Coexistence of equatorial coupled modes of ENSO

To study the regimes of leading ocean-atmosphere coupled modes of relevance to the El Nino-Southern Oscillation (ENSO) phenomenon, a comprehensive eigenmode analysis of an intermediate coupled model linearized with respect to an array of basic states is performed. Different kinds of leading modes are found to coexist and become unstable under wide ranges of basic states and parameter conditions. In particular, the two most important modes have periods of around 4 and 2 yr. They are referred to as the quasi-quadrennial (QQ) and the 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 primary mechanism for the phase transition of the QQ mode is due to the slow oceanic dynamic adjustment of equatorial heat content, which is consistent with the simple conceptual recharge oscillator, whereas anomalous advection of sea surface temperature by equatorial zonal current anomalies plays a dominant role in the phase transition of the QB mode. The coexistence of these ENSO-like coupled modes under the present climate conditions may provide an explanation for the observed rich variations in ENSO behaviors.