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We evaluate the ensemble spread at seasonal-to-interannual timescales for twoperturbation techniques implemented in the ocean component of a coupledmodel: (1) lagged initial conditions as commonly used for decadalpredictions; (2) bred vectors as commonly used for weather and seasonalforecasting. We show that relative to an uninitialized reference simulationthe implementation for bred vectors can improve the ensemble spread comparedto lagged initialization at timescales from one month up to three years.  As bred vectors have so far mostly been used at short timescales, weinitially focus on the implementation of the bred vectors in the oceancomponent. We introduce a depth-dependent vertical rescaling norm, accountingfor the vertical dependence of the variability, and extending the commonlyused upper-ocean rescaling norm to the full water column. We further showthat it is sufficient for the (sub-surface) ocean to breed temperature andsalinity (i.e., scalar quantities), and rely on the governing physics tocarry the temperature and salinity perturbations to the flow field.  Using these bred vectors with a rescaling interval of 12 months, weinitialize hindcast simulations and compare them to hindcast simulationsinitialized with lagged initial conditions. We quantify the ensemble spreadby analyzing Talagrand diagrams and spread–error ratios. For bothtemperature and salinity, the lagged initialized ensemble is particularlyunder-dispersive for the first few months of predictable lead time. Theensemble initialized with bred vectors improves the spread for temperatureand salinity for the 0–700 m and 1000–3500 m means, compared to thelagged ensemble at lead times of several months to one year. As the lead timeincreases to years, the differences between the two ensemble initializationtechniques become more difficult to discern. While the results need to beconfirmed in an initialized framework, the present analysis represents afirst step towards improved ensemble generation at the transition fromseasonal to interannual timescales, in particular at lead times up to oneyear

Ensemble initialization of the oceanic component of a coupled model through bred vectors at seasonal-to-interannual timescales