Modulation of El Niño Decay by Negative High Cloud Feedback

Understanding the physical mechanisms governing the El Niño decay phase is fundamental for simulating accurately the duration of El Niño events. This study investigates the role of negative high cloud feedback in modulating El Niño’s decay during boreal winter and spring. Utilizing ERA5 reanalysis data from 1950 to 2024, we find that peak El Niño SST anomalies in the central-eastern Pacific during boreal winter trigger a simultaneous local increase in high cloud. These high cloud anomalies exert a cooling effect on the ocean surface by reflecting incoming shortwave radiation. There is a significant correlation between wintertime surface net cloud radiative effect (CRE) and the SST tendency from winter to the subsequent spring. Heat budget diagnostics further confirm that this intense shortwave cooling effect of high cloud accounts for a substantial proportion of the net surface heat flux anomalies, acting as a critical thermodynamic factor for the decay phase. Most Coupled Model Intercomparison Project Phase 6 (CMIP6) models capture this relationship between wintertime CRE and SST tendency, validating this mechanism. However, there is a systematic bias between simulated and observed feedback sensitivity. This discrepancy likely hinders the models' ability to accurately represent the rapid decay and realistic duration of El Niño events. Our findings suggest that improving cloud-radiation parameterizations is essential for improving the simulation and prediction of ENSO lifecycles in climate models.