Two Leading ENSO Modes and El Nino Types in the Zebiak-Cane Model

Modern instrumental records reveal that El Nino events differ in their spatial patterns and temporal evolutions. Attempts have been made to categorize them roughly into two main types: eastern Pacific (EP; or cold tongue) and central Pacific (CP; or warm pool) El Nino events. In this study, a modified version of the Zebiak-Cane (MZC) coupled model is used to examine the dynamics of these two types of El Nino events. Linear eigenanalysis of the model is conducted to show that there are two leading El Nino-Southern Oscillation (ENSO) modes with their SST patterns resembling those of two types of El Nino. Thus, they are referred to as the EP and CP ENSO modes. These two modes are sensitive to changes in the mean states. The heat budget analyses demonstrate that the EP (CP) mode is dominated by thermocline (zonal advective) feedback. Therefore, the weak (strong) mean wind stress and deep (shallow) mean thermocline prefer the EP (CP) ENSO mode because of the relative dominance of thermocline (zonal advective) feedback under such a mean state. Consistent with the linear stability analysis, the occurrence ratio of CP/EP El Nino events in the nonlinear simulations generally increases toward the regime where the linear CP ENSO mode has relatively higher growth rate. These analyses suggest that the coexistence of two leading ENSO modes is responsible for two types of El Nino simulated in the MZC model. This model result may provide a plausible scenario for the observed ENSO diversity.