Abstract/Summary

The influence of El Nino Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective of the work is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone. We find a small but statistically significant relationship between ENSO and the SPV, with El Nino events occurring with weak SPV and La Nina events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. However the nature of this interference evolves through the spring and summer period, and is not aligned with the traditional seasons. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Nino events occur with weak SPV and La Nina events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Nino events occur together with a weak SPV, and when La Nina events occur together with strong SPV.