Inverse Energy Cascades in an Eddy-Induced NAO-Type Flow: Scale Interaction Mechanism

In this paper, based on a new wave–eddy interaction framework, the interaction mechanism between the North Atlantic Oscillation (NAO) and synoptic-scale eddies is revealed by using the analytical solutions of a two-scale model as a description of the inverse energy cascade from nonuniform synoptic-scale eddies to the large-scale NAO flow. It is found that the spatial shape of the eddy-induced large-scale streamfunction tendency prior to the NAO onset determines the direction of eddy energy transfer, as well as the phase and growth of the NAO. However, the feedback of the intensified NAO anomaly on synoptic eddies can affect significantly the asymmetry of the NAO between negative (NAO−) and positive (NAO+) phases in amplitude and persistence through the presence or absence of the eddy straining related to cyclonic wave breaking (CWB). For the NAO+, the stretching deformation role of the NAO+ field seems dominant in the eddy variation. Because the eddy energy generation rate (EGR) weakens and tends to be negative in the downstream side of the NAO+ region, the synoptic eddies lose their energy to the NAO+-type zonal flow, thus leading to the weakening of synoptic-scale eddies. However, for the NAO−, the EGR variation shows that synoptic eddies grow over the two upstream sides of the NAO− region by extracting energy from the NAO− shearing deformation field, while losing energy to the mean flow over the upstream middle region through the stretching deformation. This process results in the eddy straining (splitting and strengthening) associated with the CWB.