The Origin of ENSO Phase Locking: Seasonal Gap Theory

Abstract:

El Niño-Southern Oscillation (ENSO) features peak at boreal winter, which is called ENSO phase locking. Recent studies identified that seasonal modulation of ENSO growth rate is essential for such phenomena among the suggested mechanisms. Here, we investigate fundamental effect of seasonal varying growth rate to ENSO phase locking using seasonal recharge oscillator model (SROM). We revealed two regimes of phase locking along the amplitude of seasonal growth rate anomaly, (1) weak regime where only a single peak occurs and (2) strong regime where double peak arises. One of notable finding is that, there is seasonal gap on strong regime, where ENSO peak is not allowed by large scale ocean-atmosphere coupled (deterministic) process. The seasonal gap is explained by seasonal energy index (SEI) which is formulated from analytical solution of seasonal variance. Our theory provides an intuitive framework for ENSO phase locking and potentially applicable for global climate model ENSO diagnostics.