Process-Based Analysis of El Niño–Southern Oscillation Decadal Modulation

Abstract

Based on two long-term simulations using state-of-the-art coupled global climate models, we examined the physical processes that control the decadal modulation of El Niño–Southern Oscillation (ENSO) amplitude. To identify the contributions of various feedback processes to the ENSO amplitude, we used the Bjerknes stability (BJ) index, which aims to quantify the main ENSO feedbacks from a linear perspective. To start, we demonstrated that the time-varying BJ index is highly correlated with the decadal changes in the standard deviation of the ENSO index, suggesting that it provides a good representation of the decadal modulation of the ENSO amplitude. We further revealed that this phenomenon can be attributed mainly to thermocline feedback changes, particularly changes in the oceanic response to zonal wind stress. In addition, two critical features of the background state were found to contribute significantly to changes in the equatorial thermocline feedback: 1) the subtropical–tropical cells and 2) ocean stratification. It was suggested that weak (strong) background subtropical meridional overturning circulation partly contributes to regulating the narrower (wider) meridional scales of the sea surface temperature and the associated zonal wind stress anomalies. The more stratified the ocean, the stronger ocean responses to a given wind stress forcing, which affects the ENSO amplitude.