A simple GCM model study on the relationship between ENSO and the Southern Annular Mode

This study examines the relationship between (El Niño/Southern Oscillation) ENSO and Southern Annular Mode (SAM) events during austral spring with an idealized, multi-level, primitive equation model. A series of idealized model calculations are performed to address the basic dynamical questions such as why positive (negative) SAM events are substantially more frequent during La Niña (El Niño), why most stratospheric SAM events are confined mostly to El Niño and La Niña years, and how the stratospheric SAM influence the tropospheric SAM. The model simulations show that the zonal symmetric heating associated with El Niño (La Niña) produces a stronger (weaker) subtropical jet and weaker (stronger) eddy-driven jet in the troposphere, without having a large impact of planetary-scale wave propagation. For the local heating component (zonal asymmetric) of ENSO, using Eliassen-Palm flux diagnostics, it is found that the model flow is further modified by anomalous wave propagation both in the troposphere and the stratosphere. Specifically, the El Niño-like heating profile over the tropics induces anomalously strong (relative to the model's climatology) planetary-scale Rossby waves that propagate poleward and upward into the extratropical upper troposphere and stratosphere which results in a weaker stratosphere vortex. In contrast, for the La Niña-like heating profile, anomalously weak Rossby wave propagation results in smaller changes in the extratropical troposphere and stratosphere. These different wave propagation characteristics are explained by changes in the refractive index and Rossby wave source. For the El Niño experiment, negative phase SAM events dominate, whereas for the La Niña experiment, the positive phase SAM events occur more frequently. These negative phase SAM/El Niño and positive phase SAM/ La Niña events are found to persist for a longer period of time compared to SAM events during neutral years. It is found that the above coupling between the stratosphere and troposphere can account for both this asymmetry in the SAM events and the increased persistence of the SAM events during ENSO years.