Distinctive precursory air–sea signals between regular and super El Niños

Statistically different precursory air–sea signals between a super and a regular El Niño group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Niño events during 1958–2008 are first separated into two groups: a super El Niño group (S-group) and a regular El Niño group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April–May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u′). This is attributed to the difference in the thermocline depth anomaly (D′) over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D′ is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.