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A new generic perturbation scheme suitable for generation of an ensemble of ocean analysis is presented. The scheme consists of two distinct elements: perturbations to the assimilated observations, both profiles and surface observations, and perturbations to the surface forcing fields. The new scheme has been applied to the new Ocean ReAnalysis System-5 (ORAS5). The surface forcing perturbation has also been used to create oceanic surface forcing for ERA5, and in operational Ensemble Data Assimilation (EDA) from cycle 43R1.

The idea behind the observation perturbation scheme is to account for observation representativeness error. Instead of perturbing the value of the assimilated observations, the scheme perturbs the position of the observations. This is done by applying perturbations to the geographical location of the insitu temperature and salinity profiles, and by random thinning, both in the horizontal for surface observations, and in the vertical for dense profiles. This method exploits the full observation data set and uses more observations (through ensemble approach) than the previous thinning method. The impact of the perturbation scheme in the ocean reanalysis is illustrated together with selected sensitivity experiments. It is shown that the observation perturbations have little impact in global or basin wide climate indices, but they have local effect. The ensemble spread shows large errors in regions with strong mesoscale eddy activities and in areas affected by the Mediterranean Outflow waters. These are regions where departures with respect to observations are also large. It is also shown that ensemble spread in the tropical …