Resonant excitation of the quasi‐decadal oscillation by the 11‐year signal in the Sun's irradiance

Abstract

The quasidecadal oscillation (QDO) of 9‐ to 13‐year period in the Earth's climate system has been found governed by a delayed action oscillator (DAO) mechanism in the tropical Pacific Ocean similar to that governing the El Niño–Southern Oscillation (ENSO) of 3‐ to 5‐year period. It also fluctuated in phase with the ∼11‐year‐period signal in the Sun's total irradiance throughout the twentieth century. In earlier attempts to explain this association, a conceptual ocean‐atmosphere coupled model of the DAO mechanism in the tropical Pacific Basin was driven by 11‐year‐period solar forcing, producing a QDO that was in damped resonance with the solar forcing. In the present study, we likewise force a fully coupled ocean‐atmosphere general circulation model (i.e., Fast Ocean‐Atmosphere Model (FOAM)) of Jacob et al. (2001), adding an 11‐year‐period cosine signal of amplitude ∼2.0 W m−2 to the solar constant in the model. In the presence of this 11‐year‐period solar forcing the FOAM simulates both the ENSO and the QDO, while in its absence the FOAM simulates only the ENSO. We find the model QDO governed by a tropical DAO mechanism with patterns and evolution similar to those observed. We find its warm phase lagging peak solar forcing by ∼1–3 years, as observed and consistent with damped‐resonant excitation of the tropical DAO of the QDO by the 11‐year‐period solar forcing in the earlier conceptual model.