Coexistence of leading equatorial coupled modes for ENSO

A comprehensive eigen-mode analysis of an intermediate coupled model linearized with respect to arrays of basic states is performed to study the regimes of leading ocean-atmosphere coupled modes of relevance to the El Nino Southern Oscillation phenomenon. Different kinds of leading modes are found to coexist and to become unstable under wide ranges of basic states and parameter conditions. In particular, two main kinds of modes have periods around four years and two years. They are thus referred as to Quasi-Quadrennial (QQ), Quasi-Biennial (QB) modes, respectively. The positive coupled feedback destabilizes and quantizes the near-continuous spectrum for the low-frequency modes of the upper ocean dynamics giving rise to these leading modes with distinct periodicities. The QQ mode can be understood to a large extent by the mechanisms elucidated in the simple conceptual recharge oscillator which relays on slow oceanic dynamic adjustment of equatorial heat content, whereas anomalous advection of sea surface temperature by equatorial zonal current anomalies plays an important role in the QB mode. One of the findings of this study is that the QQ and QB modes may coalesce under realistic conditions through a codimension-2 degeneracy in the parameter space. The coexistence or multiplicity of ENSO-related coupled modes under present climate conditions may provide a plausible explanation for the observed dominating QQ and QB variability of rich ENSO behaviors.