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1. Introduction
El Niño-Southern Oscillation (ENSO) affects not only the tropospheric circulation but also the stratospheric circulation in the Northern Hemisphere extratropics (Horel and Wallace 1981; van Loon and Labitzke 1987). Although the stratospheric impact is relatively minor, ENSO-induced stratospheric circulation changes are well observed in the seasonal-mean polar vortex. In general, the polar vortex becomes anomalously warm and weak during El Niño winters but cold and strong during La Niña winters (Fig. 1a; van Loon and Labitzke 1987; Manzini et al. 2006; Calvo et al. 2008; Iza et al. 2016). For instance, the Arctic stratospheric temperature difference between El Niño and La Niña winters is over 4 K in the long-term radiosonde observations (Free and Seidel 2009). These ENSO-related polar vortex changes are essentially driven by vertically propagating waves from the troposphere to the stratosphere. However, they can affect the underlying troposphere as well through downward coupling, providing an important source of boreal winter seasonal prediction in the Northern Hemisphere extratropics (Cagnazzo and Manzini 2009; Ineson and Scaife 2009; Butler et al. 2014).
Recent studies have shown that the above relationship may not hold for the subseasonal variability of the polar vortex (Fig. 1b). It is well documented that stratospheric sudden warming (SSW) events occur more frequently during El Niño winters than during ENSO-neutral winters (Taguchi and Hartmann 2006). This is consistent with the anomalously weak polar vortex and enhanced planetary-scale wave activity during El Niño winters. However, Butler and Polvani (2011, hereafter BP11) and Butler et al. (2014, hereafter BPD14) reported that such an increase of SSW frequency is also found during La Niña winters. In other words, SSW events tend to occur more preferentially during both El Niño and La Niña winters (ENSO winters) than during ENSO-neutral winters. Supporting their finding, the polar vortex exhibits enhanced variability during both El Niño and La Niña winters in comparison to during ENSO-neutral winters (Fig. 1b). This is in stark contrast to a linear relationship between ENSO and the seasonal-mean polar vortex strength (Fig. 1a).
More frequent SSW events during ENSO winters than ENSO-neutral winters have been explained by different planetary-scale wave responses to ENSO (Garfinkel et al. 2012; Li and Lau 2013). During El Niño winters, zonal wavenumber 1 (k = 1)...