Using a recent 22-yr (1979-2000) record of NCEP-DOE Reanalysis 2 dataset (hereafter, NCEP2) and a two-level spherical quasi-geostrophic, quasi-linear model, we have investigated the eddy--zonal-mean flow interactions associated with the southern annular mode (SAM) variability. The main emphasis was to highlight the differing nature of the eddy-zonal mean flow interactions a) due to the high-frequency (2-8 days) eddies and medium-frequency (8-30 days) eddies, and b) during the extreme and transitional phases of the SAM. Features demonstrating this differing nature were first identified from the NCEP2 reanalyses using various statistical tools, such as EOF analysis, cross-correlation analysis, spectral and cross-spectral analysis, and latitude-lag composites. The results show that, in addition to a previously documented positive feedback due to the high-frequency eddies, the medium-frequency eddies provide an oscillatory feedback to the SAM variability, leading to an enhancement of the latter's spectral power in an intermediate frequency band. The latitude-lag composites of the transient eddy forcing (TEF) due to the high-frequency eddies during the two extreme (positive and negative) phases and two transitional phases of the SAM reveal some differences in the eddy-zonal-mean flow relationship during these phases. In particular, the composite negative phase anomalies have a longer life-time and larger magnitudes than the positive phase anomalies. Evidence is presented suggesting that the long-term variations of the zonal flow anomalies in middle and high latitudes are both forced by the high-frequency eddy forcing anomalies at high latitudes. The eddy forcing anomalies in middle latitudes vary almost concurrently with the zonal flow anomalies there.

An eddy-zonal mean flow interaction model, derived from the corresponding full nonlinear model, was then used to show that many of the features revealed in the NCEP2 analyses can be reproduced in this simple model. Numerical solutions with realistic values for the model parameters were obtained that correctly captured the meridional scales and locations of the dominant modes of the observed zonal-mean winds, high-frequency TEF and medium-frequency TEF variability, and their power spectral and cross-spectral characteristics. The differing features of the eddy-zonal-mean flow interaction during the positive and negative phases and between high and middle latitudes found in NCEP2 were also simulated well by the quasi-linear model. The sensitivity of some of these results to model parameters, such as the friction, was also discussed.