The combined effects of ENSO and the 11 year solar cycle on the Northern Hemisphere polar stratosphere

Abstract

The combined effects of El Niño-Southern Oscillation (ENSO) and the 11 year solar cycle on the Northern Hemisphere polar stratosphere have been analyzed in the Whole Atmosphere Community Climate Model version 3 in the absence of the quasi-biennial oscillation. The polar response to ENSO agrees with previous studies during solar minimum; composites of warm minus cold ENSO events show a warmer polar stratosphere and a weaker polar vortex, propagating downward as the winter evolves. During solar maximum conditions, little downward propagation of the ENSO signal is simulated, leading to colder temperatures and stronger winds in the polar lower stratosphere. The analysis of the Eliassen-Palm flux and wave index of refraction shows that this is mainly due to a reduction of upward propagating extratropical planetary wave number 1 component caused by changes in the background winds in the subtropics related to a warmer tropical upper stratosphere during solar maximum. The effect of the 11 year solar cycle variability on the polar stratosphere is not significant during cold ENSO events until February. During warm ENSO events, a statistically significant colder polar lower stratosphere and stronger polar vortex are simulated throughout the winter, and no downward propagation of this signal occurs. This is mainly due to the combined effects of solar maximum and warm ENSO conditions on the wave mean flow interaction. These results show a nonlinear behavior of the extratropical stratosphere response to the combination of the two forcings and highlight the need to stratify with respect to ENSO and solar conditions and analyze the seasonal march throughout the winter.