资源环境科技发展态势分析平台

Coupled dynamics between westerly wind events (WWEs) and the El Nino-Southern Oscillation (ENSO) is examined using an atmosphere-ocean coupled model with intermediate complexity. The model incorporates state-dependent stochastic noise that mimics observed WWEs, which occur at the edge of the Pacific warm pool when the Nino-4 sea surface temperature (SST) anomaly increases positively. The model parameter that controls the efficiency of the thermocline feedback, gamma, is perturbed to elaborate the sensitivity of the results to the system's stability. Without the noise (experiment NO), the model produces an ENSO-like regular oscillation with a 6-yr period, the variance of which increases with gamma. When additive noise is introduced over the western Pacific (experiment AD), the oscillations become irregular with a dominant period of 4-6 years and the increase in the variance relative to the NO experiment depends on gamma. When state-dependent noise is included (experiment SD), the oscillatory solution is also irregular, and its variance and asymmetry are increased irrespective of the value of gamma. Both the additive and state-dependent noise contribute to the occurrence of two types of variability, corresponding to the eastern Pacific (EP) and central Pacific (CP) El Ninos. In SD, the state dependence of the stochastic noise guarantees the existence of CP El Nino regardless of gamma since the increased likelihood of WWE occurrence with Nino-4 SSTs results in a positive feedback in the central Pacific. The above results suggest that the state dependence of WWEs plays a crucial role in the asymmetry and diversity of ENSO.