El Nino-Southern Oscillation and stratospheric sudden warmings: a re-evaluation

Several modeling studies have explored how the El Nino-Southern Oscillation (ENSO) impacts the Northern Hemisphere stratosphere, and have suggested that stratospheric sudden warmings (SSWs) may occur more frequently during the warm El Nino phase relative to the cold La Nina phase. In this study, using reanalyses and climate model output, we re-examine the relationship between the phases of ENSO and the occurrence of major SSWs, defined (following the WMO) as a reversal in the westerly zonal winds at 60°N and 10 hPa in the Northern Hemisphere wintertime. Surprisingly, we find that (1) major SSWs occur with equal probability during El Nino and La Nina winters; (2) major SSWs occur more often during both El Nino and La Nina winters relative to ENSO-neutral winters (notably, all winters in the NCEP/NCAR reanalysis record when two SSWs have been recorded are not ENSO-neutral); and (3) while major SSWs occur more frequently in mid-winter during El Nino years, they are found to occur more frequently in late winter during La Nina years.