Links between GHG fluxes and climate on the European scale

To study the interactions between CO₂ fluxes and climate variations over Europe, this subproject developed the fully coupled biogeophysics-biogeochemistry regional climate model COSMO-CLM2-CN. Compared to an earlier version of the model (Davin et al. 2011, Davin and Seneviratne 2012) the coupled system has been augmented with the OASIS3-MCT coupler (Valcke et al., 2013) and a new version of the Community Land Model version 4, CLM4, with interactive carbon-nitrogen (C/N) cycles (Lawrence et al., 2011). Furthermore, to simulate the transport of CO₂ in the atmosphere, the regional atmospheric circulation model COSMO was extended with a tracer transport module (Roches and Fuhrer, 2012). This module has been fully integrated into the latest version 5 of COSMO and was also used in one of the two inversion systems described in Subproject C.

This new coupled system has been evaluated over Europe first in its biogeophysics-only configuration. The results indicate that COSMO-CLM2 generally outperforms state-of-the-art EURO-CORDEX RCMs in simulating temperature and surface fluxes across different regions and seasons (Davin et al., to be submitted). Overall this highlights the added value of using a more comprehensive land surface scheme for regional climate simulations. As a further step, the coupled system has been also evaluated with C/N dynamics activated with a focus on biogeochemical processes (Mystakidis et al., in prep). The evaluation shows that the European carbon cycle is overall well represented and that CLM4.0 performance is not degraded compared to corresponding offline experiments with an observation-based meteorological forcing. The model has been then further applied to study the sensitivity of the carbon cycle to soil moisture. The results underline the important role of soil moisture on the interannual variability of the European carbon balance.

In parallel to these model developments, a synthesis of observation-based products of carbon fluxes and pools has been made. These products were used not only in the aforementioned evaluation but also to evaluate and constrain the CMIP5 multi-model ensemble. Systematic carbon cycle biases common to many models have been identified and observations of carbon and water fluxes have been combined with models to provide revised estimates of future carbon cycle projections (Mystakidis et al., 2016). Another study is applying this approach to constrain the so-called carbon cycle feedbacks (Mystakidis et al., in prep).

References:

• Davin, E. L., R. Stoeckli, E. B. Jaeger, S. Levis and S.I. Seneviratne (2011), COSMO-CLM2: A new version of the COSMO-CLM model coupled to the Community Land Model, Clim. Dyn., 37, 9, 1889-1907, doi: 10.1007/s00382-011-1019-z.
• Davin, E.L. and S.I. Seneviratne (2012), Role of land surface processes and diffuse/direct radiation partitioning in simulating the European climate, Biogeosciences, 9, 1695-1707, doi:10.5194/bg-9-1695-2012.
• Mystakidis, S., Davin, E. L., Gruber, N., and Seneviratne, S. I. (2016), Constraining future terrestrial carbon cycle projections using observation-based water and carbon flux estimates, Global Change Biology, doi: 10.1111/gcb.13217.
• Valcke, S., Craig, T., Coquart, L. (2013): OASIS3-MCT User Guide, OASIS3-MCT 2.0, Technical Report, TR/CMGC/13/17, CERFACS/CNRS SUC URA No 1875, Toulouse, France.