The Salience of Nonlinearities in the Boreal Winter Response to ENSO: Arctic Stratosphere and Europe

The salience of nonlinearities in the Arctic stratospheric response to El Nino - Southern Oscillation (ENSO) is evaluated in a 41 member ensemble of the period 1980 to 2009 in the the NASA Goddard Earth Observing System Chemistry-Climate Model. Central Pacific El Nino (CP EN) events preferentially increase zonal wavenumber 2, while Eastern Pacific El Nino (EP EN) events preferentially increase zonal wavenumber 1, though these nonlinearities become statistically robust only after more than ~50 events are considered. This leads to a difference in the morphology of SSW events preferentially forced by each ENSO flavor. While the zonal-mean Arctic stratospheric response is little different between EP and CP EN once one accounts for the relative weakness of CP events, the tropospheric subpolar negative North Atlantic Oscillation response is stronger for CP events. Other nonlinearities are even less salient: there is no indication of any nonlinearities in the wave driving between moderate EP EN and extreme EP EN, nor between EN and La Nina (LN). However in approximately 15% of the integrations within our ensemble, there is no difference in SSW frequency between EN and LN, suggesting that a similar SSW frequency for both EN and LN, as has occurred over the past ~60 years, can occur by chance. The intra-ensemble variability in LN SSW frequency can be related back to intra-ensemble variability in the tropospheric response to LN.