Revisiting the ENSO–SSW Relationship

Stratospheric sudden warming (SSW) events exhibit pronounced interannual variability. Based on zonal wind reversals at 60°N and 10 hPa, it has been suggested that SSW events occur more preferentially during El Niño–Southern Oscillation (ENSO) winters (both El Niño and La Niña winters) than during ENSO-neutral winters. This relationship is reevaluated here by considering seven different SSW definitions. For all definitions, SSW events are detected more frequently during El Niño winters than during ENSO-neutral winters, in agreement with a strengthened planetary-scale wave activity. However, such a systematic relationship is not found during La Niña winters. While three SSW definitions, including the wind-reversal definition, show a higher SSW frequency during La Niña winters than during ENSO-neutral winters, other definitions show no difference or even lower SSW frequency during La Niña winters. This result, which is qualitatively insensitive to the choice of reanalysis datasets, ENSO indices, and SST datasets, indicates that the reported ENSO–SSW relationship is dependent on the details of the SSW definition. This result is interpreted in terms of different background wind, latitudinal extent of wind reversal, and planetary-scale wave activity during El Niño and La Niña winter SSW events.