Amplitude-dependent relationship between the Southern Annular Mode and the El Nino Southern Oscillation in austral summer

Abstract

Co-variability between the Southern Annular Mode (SAM) and the El Nino Southern Oscillation (ENSO) during the austral summer is examined, and it is found that there exists an apparent co-variability of a negative (positive) SAM during the mature period of El Nino (La Nina). However, this co-variability is largely controlled by the small number of strong ENSO cases. When strong ENSO cases are excluded, the correlation becomes nonsignificant. This behavior in the relationship between SAM and ENSO is supported by a series of general circulation model experiments with prescribed sea surface temperature boundary conditions that represent the incremental strengthening of El Nino (La Nina) conditions. The modeled Antarctic sub-polar jet exhibits similar behavior to that identified through observational analysis. Marked changes in both the magnitude and position of the sub-polar jet are largely controlled by particularly strong transient eddy forcing. Planetary wave forcing plays only a minor role in the co-variability, but it can explain in part the asymmetric response of the sub-polar jet between El Nino and La Nina.