Intraseasonal dynamical evolution of the Northern Annular Mode

Recent observational and modeling studies indicate a robust dynamical coupling between the stratosphere and troposphere during boreal winter. This coupling occurs in association with the Northern Annular Mode (NAM), which itself accounts for a significant fraction of the variability of the extratropical circulation. While monthly NAM dynamics have been studied previously, the mechanisms that give rise to NAM variability on short intraseasonal timescales are still unclear.

We perform composite diagnostic analyses based on daily NAM indices with a goal of identifying the dominant processes responsible for the growth and decay of large amplitude positive and negative NAM events on short intraseasonal time scales. More specifically, composite analyses of NAM tendencies are performed to isolate the structural and dynamical evolution of NAM events. Zonal-mean and three-dimensional eddy-flux diagnoses are used to examine the role of eddy-mean flow interaction in driving the wind tendencies characteristic of the NAM. In particular, Plumb flux analyses are employed to quantify the contribution of regional stationary wave anomalies toward the zonal mean wind tendency field. Potential vorticity inversions are also used to determine the role of stratospheric anomalies in inducing tropospheric circulations. Emphasis is placed on the demise of mature NAM events in an attempt to determine the physical mechanisms behind such a transition.

A remarkable degree of reverse symmetry is observed between the zonal-mean dynamical evolution of positive and negative NAM events. Anomalous equatorward and downward (poleward and upward) Eliassen-Palm fluxes are observed during the maturation of positive (negative) NAM events, consistent with index of refraction considerations and an indirect downward influence from the stratosphere. The associated patterns of anomalous wave driving provide the main forcing of the zonal wind tendency field. Spectral analyses reveal that both the stratospheric and tropospheric patterns of wave driving are primarily due to low frequency planetary scale eddies. Regional wave activity flux diagnoses further illustrate that this wave driving pattern represents the zonal mean manifestation of planetary scale anomalies over the North Atlantic that are linked to local anomalies in stationary wave forcing. The decay of NAM events coincides with the collapse in the pattern of anomalous stationary wave forcing over the North Atlantic region. Our diagnostic results indicate that both synoptic eddies and direct downward stratospheric forcing provide second order reinforcing contributions to the intraseasonal dynamical evolution of NAM events