Dynamics of El Niño-Southern Oscillation Diversity in an Intermediate Coupled Model
| dc.contributor.advisor | Jin, Fei-Fei | |
| dc.contributor.author | Geng, Licheng | |
| dc.contributor.department | Atmospheric Sciences | |
| dc.date.accessioned | 2021-09-30T18:01:55Z | |
| dc.date.issued | 2021 | |
| dc.description.abstract | The El Niño-Southern Oscillation (ENSO) phenomenon features rich sea surface temperature (SST) spatial pattern variations dominated by the central Pacific (CP) and eastern Pacific (EP) patterns at its warm phase. Understanding of such ENSO pattern diversity has been a subject under extensive research activity. The fundamental dynamics for ENSO diversity, however, remains elusive after decades of effort. In this dissertation, an intermediate coupled model based on the Cane-Zebiak-type framework whereas with revised model formulation and well-tuned parameterization schemes, denoted as R-CZ, has been independently built to unveil the dynamics and sensitivity of ENSO diversity. The main findings of this dissertation are as follows. Firstly, through the linear stability analysis, it is demonstrated that there exists a unique ENSO-like leading oscillatory mode within the R-CZ framework. This demonstration precludes the possibility suggested in some earlier studies that the observed CP and EP ENSO may be randomly excited from two coexisting linear ENSO modes under the same climate conditions. The ENSO mode derived in R-CZ exhibits significant sensitivity to feedback processes and mean states. As noted in earlier studies, the ENSO mode is rooted in either the recharge-oscillator (RO) mode or the wave-oscillator (WO) mode. This study further illustrates that the RO and WO modes compete for predominance depending on the relative intensity of the zonal advective feedback and the thermocline feedback. It is the competition between these two generic modes that determine the uniqueness of the ENSO mode. Secondly, a generalized nonlinearity/noise-induced regime transition (NIRT) mechanism is proposed for the pathway from the linear ENSO mode to ENSO diversity. In the subcritical regime where the ENSO mode is stable, ENSO events are excited with external forcing and feature spatial patterns similar to that of the ENSO mode characterized by maximum SSTA over the central-eastern Pacific. In the strongly supercritical regime where the ENSO mode has a large growth rate, the nonlinear growth of the ENSO mode leads to strong ENSO cycles and induces a mean climate drift due to the nonlinear rectification effect. The mean climate drift features a west (cold)-east (warm) dipole of SST and weakens the climatological cold tongue. EP ENSO-like oscillation with a longer period than expected from the ENSO mode is favored under the drifted mean climate. Consequently, EP ENSO manifests as a strong attractor in this supercritical regime. Within the in-between near-critical regime, also named the diversity regime, CP and EP ENSO resembling those observed coexist as nonlinearity allows transition between the two ENSO regimes (i.e., the linear ENSO regime and the nonlinear EP ENSO regime). Such a diversity regime is broadened with stochastic processes. NIRT serves as a unified paradigm for ENSO diversity as it encompasses the most relevant processes suggested in the literature, namely the atmospheric nonlinear convective heating, the oceanic nonlinear dynamical heating, and the stochastic excitation. Thirdly, the sensitivity of ENSO diversity is examined in idealized mean state space. With global warming-like mean state change, the ENSO mode tends to shift towards being more unstable, primarily attributed to the weakened zonal advective feedback and the strengthened Ekman feedback. Correspondingly, EP (CP) ENSO is projected to experience more (less) frequent occurrences. However, the exact impact of global warming on ENSO diversity depends on how close the ENSO mode stability of the climate system is to the criticality and deserves further study. | |
| dc.description.degree | Ph.D. | |
| dc.embargo.liftdate | 2022-09-29 | |
| dc.identifier.uri | http://hdl.handle.net/10125/76274 | |
| dc.language | eng | |
| dc.publisher | University of Hawaii at Manoa | |
| dc.subject | Atmospheric sciences | |
| dc.subject | Physical oceanography | |
| dc.title | Dynamics of El Niño-Southern Oscillation Diversity in an Intermediate Coupled Model | |
| dc.type | Thesis | |
| dc.type.dcmi | Text | |
| local.identifier.alturi | http://dissertations.umi.com/hawii:11150 |
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