Understanding ENSO Regime Behavior under the Increase of Warm-pool Temperature using a Simple ENSO Model

The regime behavior of low-order El Niño-Southern Oscillation (ENSO) model according to the increasing radiative-convective equilibrium sea surface temperature (SST; Tr) has been studied to provide a possible explanation on the observed increase of ENSO irregularity involving decadal modulation. During the recent decades, a clear increasing trend of warm-pool SST and the associated increasing heat fluxes into the tropical ocean are observed. In this study, the increase of warm-pool maximum SST has been interpreted as an increase of Tr following the previous studies. A bifurcation analysis with Tr as a control parameter has been 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 revealed that the period doubling bifurcation occurs and amplitude modulated ENSO emerges. At this point, distinguished sub-cycle appears within the preexisting ENSO cycle which initiates decadal modulation of ENSO. As Tr increases further, nested oscillation is successively generated constituting a clear decadal modulation of ENSO. The qualitative regime change revealed in this study seems being supported by the observation that shows regime shift around 1970s. With the increasing Tr, mean zonal SST gradient increases and model adjusts toward ‘La Niña-like' mean state. Further constraint with shoaling of mean thermocline depth and increasing stratification cause ENSO to exhibit stronger amplitude modulation. Also the timing of the period doubling bifurcation advances with these two effects