The Connection between the Boreal Spring Southern Oscillation Persistence Barrier and Biennial Variability

Previous work has shown that the widely used Tahiti minus Darwin Southern Oscillation index has a sharp decrease in persistence in the boreal spring. It is suggested that this persistence barrier is due to a purely biennial oscillation B(t) embedded in the Southern Oscillation index. The analysis shows that the Southern Oscillation index S(t) may be written StBtLtMt where B(t) has a zero in May and L(t) and M(t) vary interannually and decadally. Since L is time dependent with negligible mean S(t) does not have a biennial peak even though B(t) is present. The persistence barrier strengthens and weakens decadally as L and M vary decadally. For example, during 1975–84 BL dominates M and the persistence barrier is strong while for the decade 1940–49 BL is negligible and the persistence barrier disappears.