The aim of Module B is to enhance the understanding of decadal variability, to improve existing model components and to incorporate additional climate subsystems that are relevant for decadal climate predictions. This is done by concentrating on different processes like soil-atmosphere interactions, Arctic processes, atmosphere-ocean coupling, impact of natural external forcing, chemical processes and stratospheric processes.
A new parameterization was developed in project SPARCS accounting for surface fluxes over inhomogeneous sea ice (Lüpkes et al. 2013, Lüpkes and Gryanik 2015). Two different regimes were considered: the marginal sea ice zone with drifting floes and the inner Arctic regions during summer with melt ponds and leads. SPARCS showed that the differences in the morphology of these regimes require in general different parameterizations of the transfer coefficients for the turbulent transport of heat and momentum in the atmospheric boundary layer over sea ice (Fig. 1). As the most detailed parameterizations can only be applied by atmospheric climate models when they are coupled with sophisticated sea ice models, SPARCS developed also a simplified parameterization, which describes the transfer coefficients as a nonlinear function only of sea ice and melt pond concentrations. The new parameterization, which involves also a new stability correction, has been tested in cooperation with project TORUS in the atmosphere-ocean-sea ice model ECHAM6-FESOM. It was shown that the new parameterization has a large impact, especially on the momentum fluxes. But also other atmospheric variables (2m air temperature, surface pressure) as well as the sea ice concentration are influenced (Fig. 2). The impact on the ensemble average of the sea ice concentration was relatively small, however, in some regions statistically significant effects were found and there was a large variability between ensemble members.
The dependence of decadal predictions on the employed ocean component is tested and analysed within the project TORUS by the application of an ocean model alternative to the MPI-OM. Their ocean-sea ice model FESOM has an unstructured grid that allows for increased horizontal resolution in key regions like the Arctic and the tropics (Sidorenko et al. 2015). The long control run under present-day forcing showed that ECHAM6-FESOM performs as well as other climate models and has similar shortcomings in the North Atlantic circulation, leading to a too weak deep convection in the Labrador Sea, with phases of stronger convection related to ice-free conditions and weaker convection related to ice-covered conditions (Fig. 3, blue curve, R1). The decadal climate variability of global mean temperature, atmospheric teleconnection patterns, large-scale oceanic variability patterns and El Niño Southern Oscillation (ENSO) is well reproduced. An increased horizontal resolution in the tropics leads to an even more realistic representation of ENSO. First results from a long simulation under present-day forcing over 250 years with a new version of FESOM with better refinement in the northern North-Atlantic, the Canadian Arctic archipelago, in the Labrador Sea, and over the whole Arctic in general showed an improvement (strengthening) of the deep convection in the Labrador Sea (Fig. 3, red curve, R2).
The memory effect of soil moisture has a strong impact on the hydrological and energy cycles at the land surface on decadal time scales. To account for these soil-atmosphere interactions a realistic representation of subsurface hydrodynamics is needed but it is computationally expensive. Therefore, project MCRA developed a model complexity reduction approach to correct the simplified climate model with the information from a full-physics model that was applied over specified catchments (Shrestha et al. 2014). They have run their simplified regional climate model for the European CORDEX domain at a high resolution of 0.11° (Fig. 4) for short time periods, e.g. including the European heat wave in August 2003, and analysed the influence of the lower energy boundary conditions on the water and energy cycle at the land surface. They also developed similarity indices that are needed for the hydrologic parameterizations in the complexity reduction approach.
To consider chemical processes in the atmosphere in detail, chemistry climate models need to be applied. In Module B MAECHAM5/HAM and EMAC were used for different problems. They are based on the atmosphere model ECHAM that is part of the MiKlip prediction system. As the inclusion of interactions between chemistry and climate is computationally expensive, the project LiCoS optimized a Rosenbrock integrator for chemical reactions achieving a speed-up of 2 to 3 times in EMAC compared to the standard integrator.
Another possibility to drastically reduce computation time when accounting for stratospheric chemistry is realized in the project FAST-O3 by developing the fast stratospheric chemistry scheme SWIFT to simulate important interactions between climate and ozone (Rex et al. 2014). They implemented SWIFT as an alternative chemistry module into a chemistry and transport model and compared it to the full chemistry version and to satellite observations. The comparison indicated that the computationally much faster parameterization yields similarly good results as the full chemistry version (Fig. 5). In the last year of the project SWIFT was also implemented in EMAC.
