A Multiscale Interaction Model for the Origin of the Tropospheric QBO

Abstract

A conceptual model for the origin of the tropospheric quasi-biennial oscillation (QBO) is presented. It is argued that the tropospheric QBO may not be a fundamental mode of oscillation of the tropical coupled system. It is proposed that it may arise due to multiscale interactions between high-frequency synoptic and intraseasonal oscillations of the atmosphere and a low-frequency oscillation of the coupled system in the presence of the annual cycle. This is demonstrated using a conceptual low-order system consisting of three variables representing the nonlinear atmospheric oscillations and a linear oscillator representing the low-frequency coupled mode. The annual cycle and coupling to the low-frequency linear oscillator provide slowly varying forcings for the atmospheric high-frequency oscillations. The atmospheric oscillations go through a chaotic regime during a certain part of the slowly varying forcing. Such variable forcing introduces a low-frequency tail in the spectrum of the atmospheric high-frequency oscillations. This low-frequency tail resonantly interacts with the low-frequency oscillation and produces the QBO in addition to broadening the spectrum of the low-frequency oscillator. The conceptual model simulates features similar to many observed features of the tropospheric QBO but depends on the assumption that there is an inherent low-frequency El Niño-Southern Oscillation oscillation with a four-year period that occurs independently of the high-frequency forcing or the QBO.