Coupled bred vectors in the tropical Pacific and their application to ENSO prediction

Two independent coupled bred vectors (BVs) were generated to capture growing errors related to a high-frequency mode and typical ENSO mode by applying different rescaling cycles using the SNU Coupled GCM. We showed that zonal advection (vertical advection) is primary mechanism to develop SST perturbation with 1-month (6-month) rescaling cycle, which implies that the physical mechanisms of two coupled BVs are different. In addition, the dominant pattern of a 1-month (6-month) rescaling BV thermocline depth showed a large positive anomaly over the central Pacific (eastern Pacific). Twenty-year ensemble hindcast with BVs showed a significant correlation improvement over the tropical Pacific regions when compared to lagged averaged forecast perturbations. The improvement was greatest after taking ensemble average of two pairs of BVs, providing evidence that BV1 and BV6 exhibit dynamically independent modes.

► Two independent coupled bred vectors (BVs) were generated to capture growing errors.
► Zonal advection (vertical advection) is primary mechanism to develop SST perturbation with 1-month (6-month) rescaling cycle.
► Ensemble forecasts with BVs showed a significant correlation improvement over the tropical Pacific regions.