Optimal Spatiotemporal Perturbation for MJO in Triggering El Niño Events: An Intermediate Coupled Model Study

Abstract The Madden‐Julian Oscillation (MJO) can potentially excite El Niño events by perturbing the low‐frequency air‐sea coupled system through its intraseasonal wind stress anomalies. To fully exploit this dynamic triggering effect, a temporally extended conditional nonlinear optimal perturbation (CNOP) method is implemented within a recently developed, statistically accurate intermediate coupled model that uniquely captures the diverse spatiotemporal features of El Niño. By optimizing additive MJO wind stress anomalies over the constrained space spanned by historical MJO forcings and across twelve neutral reference states from the model’s free integration, the most effective spatiotemporal structures of MJO forcings for triggering El Niño events are identified. When the MJO wind stress forcing is organized with the optimal perturbation represented by the specific spatiotemporal structure of CNOP, it can induce the maximum persistent warming of sea surface temperature anomalies (SSTAs), thereby successfully triggering El Niño events under the operational criterion. Across all optimal spatiotemporal perturbations, strong wind stress anomalies are consistently concentrated in the early stage (January‐March) of the calendar year, even though the optimization target is to achieve the long‐standing SSTA warming in the latter months. Such an early‐year segment of wind forcings in the CNOP is a crucial structure for subsequent SSTA development, as it accounts for the majority of the perturbation’s warming effect. The analysis of optimal spatiotemporal perturbations indicates that the MJO wind stress plays a limited yet non‐negligible role in triggering El Niño events from neutral states, with its effectiveness constrained by the oscillating nature of MJO.