Dynamical relationship between the phase of North Atlantic oscillations and the meridional excursion of a preexisting jet: An analytical study

In this paper, it is shown from an analytical solution that in the presence of a preexisting jet the interaction between the zonal jet and the topography of the land-sea contrast (LSC) in the Northern Hemisphere (NH) tends to induce a dipole component that depends crucially upon whether this zonal jet exhibits a north-south excursion. This phenomenon cannot be observed if the zonal jet has no north-south shift. When the preexisting jet is located more northward (southward), the induced dipole can have a low-over-high (high-over-low) structure and thus can make the center of the stationary wave anomaly shift southward (northward), which can be regarded as an initial state or embryo of a positive (negative) phase North Atlantic Oscillation (NAO). This dipole component can be amplified into a typical NAO event under the forcing of synoptic-scale eddies. To some extent, this result provides an explanation for why the positive (negative) phase of the NAO can be controlled by the northward (southward) shift of the zonal jet prior to the NAO. In addition, the impact of the jet shift on the occurrence of NAO is examined in a weakly nonlinear NAO model if the initial state of an NAO is prespecified. It is found that the northward (southward) shift of a zonal jet favors the occurrence of the subsequent positive (negative) phase NAO event and then results in a northward (southward)-intensified jet relative to the preexisting jet. In addition, during the decaying of the positive phase NAO, a strong blocking activity is easily observed over Europe as the jet is moved to the north.