EMAC was also applied in the project STRATO to analyse the response of the coupled stratosphere-troposphere-ocean system to decadal variability of the stratosphere including the natural external solar forcing. The comparison between EMAC and Baseline1-LR showed a good agreement in structure and magnitude of the decadal solar signal. However, the ozone and solar temperature signals are overestimated in both Baseline1-LR and MR compared to EMAC and to observations (Fig. 6). In the lower stratosphere the ENSO temperature signal is reproduced in both Baseline1 versions but weaker than in EMAC and in observations.
As a further natural external forcing the impact of large volcanic eruptions is analysed in the project ALARM. They provided a volcano module with a two-step approach. First, the volcanic radiative forcing is calculated with MAECHAM5/HAM. Second, this forcing is used in the MiKlip prediction system. With this setup they performed simulations with a Pinatubo-like eruption in 2013 and showed that the impact is similar to the reconstructed patterns after large historic eruptions. To assess the impact of volcanic eruptions on the predictive skill, they analysed Baseline0-LR decadal hindcasts with and without major volcanic eruptions and showed that especially in the first year the prediction skill over Eurasia is significantly improved if the eruptions are considered (Fig. 7).
The impact of clouds on decadal climate variability was analysed by the project LiCoS. They found that key players in the interannual to decadal variability of large-scale climate patterns are atmospheric processes that are caused by cloud feedbacks (Bellomo et al. 2014, 2015). These feedbacks are likely to be responsible for a big part of the ENSO variability by changing global circulation. They also showed that a positive feedback among sea surface temperature (SST), cloud cover, and large-scale atmospheric circulation can explain decadal climate variability in the Pacific Ocean. In addition, LiCoS quantified the ozone pollution prediction skills of the MiKlip prediction system by identifying the atmospheric stagnation index as a good indicator for the occurrence of high ozone days in industrialized regions (Fig. 8). As all necessary parameters are part of the standard output of the MiKlip prediction system, the atmospheric stagnation index can be used as a proxy for an ozone pollution index.
For a better understanding of atmosphere-ocean processes the project ATMOS concentrated on the North Atlantic and found in atmosphere-only simulations that a significant fraction of the convective precipitation over and south of the Gulf Stream can be explained by the variability of the underlying SST, especially in summer (Fig. 9, Hand et al. 2014). In winter nearly all of the anomalous precipitation is connected to passing atmospheric fronts. They also found a resolution dependence of the intensity, tilt and north-eastward extension of the North Atlantic storm tracks and of precipitation along the path of the Gulf Stream. This demonstrates the importance of reducing the misplacement of the Gulf Stream and the North Atlantic Current and therefore the cold bias in coupled climate models like the MiKlip prediction system. The need to reduce the coupled model bias was also shown in the project MultiCliP by the importance of the subpolar North Atlantic and its interaction between gyre and overturning circulation for the northward oceanic heat transport (Müller et al. 2015). Beside the North Atlantic they also focused on the Pacific and found a sensitivity of North Atlantic and tropical SSTs to external natural forcing like volcanic eruptions, which is absent for the extratropical North Pacific SSTs. The understanding of the circumstances and mechanisms favouring specific phasing between Atlantic and Pacific SST modes could lead to a reduction of uncertainty in decadal climate predictions.
The analysis of the impact of volcanic eruptions by MultiCliP revealed that Northern Hemisphere temperature anomalies around eruptions depend on the initial state (Fig. 10, Zanchettin et al. 2013). Together with the project ALARM they expect an improvement of decadal predictions including volcanic forcing when sea ice is initialised. Both projects also showed that the expansion of Arctic sea ice extent after a volcanic eruption is robustly simulated. According to ALARM quite accurate aerosol forcing fields would be necessary to improve predictions of the dynamical response to stratospheric sulphate aerosol loading for a Pinatubo-like eruption (Fig. 11, Toohey et al. 2014). They also showed that the variability of the temperature anomalies in mid- and late winter in the northern polar stratosphere is significantly reduced under the strong Tabora-like eruption and not significantly changed under the weaker Pinatubo-like eruption. However, weaker eruptions do produce significant global cooling.
Several projects in Module B investigated the effect of the different resolutions in Baseline1-LR and MR. In general, the stratosphere should be included, as MultiCliP found that this improves the simulation of extra-tropical tropospheric variability. This is the case for both versions, LR and MR. However, the higher resolution in MR is necessary if an improved representation of specific processes like the quasi-biennial oscillation is required. Concerning stratospheric variability STRATO found a gradual improvement from LR to MR, but both versions overestimate variability during winter in southern hemisphere polar regions.
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