The structure, dynamics and tropospheric signature of the Polar Annular Mode

Authors


Abstract

[1] Intraseasonal variability in the stratospheric polar vortex is intrinsically linked to concomitant tropospheric variability via the annular modes. The primary mode, the Northern Annular Mode (NAM), represents variability in the polar vortex strength while the second mode, the lesser studied Polar Annular Mode (PAM), represents variability in the latitudinal position of the polar vortex. The current study examines the structural evolution, zonal-mean dynamical forcing and tropospheric impact of discrete PAM episodes occurring during boreal winter. The analysis identifies large amplitude positive and negative PAM events occurring in the absence of NAM or sudden stratospheric warming events. Composite time evolution analyses are performed for both positive and negative PAM events. The onset of positive (negative) PAM events is preceded by a weak and broad acceleration (deceleration) within the stratospheric polar vortex at midlatitudes. During PAM onset the canonical PAM structure in zonal-mean zonal wind develops over a 5 day period in association with robust stratosphere-troposphere coupling. Zonal wind changes occurring during onset are induced by anomalous wave driving associated with low frequency Rossby wave activity. After PAM onset, the zonal wind anomaly features at high latitudes quickly weaken while those in the subtropics and midlatitudes persist for a period of 2–3 weeks. During this time, PAM events provide a statistically significant impact upon the tropospheric circulation, with large-scale perturbations in (a) surface air temperature over North America and Eurasia and (b) the structure and amplitude of the primary storm tracks.