Impact of initialization procedures on the predictive skill of a coupled ocean-atmosphere model and related mechanisms for predictability

The sensitivity of the predictive skill of a decadal climate prediction system is investigated with respect to details of the initialization procedure. For this purpose, the coupled ocean-atmosphere UCLA/MITgcm climate model is initialized using the following three different initialization approaches: full state initialization (FSI), anomaly initialization (AI) and full state initialization employing heat flux and freshwater flux corrections (FC). The ocean initial conditions are provided by the GECCO state estimate (the German contribution to Estimating the Circulation and Climate of the Ocean project), from which ensembles of decadal hindcasts are started every 5 years from 1961 to 2001. To evaluate the performance of the initialized hindcasts, they are compared with the persistence forecasts and an ensemble of twentieth-century simulations (un-initialized hindcasts). The study is divided in two main parts. In the first part the primary focus is to estimate the predictive skill for differently initialized hindcasts of sea surface temperature (SST), sea surface height (SSH) and the Atlantic meridional overturning circulation (AMOC). The predictive skill for SST, SSH and AMOC is assessed against the GECCO synthesis using anomaly correlation coefficient and root-mean-squared-error skill score. In regions with a deep mixed layer the predictive skill for SST anomalies remains significant for up to a decade in the FC experiment. By contrast, FSI shows less persistent skill in the North Atlantic and AI does not show high skill in the extratropical Southern Hemisphere, but appears to be more skillful in the tropics. In the extratropics, the improved skill is related to the ability of the FC initialization method to better represent the mixed layer depth, and the highest skill occurs during wintertime. The correlation skill for the spatially averaged North Atlantic SSH hindcasts remains significant up to a decade only for FC. The North Atlantic MOC initialized hindcasts show high correlation values in the first pentad while correlation remains significant in the following pentad too for FSI and FC. Overall, for the current setup, the FC approach appears to lead to the best results, followed by the FSI and AI procedures. An extended analysis of predictive skill for SSH hindcasts from different initialization experiments is performed in the second part. The analysis employs a method that allows to distinguish different contributions to steric SSH changes, namely those related to density changes imposed by temperature or salinity anomalies beneath the mixed layer (thermosteric and halosteric heave terms) and those related to processes of density compensated temperature–salinity changes (spice term). The spice and heave contributions in the mixed layer are not separated (mixed layer term). The patterns of the predictive skill suggest significant improvement of initialization which is related to the thermosteric heave term (in the subtropical Pacific, western North Atlantic), thermosteric mixed layer term (in the subtropical Atlantic) and spice term (in the eastern and subpolar North Atlantic and Southern Ocean). These contributions imply useful predictive skill as they occur in

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