Two Leading ENSO Modes and El Niño Types in the Zebiak–Cane Model

Abstract

Modern instrumental records reveal that El Niño 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 Niño 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 Niño events. Linear eigenanalysis of the model is conducted to show that there are two leading El Niño-Southern Oscillation (ENSO) modes with their SST patterns resembling those of two types of El Niño. 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 Niño 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 Niño simulated in the MZC model. This model result may provide a plausible scenario for the observed ENSO diversity.

ENSO Regime Changes Responsible for Decadal Phase Relationship Variations Between ENSO Sea Surface Temperature and Warm Water Volume

“…It can be seen that the EP ENSO index is dominated by power at ~3‐ to 4‐year periodicities, whereas the CP ENSO index displays a major peak at ~2–3 years. These observed periodicities are consistent with those of the theoretical unstable quasi‐quadrennial and quasi‐biennial ENSO modes in the Zebiak‐Cane model (Bejarano & Jin, ; Xie & Jin, ).…”

“…Our conclusions remain the same, with the characteristics in the nonlinear simulations looking closer to the observations for the periodicity (Figure S5). The modeling results of the MZC model also suggest that the background state changes are one potential source for different ENSO types, as shown in the previous studies (Bejarano & Jin, ; Xie & Jin, ).…”

The influence of wintertime SST variability in the Western North Pacific on ENSO diversity

“…Likewise only the statistically significant differences in the surface winds are plotted, also at the 95% confidence level. The vector scale is shown at the bottom right side of the figure western equatorial Pacific-a deep (shallow) thermocline is said to favor the development of EP (CP) ENSO because of the relative dominance of thermocline (zonal advective) feedbacks under such a mean state (Fedorov et al 2015;Capotondi and Sardeshmukh 2015;Xie and Jin 2018).…”

ENSO Diversity from an Atmospheric Perspective

“…In these non-canonical El Niño events, SST warming is confined to the central equatorial Pacific and peaks near the dateline. The SST warming of this central Pacific (CP) El Niño is primarily driven by changes in zonal advection of heat by ocean currents, whereas changes in vertical advection play an important role in canonical, eastern Pacific (EP) El Niño [10,11]. The different roles of ocean currents in generating EP and CP El Niños reflect different climatological oceanic conditions in the eastern and central equatorial Pacific: shallow thermocline in the eastern Pacific and large zonal SST gradient in the central Pacific.…”