Interdecadal wind stress variability over the tropical Pacific causes ENSO diversity in an intermediate coupled model

The role of interdecadal wind stress variability in the genesis of ENSO diversity is examined by using an intermediate coupled model (ICM) in the tropical Pacific; two types of experiments are performed, one with the original ICM, and the other with interdecadal wind stress (tau(interde)) effect being explicitly represented. The tau(interde) component is derived from NCEP/NCAR reanalysis dataset as follows. First, the ensemble empirical mode decomposition (EEMD) is used to extract the interdecadal component of wind stress anomalies on about a 10-40 yr timescale. Next, an idealized interdecadal cycle of tau(interde) is reconstructed by a principal oscillation pattern (POP) analysis based on the EEMD-extracted interdecadal wind component. A 110-yr model integration is then performed by explicitly incorporating the reconstructed tau(interde) cycle into the ICM. Compared with the regular interannual oscillation in the original ICM, the simulated ENSO events become highly irregular with interdecadal variations in the amplitude and asymmetry when the tau(interde) effect is included. Especially, the model reproduces two types of El Nino with different spatial distribution and temporal evolution of SST anomalies, namely Eastern-Pacific (EP) and Central-Pacific (CP) types. Further attribution analyses are performed to understand the modulating effects of tau(interde) in the tropical Pacific using the ocean component of the ICM, forced by the added tau(interde) effect. Two different roles of the Interdecadal Pacific Oscillation (IPO) in modulating different types of El Nino are illustrated. On the one hand, the warm phase of IPO favors for the emergence of EP-El Nino events, in association with the initial warming signals occurring in the eastern equatorial Pacific, which are absent in the original ICM. On the other hand, the cold phase of IPO tends to shift the El Nino (i.e., the single type of El Nino in the original ICM) to an CP type, with which the SST anomalies propagate eastward along the equator but cannot extend into the eastern boundary. This simple modeling study highlights the significant contributions of interdecadal wind variability to the genesis of ENSO irregularity and diversity theoretically.

Automated summary

This study examines the role of interdecadal wind stress variability in the genesis of ENSO diversity using an intermediate coupled model (ICM) in the tropical Pacific. The results show that when the interdecadal wind stress effect is included, the simulated ENSO events become highly irregular with interdecadal variations in the amplitude and asymmetry. Furthermore, the study illustrates the different roles of the Interdecadal Pacific Oscillation (IPO) in modulating different types of El Nino.