Predictability and prediction skill of the Southern Annular Mode based on its relationship with ENSO

The Southern Annular Mode (SAM) is the dominant mode of variability of the extratropical atmospheric circulation in the Southern Hemisphere (SH) throughout different time-scales, ranging from weeks to decades. The positive index of SAM is characterised by lower than average pressure over the high latitudes and higher than average pressure over the mid latitudes, and its variability explains ~30% of the total variance of the circulation over the SH extratropics. SAM is driven by internal atmospheric dynamics as evidenced by its typical decorrelation time scale of ~10 days. Nevertheless, in the current study we demonstrate that seasonal variability of SAM can be predicted with good skill (proportion correct > 60%, correlation > 0.4) at lead times of up to 3 months by the Australian Bureau of Meteorology's dynamical seasonal forecast system, POAMA.

The predictability of seasonal SAM stems from its relationship with tropical sea surface temperatures associated with ENSO. Our study shows two distinctive periods of high predictability: the SH late autumn to winter and the SH late spring to early summer. Predictability of the SAM in austral winter stems from the association of the SAM with warm-pool (or Modoki/central Pacific) El Niņo/La Niņa, whereas predictability in austral early summer stems from the association of the SAM with cold-tongue (or eastern Pacific) ENSO. As POAMA is capable of predicting ENSO with high skill and simulating the SAM and ENSO relationship, it demonstrates good skill in predicting the phase and amplitude of seasonal anomalies of the SAM beyond a season in advance for austral early summer and late autumn.