Quantifying the residual effects of ENSO on low-frequency variability in the tropical Pacific

The asymmetry of El Niño-La Niña, one of the well-known characteristics of the El Niño-Southern Oscillation (ENSO), is suggested to produce a non-zero residual effect that could rectify the background state, and thereby generates the low-frequency variability in the tropical Pacific. So far, this rectification effect has been hardly quantified apart from the low-frequency variability because the low-frequency variability captured via conventional methods represents the mixture of both the residual effects of ENSO and the no-ENSO-related natural decadal variability. Here we separate the residual effects of ENSO from the natural decadal variability that appears in four historical sea surface temperature datasets during the last century by applying a long-term moving average. A significant correlation between the computed residual effect and the decadal change in the ENSO skewness (i.e. the measure of the El Niño-La Niña asymmetry) confirmed the applicability of our computational method. Quantitatively, the residual effects of ENSO consistently account for at least 15% of the total low-frequency variability in four datasets, especially over the eastern and central tropical Pacific. This implies that the asymmetry of ENSO enhances the tropical Pacific decadal variability for the last century.