Sub-Monthly Polar Vortex Variability and Stratosphere-Troposphere Coupling in the Arctic

The annual breakdown of the stratospheric polar vortex is known as the stratospheric final warming [SFW]. A recent observational study of SFW events found that these events are associated with a vertically coherent north-south dipole pattern in the zonal wind anomaly field extending from the mid-stratosphere downward to Earth's surface at mid to high latitudes. However, this pattern is distinct from the canonical Northern Annular Mode (NAM) structure as the primary centers in the north-south anomaly dipole are retracted northward compared to the NAM. These results suggest that SFW events may be associated with distinct and previously unrecognized annular modes of variability at high latitudes. Here we explore this idea with a more general characterization of the primary modes of intraseasonal variability in the wintertime stratospheric polar vortex. In contrast to previous studies, the current analysis concentrates on zonal-mean variability on short [daily] time scales within a limited spatial domain [the lower stratosphere at high latitudes] encompassing the stratospheric polar vortex.

An empirical orthogonal function analysis is used to characterize daily variability in the boreal stratospheric polar vortex. The leading EOF modes consist of vertically coherent north-south dipoles in the zonal-mean zonal wind extending from the mid-stratosphere to the Earth's surface. The first mode represents intraseasonal variability in the polar vortex strength and is highly correlated with the stratospheric NAM. The second mode, referred to here as the Polar Annular Mode [PAM], represents intraseasonal variability in the latitudinal position of the polar vortex and is structurally and statistically distinct from NAM, with the former having a northward retracted spatial structure. Composite analyses of the respective PC time series indicate that PAM events are relatively short-lived [1-2 weeks] compared to their NAM counterparts. We further find that the evolution of sudden stratospheric warming events is dominated by NAM variability whereas NAM and PAM play approximately equal roles in stratospheric final warming events. Finally, regression analyses reveal that the PAM is associated with circumpolar circulation and temperature anomalies within the high latitude troposphere. It is concluded that PAM represents a previously unrecognized intraseasonal annular mode that is orthogonal to the NAM and strongly couples the stratosphere and troposphere on submonthly time scales at mid to high latitudes