Reconsidering the oscillation center of the El Niño Southern Oscillation

Abstract

Analysis of the El Ninño-Southern Oscillation (ENSO) phenomena has conventionally been performed using deviations from the mean. However, the question remains as to whether the mean is appropriate as the reference state, since it is sensitive to the existence of a few extreme events. In this study, the validity of the mean and mode as a reference is compared using an idealized simulation model. We show that the mode is not affected by the asymmetry of El Ninño and La Ninña and is more stable as a reference than the mean. Then, this result is also demonstrated using observed data. Observations also show that the relationship between the ENSO amplitude and the zonal shifts of the ENSO anomalies is more emphasized if the mode is employed. For the variables in the ocean interior, differences between the mode-based and mean-based methods appear throughout all seasons.