Two tales of initializing decadal climate predictions experiments with the ECHAM5/MPI-OM model

This paper investigates the impact of different ocean initialization strategies on the forecast skill of decadalprediction experiments performed with the ECHAM5/Max Planck Institute Ocean Model (MPI-OM) coupledmodel. The ocean initializations assimilate three-dimensional temperature and salinity anomalies from two differentocean state estimates, the ocean reanalysis of the German contribution to Estimating the Circulation andClimate of theOcean (GECCO) and an ensemble ofMPI-OM ocean experiments forced with the NCEP–NCARatmospheric reanalysis. The results show that North Atlantic and Mediterranean sea surface temperature (SST)variations can be skillfully predicted up to a decade ahead and with greater skill than by both uninitialized simulationsand persistence forecasts. The regional distribution of SST predictive skill is similar in both initializationapproaches; however, higher skill is found for the NCEP hindcasts than for the GECCO hindcasts when a combinationof predictive skill measures is used. Skillful predictions of surface air temperature are obtained overnorthwestern Europe, northern Africa, and central-eastern Asia. The North Atlantic subpolar gyre region standsout as the region with the highest predictive skill beyond the warming trend, in both SST and upper-ocean heatcontentpredictions. Here the NCEP hindcasts deliver the best results due to a more accurate initialization of theobserved variability. The dominantmechanism for North Atlantic climate predictability is of dynamical origin andcan be attributed to the initialization of the Atlantic meridional overturning circulation, thus explaining the reoccurrenceof high predictive skill within the second pentad of the hindcasts experiments. The results hereindemonstrate that ocean experiments forced with the observed history of the atmospheric state constitute a simplebut successful alternative strategy for the initialization of skillful climate predictions over the next decade