Abstract
In the present paper, we explore the manner in which nonlinearities modulate El Niño events by investigating the optimal precursory disturbance for El Niño events in the Zebiak-Cane model. The initial anomalies of conditional nonlinear optimal perturbations (CNOPs) and linear singular vectors (LSVs) are investigated. The CNOPs evolve into stronger El Niño events than the LSVs and act as the optimal precursor for El Niño events. By examining the role of nonlinearities in El Niño events induced by CNOPs and LSVs, we determined that, when the initial anomalies of the CNOP and LSV structures are large, the nonlinearities enhance CNOP-El Niño events but suppress LSV-El Niño events. Nonlinearities in the Zebiak-Cane model arise from nonlinear temperature advection (NTA), sub-surface temperature parameterization (STP), and wind stress anomalies (WSA). Using these types of nonlinearities, we trace the approach of the nonlinearities modulating the CNOP- and LSV-El Niño events. The results demonstrate that nonlinearities that originate from NTA enhance both CNOP-El Niño events and LSV-El Niño events, while nonlinearities originating from STP and WSA suppress these events. For the CNOP-El Niño events, the enhancement effect of NTA is larger than the suppression effect of STP and WSA, resulting in the combined effect of the nonlinearities in the Zebiak-Cane model being an enhancement of the CNOP-El Niño events. However, for the LSV-El Niño events, the enhancement effect of NTA is smaller than the suppression effect of WSA and STP. Consequently, the combined effect of the nonlinearities in the Zebiak-Cane model suppresses the LSV-El Niño events.
Highlights
The El Nino-Southern Oscillation (ENSO) is a global coupled ocean–atmosphere phenomenon
The results demonstrate that nonlinearities that originate from nonlinear temperature advection (NTA) enhance both conditional nonlinear optimal perturbations (CNOPs)-El Nino events and linear singular vectors (LSVs)-El Nino events, while nonlinearities originating from surface temperature parameterization (STP) and wind stress anomalies (WSA) suppress these events
We demonstrate that the NTA in the LSV-El Nino case favors stronger El Nino events and the WSA and STP favor weaker El Nino events, the combined effect of the WSA and STP is more significant than the effect of the NTA when the Nino3 sea surface temperature anomalies (SSTA) evolves to become large
Summary
The El Nino-Southern Oscillation (ENSO) is a global coupled ocean–atmosphere phenomenon. Duan et al (2009) examined the decaying behavior of El Nino events and illustrated that nonlinearity suppresses the amplitude of El Nino during the decaying phase and favors the decaying of El Nino events; the stronger the El Nino event, the more significant the nonlinear effect and the shorter the duration of the decaying phase These studies emphasized that the ENSO may be controlled by a nonlinear system and fall in the dynamical nonlinearly oscillatory mode of a coupled ocean–atmosphere system. We will apply the physics of the CNOP as an optimal precursor for El Nino events in the Zebiak-Cane model of intermediate complexity to reveal the spatial pattern of the optimal precursor for El Nino events, and compare the results with those of the LSV to study the effect of nonlinearity on El Nino events; new results are anticipated.
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