Polar vortex shape-transition during SSW depending on preceding NAO conditions

Sudden stratospheric warming (SSW) characterized by a rapid increase in polar stratospheric temperature and an abrupt decrease in circumpolar westerly wind is accompanied by deformation in the shape of the polar vortex. The SSW type can be distinguished depending on the vortex shape. SSW events preceded by a displacement in polar vortex center are characterized by whether they retain their displaced form (displacement-displacement type) or split into two vortices (displacement-split type) after onset. Here, we show that existence of a polar vortex shape-transition during the course of the SSW life cycle can be attributable to the condition of North Atlantic Oscillation (NAO) preceding before onset: Positive NAO favors SSW of displacement-displacement type with no transition while negative NAO favors the displacement-split type. We show that, in positive NAO precondition, vertical flux of wave activity immediately before onset is mostly contributed only by wavenumber 1 component, which contrasts with the relatively stronger contribution of wavenumber 2 in negative NAO pre-condition. This study provides probability that the North Atlantic anomaly can induce a favorable condition for the development of small scale waves and lead to the occurrence of SSW type-transition. Whole Atmosphere Community Climate Model (WACCM) simulation results reproduce well the observational findings. Therefore, NAO can be regarded as a useful predictor for distinguishing the type of forthcoming SSW events.