제목: Regime Behaviour in the Upper Stratosphere as a Precursor of Stratosphere–Troposphere Coupling

Every winter, a band of strong westerly winds forms in the stratosphere (15-50 km above the ground) and encloses a large pool of extremely cold air over the Pole. This so-call stratospheric polar vortex is sporadically disrupted by upward propagating waves from the troposphere. Under certain conditions, anomalous disruptions or strengthening of the polar vortex are connected all the way from the upper stratosphere back into the troposphere. Via this connection, the stratospheric polar vortex plays an important role in influencing near surface weather on sub-seasonal to seasonal time scale. Climate models however have limited success in capturing such a connection as the relevant mechanisms remain unclear. 

In this talk, we introduce a newly derived regime index, which captures the seasonal development of the stratospheric polar vortex in the northern winter. The index is based on variability in the early winter upper stratosphere and discerns the early winter stratosphere into two types of configurations: the narrow- and wide-jet regimes. With the narrow-jet configuration, the polar vortex is more susceptible to the disturbances of upward propagating planetary-scale Rossby waves; it weakens earlier and becomes vertically shallower since January. With the wide-jet configuration, the polar vortex is more susceptible for filamentation induced by Rossby wave breaking. These two flow regimes differ in both the occurrence and wavenumber disturbances of extreme stratospheric events, i.e. sudden stratospheric warmings (SSWs). They also differ in terms of circulation anomalies in the lower stratosphere and the troposphere in middle winter, with marked fingerprints in the large-scale atmospheric modes, including the North Atlantic Oscillation (NAO), the Pacific/North American teleconnection pattern (PNA), and the Arctic Oscillation (AO). The midwinter storm tracks over western Europe and the east coast of North America are also found to be sensitive to the early-winter-upper-stratospheric flow regimes, which may have implications for extreme weather events in those regions. Possible mechanisms relating to solar forcing, internal wave reflection, and resonance will be discussed.