Non-linear alignment of El Niño to the 11-yr solar cycle

Abstract

El Nino/La Nina episodes represent warm/cool phases of 2- to 7-yr period El Nino-Southern Oscillation (ENSO) in the tropical Pacific ocean-atmosphere system. Modeling studies find ENSO self-excited or driven by ambient noise. Here we find most El Nino and La Nina episodes from 1900-2005 grouped into non-commuting pairs that repeat every ∼11 yrs, aligned with rising and falling transition phases of the ∼11-yr period quasi-decadal oscillation (QDO). These alignments arise from non-linear phase locking of 3rd and 5th harmonics near 3.6- and 2.2-yr to the 1st harmonic near 11-yr period. Here we find these alignments replicated in both coupled general circulation model and conceptual model driven by 11-yr solar forcing, wherein the solar-forced 1st harmonic initiates a non-linear cascade of higher odd harmonics that are phase-locked with the same alignments as observed. These solar-forced 3rd and 5th harmonics explain ∼52% of inter-annual variance in the Nino-3 temperature index from 1900-2005. Copyright 2008 by the American Geophysical Union.