The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrennial timescale

A correlation analysis is performed to investigate the relationship between El Niño-Southern Oscillation (ENSO) and the Antarctic oscillation (AAO) at the quasi-quadrennial (QQ) timescale. It is found that the cold tongue index (CTI) and the AAO index (AAOI) are negatively correlated with about a 7-month lead-time, while they are positively correlated with about a 15-month lag-time. To further explore this relationship, complex empirical orthogonal function analysis is employed in the QQ sea level pressure (SLP) anomalies from 1951 to 2002. The results indicate that, during the ENSO cycle, there exists one kind of global tropical wave of wavenumber 1 (GTW1) propagating eastward. With the traveling of GTW1, the tropical SLP anomaly tends to intrude into the southern mid-latitudes. Accordingly, three strong signals travel synchronously along the circum-South-Pacific path, and a relatively weak signal extends eastward and poleward over the South Ocean in the Atlantic-Indian Ocean sector. Following the propagation of these signals, the AAO phase tends to be reversed progressively. As a result, there exists an evident lead-lag correlation between CTI and AAOI. It can be concluded that ENSO plays a key role in the phase transition of AAO at the QQ timescale. It is also noticed that this regular relationship is only evident in the canonical ENSO events, for which sea surface temperature (SST) anomalies extend westward from the tropical eastern Pacific. On the other hand, the similar relationships are not found among those atypical ENSO events for which SST anomalies spread eastward from the central Pacific, such as the 1982–1983 ENSO event.