A principal component analysis is performed to characterize intraseasonal variability in the boreal stratospheric polar vortex. In contrast to previous studies, the current analysis examines daily zonal-mean variability within a limited spatial domain encompassing the stratospheric polar vortex. The leading EOFs are vertically coherent north–south dipoles in the zonal-mean zonal wind extending through the lower stratosphere. The first mode represents variability in polar vortexstrength and is highly correlated with the stratospheric northern annular mode (SNAM). The second mode, the polar annular mode (PAM), represents variability in the latitudinal position of the polar vortex and possesses a poleward-retracted dipole anomaly structure. Composite analyses indicate that large-amplitude PAM events are relatively short lived (1–2 weeks) compared to SNAM events (1 month or longer). Trend analyses further reveal that recent decadal trends in the boreal polar vortex project more strongly onto PAM than SNAM.
Composite analyses illustrate that the time evolution of sudden stratospheric warming events is dominated by SNAM, whereas SNAM and PAM play approximately equal roles in final warming events. Linear regression analyses reveal that SNAM and PAM result in circumpolar circulation and temperature anomalies of similar magnitudes within the high-latitude troposphere. It is concluded that PAM represents a previously unrecognized annular mode that strongly couples the stratosphere and troposphere on submonthly time scales at mid- to high latitudes. It is further suggested that the SNAM/PAM framework provides a means for isolating the proximate tropospheric response to respective variations in the strength and position of the stratospheric polar vortex.
Journal of Climate