On the linearity of the stratospheric and Euro-Atlantic sector response to ENSO

The dependence of the winter stratospheric and Euro-Atlantic climate response on ENSO amplitude is investigated using the HadGEM3 model. Experiments are performed with imposed east Pacific sea surface temperature perturbations corresponding to Nino-3.4 anomalies of +/- 0.75, 1.5, 2.25, and 3.0 K. In the North Pacific, El Nino (EN) deepens and shifts the Aleutian low eastward, while the equivalent magnitude La Nina (LN) perturbations drive anomalies of opposite sign that are around 4 times weaker. The muted North Pacific response to LN can be traced back to the weaker response of tropical convection and the associated anomalous Rossby wave source. The EN perturbations weaken the Arctic polar vortex, with the winter mean zonal mean zonal wind at 60 degrees N and 10 hPa decreasing approximately linearly with Nino-3.4 anomaly by around -3.6 m s(-1) K-1. For the strongest EN case (+3 K), the frequency of sudden stratospheric warmings (SSWs) increases by similar to 60% compared to the control experiment. Hence the results do not support a saturation of the stratospheric pathway for strong EN as suggested in previous literature. The equivalent amplitude LN perturbations cause a weak strengthening of the polar vortex and no substantial change in SSW frequency, in contrast to some reanalysis-based studies. EN induces a negative North Atlantic Oscillation (NAO) index throughout boreal winter, which increases approximately linearly with the Nino-3.4 anomaly by around -0.6 standard deviations K-1. Only the response to the strongest LN perturbations projects onto a weak positive NAO in November, suggesting that the mechanism for the Euro-Atlantic response to LN may be distinct from EN.