Nonlinearity Modulating Intensities and Spatial Structures of Central Pacific and Eastern Pacific El Niño Events

This paper compares data from linearized and nonlinear Zebiak-Cane model, as constrained by observed sea surface temperature anomaly (SSTA), in simulating central Pacific (CP) and eastern Pacific (EP) El Niño. The difference between the temperature advections (determined by subtracting those of the linearized model from those of the nonlinear model), referred to here as the nonlinearly induced temperature advection change (NTA), is analyzed. The results demonstrate that the NTA records warming in the central equatorial Pacific during CP El Niño and makes fewer contributions to the structural distinctions of the CP El Niño, whereas it records warming in the eastern equatorial Pacific during EP El Niño, and thus significantly promotes EP El Niño during El Niño-type selection. The NTA for CP and EP El Niño varies in its amplitude, and is smaller in CP El Niño than it is in EP El Niño. These results demonstrate that CP El Niño are weakly modulated by small intensities of NTA, and may be controlled by weak nonlinearity; whereas, EP El Niño are significantly enhanced by large amplitudes of NTA, and are therefore likely to be modulated by relatively strong nonlinearity. These data could explain why CP El Niño are weaker than EP El Niño. Because the NTA for CP and EP El Niño differs in spatial structures and intensities, as well as their roles within different El Niño modes, the diversity of El Niño may be closely related to changes in the nonlinear characteristics of the tropical Pacific.