The Impact of ENSO on Wave Breaking and Southern Annular Mode Events

This study examines the relationship between intraseasonal southern annular mode (SAM) events and the El Nino-Southern Oscillation (ENSO) using daily 40-yr ECMWF Re-Analysis (ERA-40) data. The data coverage spans the years 1979-2002, for the austral spring and summer seasons. The focus of this study is on the question of why positive SAM events dominate during La Nina and negative SAM events during El Nino. A composite analysis is performed on the zonal-mean zonal wind. Eliassen-Palm fluxes, and two diagnostic variables: the meridional potential vorticity gradient, a zonal-mean quantity that is used to estimate the likelihood of wave breaking, and the wave breaking index (WBI), which is used to evaluate the strength of the wave breaking. The results of this investigation suggest that the background zonal-mean flow associated with La Nina (El Nino) is preconditioned for strong (weak) anticyclonic wave breaking on the equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive positive (negative) SAM events. A probability density function analysis of the WBI, for both La Nina and El Nino, indicates that strong anticyclonic wave breaking takes place much more frequently during La Nina and weak anticyclonic wave breaking during El Nino. It is suggested that these wave breaking characteristics, and their dependency on the background flow, can explain the strong preference for SAM events of one phase during ENSO. The analysis also shows that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies and then are followed by a prolonged period of wave breaking that lasts for approximately 30 days. These findings suggest that the ENSO background flow also plays a role in the excitation of stratospheric SAM anomalies and that the presence of these stratospheric SAM anomalies in turn excites and then maintains the tropospheric SAM anomalies via a positive eddy feedback.