ENSO and meridional modes: A null hypothesis for Pacific climate variability

Pacific low-frequency variability (timescale > 8 years) exhibits a well-known El Nino-like pattern of basin-scale sea surface temperature, which is found in all the major modes of Pacific decadal climate. Using a set of climate model experiments and observations, we decompose the mechanisms contributing to the growth, peak, and decay of the Pacific low-frequency spatial variance. We find that the El Nino-like interdecadal pattern is established through the combined actions of Pacific meridional modes (MM) and the El Nino-Southern Oscillation (ENSO). Specifically, in the growth phase of the pattern, subtropical stochastic excitation of the MM energizes the tropical low-frequency variance acting as a red noise process. Once in the tropics, this low-frequency variance is amplified by ocean-atmospheric feedbacks as the pattern reaches its peak phase. At the same time, atmospheric teleconnections distribute the variance from the tropics to the extratropics, where the pattern ultimately decays. In this stochastic red noise model of Pacific climate, the timescale of the extra-tropical/tropical interactions (1-2 years) permits the stochastic excitation of the ENSO-like pattern of decadal and interdecadal variance.