| This study examines the relationship between Southern Annular Mode (SAM) events and the El Nino-Southern Oscillation (ENSO) using daily ERA-40 data. The data coverage spans the years 1979 through 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 some basic variables such as zonal-mean zonal wind, Elliassen-Palm fluxes, and two diagnostic Rossby wave breaking indices.The results of this investigation suggest that the triggering mechanism of SAM is eddy-mean flow interaction. Prior to SAM events about 10 days, there is tropospheric background flow change which projects on SAM-positive/negative-like pattern. Due to the basic flow structure change, transient Rossby eddies play an important role in developing SAM, as the momentum flux transportation during Rossby wave breaking strengthen the original wind field anomalies. As a result, this leads'to more Rossby wave breaking. Such processes happen recurrently and lead to the SAM dipole pattern eventually. Thus in this study, we take the view point that it is the Rossby wave breaking excites SAM pattern, and the phase of SAM is decided by the original background flow.In this study, we defined two diagnostic variables to study the synoptic-scale Rossby wave breaking. One is the wave breaking potential (WBP), a zonal mean quantity that is used to estimate the likelihood of wave breaking. The WBP index, based on the zonal-mean flow characteristics, combines the horizontal wind shear and Potential Vorticity gradient and measures the non-dimensional time scale of Rossby wave breaking. The other one is the wave breaking index (WBI), which is based on the sign reversal of the potential temperature gradient on 2PVU surface and used to evaluate the strength of the wave breaking. The results of composite WBI for SAM events are consistent with E-P flux analysis, which shows the dominant factor of SAM formation is the anti-cyclonic (LC1) wave breaking on the equatorward side of the eddy-driven jet, while the cyclonic (LC2) wave breaking on the poleward side of the jet plays relatively weak role.The results of this investigation suggest that during ENSO there are significant changes in zonam-mean flow, E-P flux, WBI and WBP index. The background zonal-mean flow associated with La Nina (El Nino) is preconditioned for strong (weak) anti-cyclonic 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 indicates that strong (weak) anti-cyclonic wave breaking takes place with a much higher frequency during La Nina (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 showed that austral spring SAM events that coincide with ENSO are preceded by strong stratospheric SAM anomalies. During El Nino, the cyclonic wave breaking on the poleward side of the eddy-driven jet (60°S) occur more frequently. Via upscale energy cascade, energy is transferred to lower wave number component, which is planetary-scale wave propagation between the troposphere and stratosphere. The increasing vertical wave activities disturb the polar vortex and lead to the stratosphere zonal-mean flow deceleration (easterly wind anomalies). In contrast, during La Nina, the cyclonic wave breaking activities reduce dramatically, leading to weaker vertical wave propagation. Besides, the austral spring SAM events also correspond to a prolonged period of wave breaking that lasts for about 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.
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