A pilot variational coupled reanalysis based on the CESAM climate model

Abstract A new Earth system reanalysis framework is being introduced consisting of the intermediate complexity coupled CESAM model together with its adjoint. The results presented are a first pilot demonstration of an Earth system reanalysis that shows the potential of such an approach to (a) improving model biases through parameter estimation and (b) accomplishing a fully coupled reanalysis over long time windows. Variational data assimilation applied to nonlinear chaotic atmospheric models is limited by predictability, and therefore restricted to short assimilation windows. Applications with ocean models, however, require much longer assimilation windows to propagate sparse data information through time and space efficiently. To overcome this contradiction we employ the adjoint method together with synchronization to atmospheric data. The first application presented requires nearly complete information on the atmospheric state for synchronization, for which it relies on European Centre for Medium‐Range Weather Forecasts Reanalysis v5 data, and a simple nudging technique to achieve synchronization. We demonstrate that, over 39 years, efficient assimilation of in‐situ and satellite ocean data and atmospheric reanalysis data is possible by adjusting the surface fluxes and internal model parameters. Given the coarse resolution of the model of , overturning and meridional transports of heat and fresh water are less realistic than previous higher resolution ocean syntheses based on the adjoint method. However, unsynchronized model runs with adjusted parameters show an improved climate and circulation consistent with the reanalysis, suggesting low initialization shocks if the reanalysis is used for initializing decadal predictions.