Historical ocean reanalyses (1900-2010) using different data assimilation strategies

Two historical ocean reanalyses that cover the period from 1900 to 2010 are performed. One ocean reanalysis (CHOR_AS) assimilates both sea-surface temperature (SST) and vertical profile data with a 3D-Var assimilation scheme, while the other one (CHOR_RL) assimilates vertical profile data, but is nudged to monthly Hadley Centre global sea Ice coverage and SST (HadISST) reconstructed fields. The reanalyses are thus designed to investigate the impact of different strategies for constraining the SST in long-term assimilation experiments and the feasibility of assimilating subsurface observations in the same kind of experiments. The atmospheric forcing comes from the ensemble mean of the Twentieth-Century Reanalysis version 2 (20CRv2). Due to biases being particularly large at high latitudes, corrections are applied to the atmospheric forcing, based on the comparison with ECMWF's ERA-Interim during the overlapped period. In general, these ocean reanalyses capture trends and variability of key ocean parameters. A warm bias of SST in CHOR_AS in the first half of the century leads to incorrect long-term trends. However, SST anomalies in both CHOR_AS and CHOR_RL are realistically captured. Heat contents at 0-300, 0-700 and 0-2000 m have an increasing trend in both CHOR_RL and CHOR_AS, consistent with available datasets for the second half of the century. In order to reproduce realistic mass transport, mixed-layer depth and eddy kinetic energy, it is necessary to assimilate vertical profile data, although this leads to some discontinuities in the reanalysis time series. Based on these findings, we suggest ideas for the requirements of future historical ocean reanalyses, such as related to the necessity of applying bias correction and minimizing the effects of the observing network discontinuities. Meanwhile, the comparison between CHOR_RL and CHOR_AS shows that both direct assimilation and the nudging scheme have their advantages and disadvantages. A combination of these two schemes is desirable in future work.