The Polar Annular Mode: Submonthly Stratosphere-Troposphere Coupling in the Arctic

An empirical orthogonal function analysis is used to characterize daily variability in the boreal stratospheric polar vortex. The leading modes consist of vertically coherent north-south dipoles in the zonal-mean zonal wind field extending from the middle stratosphere down to the Earth's surface. The first mode represents intraseasonal variability in polar vortex strength and is highly correlated with the Northern Annular Mode [NAM] defined at stratospheric altitudes. The second mode, introduced as the Polar Annular Mode [PAM], represents variability in the latitudinal position of the stratospheric polar vortex and is significantly distinct from the NAM both structurally and statistically. PAM is characterized by short intrinsic timescales (1-2 weeks) and a northwardly retracted polar vortex. It is concluded that the PAM represents a newly recognized annular mode that (a) strongly couples the stratosphere-troposphere system on submonthly timescales, (b) provides a robust cyclonic circulation in the high-latitude troposphere, and (c) plays an essential role in stratospheric final warming events (although occurence of PAM events is not confined to boreal spring).