The recent carbon cycle balance of the northern hemisphere derived with CarbonTracker Europe

The CarbonTracker Europe data assimilation system for atmospheric CO2 has been substantially overhauled to allow more flexibility in the specification of individual components such as first-guess fluxes, the set of observations to be assimilated, and the nature of the unknowns to be estimated. Amongst others, this allows an easy switch from 'ecoregion' inversions to 'gridded' inversions in which many more elements are added to the state vector, but coupled through a covariance structure that limits the number of degrees of freedom. Moreover, the state vector can be augmented with extra parameters, for instance to optimize the vertical exchange in the underlying TM5 model along with the surface fluxes. The new system (documented at www.carbontracker.eu/ctdas) follows an open-source policy, and is one of the first to integrate with the new ObsPack product created at NOAA ESRL. We will present the recent carbon balance of the northern hemisphere estimated with the new CarbonTracker system, with a special focus on some of the known shortcomings of earlier versions such as summer time biases in simulated CO2 mole fractions at the surface, and a lack of flexibility in its spatial allocation of anomalous carbon fluxes.