Lagged and Instantaneous Impacts of the NAO on Subdecadal Variability of the Norwegian Sea Temperature

Abstract We disentangle the role of the North Atlantic Oscillation (NAO) in the subdecadal variability of the Norwegian Sea upper ocean temperature. With emphasis on the subdecadal time scale, we are able to detect lagged and instantaneous impacts of the NAO on the Norwegian Sea temperature in a MPI-ESM-LR1.2 assimilation product. By inducing temperature anomalies in the subpolar North Atlantic via buoyancy forcing, the lagged impact of the NAO manifests itself through the oceanic pathway. The resulting temperature anomalies in the subpolar North Atlantic show a clear poleward propagation and spread across the Norwegian Sea in the following 4-5 years. The NAO also exerts an instantaneous impact on the Norwegian Sea temperature by modulating turbulent heat flux and wind-driven transport into the Norwegian Sea. The positive (negative) NAO elevates (lowers) sea surface height along the Norwegian continental shelf and enhances (reduces) temperature transport into the Norwegian Sea simultaneously. The twofold, lagged and instantaneous impact of the NAO limits the predictability of the Norwegian Sea temperature. Although the lagged impact of the NAO acts as ocean memory and emerges 4-5 years later, the instantaneous impact may damp and counteract the lagged oceanic signal, leading to only 1-year predictability of the Norwegian Sea temperature. We find that the initialized positive NAO events largely contribute to this 1-year predictability, and we demonstrate that the lagged impact of the NAO via slow ocean dynamics is potentially predictable. The emergence of ocean memory reveals the possibility to improve subdecadal prediction of the Norwegian Sea temperature.