Impacts of the Thermocline Feedback Uncertainty on El Niño Simulations in the Tropical Pacific

As a key dynamic element of the Bjerknes feedback mechanism, the thermocline effect (TE) is critically important to El Niño modeling. In this study, the potential influence of TE‐related parametric uncertainties on El Niño is investigated using the conditional nonlinear optimal parametric perturbation (CNOP) method based on an intermediate coupled model (ICM). The optimal perturbation of the TE‐related parameter (OTEP), which substantially affects El Niño simulations, is estimated through the CNOP approach. Results reveal that the El Niño simulation is highly sensitive to the TE uncertainty in the eastern equatorial Pacific, with OTEP‐induced simulation errors demonstrating an El Niño‐like growth trend. On one hand, as indicated by the simulated El Niño intensity, the uncertainty in the TE in the eastern region can easily affect the strength of the Bjerknes feedback‐related thermocline effect and atmospheric circulation. On the other hand, the enhanced TE is highly favored to accelerate the growth of the SST error due to the air–sea interaction, thus severely affecting the El Niño simulations. Therefore, adequately representing the TE in the equatorial eastern Pacific is emphasized for effectively improving El Niño simulations. Plain Language Summary Thermocline effect (TE) uncertainty seriously challenges numerous climate models. Focusing on the TE‐related parameter in an intermediate coupled model, this study quantitatively examines the impact of the TE on El Niño simulations. The spatial structure of the TE‐related parameters that cause the largest simulation errors is determined by employing an optimization algorithm (conditional nonlinear optimal parametric perturbation). Subsequently, a systematic analysis of the error scenarios resulting from these optimal TE‐related parameters is conducted with their underlying mechanisms being explored. Results indicate that the uncertainties in the TE in the eastern tropical Pacific tend to alter the intensity of the positive Bjerknes feedback, leading to considerable errors in El Niño simulations. Furthermore, these simulation errors are intensified by air–sea interactions. This study provides vital theoretical insights for augmenting the precision of ENSO predictions. Key Points El Niño simulation is sensitive to the model uncertainty in the thermocline effect (TE) in the eastern equatorial Pacific The TE uncertainty‐related effects on vertical advection and Kelvin wave act to amplify El Niño warming, thus caus