Dynamics of eddy-driven North Atlantic Oscillations in a localized shifting jet: zonal structure and downstream blocking

Abstract

In a weakly nonlinear model how an initial dipole mode develops to the North Atlantic Oscillation (NAO) in a localized shifting jet under the prescribed eddy forcing is examined. It is found that the zonal structure of the eddy-driven NAO anomaly is not only dominated by the longitudinal distribution of the preexisting Atlantic storm track, but also by the initial condition of the NAO anomaly itself associated with the interaction between a localized shifting jet and a topographic standing wave over the Atlantic basin. When both the initial NAO anomaly and the eddy vorticity forcing in the prior Atlantic storm track are more zonally localized, the subsequent eddy-driven NAO anomaly can be more zonally isolated and asymmetric. But, it seems that the shape of the initial NAO anomaly associated with the latitudinal shift of a prior Atlantic jet plays a more important role in producing the zonal asymmetry of subsequent NAO patterns. The zonal asymmetry of the NAO anomaly can be enhanced as the height of topography increases. In addition, it is further found that blocking events occur easily over the Europe continent through the decaying of positive-phase NAO events. However, prior to the positive-phase NAO life cycle the variability in each of three factors: the Atlantic jet, the eddy vorticity forcing in the Atlantic storm track and the initial NAO anomaly can result in a variation in the blocking activity over the Europe sector in strength, duration, position and pattern.