Understanding ENSO Regime Behavior upon an Increase in the Warm-Pool

Abstract

The regime behavior of the low-order ENSO model, according to an increase in the radiative？convective equilibrium sea surface temperature (SST;Tr) is studied to provide a possible explanation for the observed increase in ENSO irregularity characterized by decadal modulation. A bifurcation analysis with Tr as a control parameter is conducted to reveal that the degree of ENSO irregularity in the model is effectively controlled by the equilibrium states of the model. At a critical value of Tr, bifurcation analysis reveals that period-doubling bifurcation occurs and an amplitude-modulated ENSO emerges. As Tr increases further, nested oscillations are successively generated, illustrating clear decadal modulation of ENSO. The qualitative regime changes revealed in this study are supported by the observation of regime shifts in the 1970s.