ENSO impact in the Wave Forcing of the Stratospheric Zonal-Mean Flow

The impact of El Niño/Southern Oscillation (ENSO) on the stratospheric dynamics variability is investigated using the momentum equation in the Transformed-Mean Eulerian (TEM) framework with the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). This study reveals that the response in the stratospheric zonal-mean flow to ENSO is complex, only moderately statistically significant, and varies greatly from month to month. In particular, the response of the polar vortex to ENSO resembles a Stratospheric Sudden Warming, with a brief weakening of the vortex in January and February and quick recovery in March. The weakening of the polar vortex is due to more vertical propagation of stationary planetary waves in January during El Niño events compared to La Niña events. In February, the budget analysis reveals that the recovery of the polar vortex is due to a significant decrease in the planetary waves, with contribution from both stationary and transient waves.

Since ENSO is very much a zonally asymmetric phenomenon, it is likely that a large part of the noise and lack of strong statistical significance seen in this study are due to the two-dimensionality of the TEM framework. As a result, this study is extended to a three-dimensional analysis of the wave forcing of the zonal-mean flow, revealing the zonally asymmetric impact of ENSO on the stratosphere and displaying higher statistical significance. Overall, the results underline the shortcomings of the TEM framework when investigating zonally asymmetric teleconnections, and points out the need for a three-dimensional analysis for ENSO studies.