Understanding the Frequency of SSW Events During La Nina in a Large Ensemble of Integrations from a Chemistry-Climate Model

The effect of El Nino-Southern Oscillation (ENSO) on the frequency and character of Northern Hemisphere major mid-winter stratospheric sudden warmings (SSWs) is evaluated using a meteorological reanalysis data set and 41 integrations of the GEOSCCM chemistry-climate model covering the period 1980 to 2009.
The frequency of SSW events during La Nina ranges from 0.1 to 0.9 SSW per La Nina year, with this latter frequency in agreement with observations. The causes of these differences among the integrations are examined. The parameter most strongly associated with the relative frequency of SSWs is extreme negative height variability in the Northwest Pacific (i.e. the SSW precursor region from Garfinkel et al 2012).