Earth’s climate by shaping large-scale atmospheric circulation patterns. In this study, we investigate the effect of early Eocene paleogeography, specifically from the Ypresian stage, on extratropical eddies. We analyse the influence of the epicontinental West Siberian Sea as well as the impact of the absence of the Antarctic Circumpolar Current on mid-latitude cyclones and blocking events. Previous work from the Deep-Time Model Intercomparison Project (DeepMIP) has shown that during the early Eocene, heat transport through cyclonic systems was more intense than under modern conditions at the northern mid-latitudes, and less intense at the southern mid-latitudes. We analyse cyclone tracks and blocking systems of the early Eocene in an atmosphere-only CESM1.2 simulation, continuing the DeepMIP 1xCO₂ experiment. Sea surface temperatures from the DeepMIP experiment are used as boundary conditions. The simulation output is six-hourly, which enables direct cyclone tracking in the pressure field. In parallel, a decrease in heat transport of stationary eddies at the northern mid-latitudes in DeepMIP data, motivates the analysis of blocking climatology based on the 500 hPa geopotential height field. Our results show that, through air–sea interactions, paleogeographic features of the early Eocene enhance cyclonic activity at northern mid-latitudes while reducing it at southern mid-latitudes compared to modern conditions.