Importance of realistic zonal currents in depicting the evolution of tropical central Pacific sea surface temperature

Abstract

Classical El Niño–Southern Oscillation (ENSO) theories mainly consider the vertical process-related Ekman and thermocline feedbacks. However, the zonal current-related zonal advective feedback has been suggested to play a crucial role in the evolution of central Pacific (CP) El Niño and La Niña events. Also, the simulation of a realistic current is complex and not the focus of the classical ENSO theories. Using reanalysis datasets and a statistical model, this study emphasizes the importance of the zonal currents in the sea surface temperature anomaly (SSTA) evolution in the Niño4 region (160° E–150° W, 5° S–5° N). Specifically, in addition to the widely used predictors for the ENSO evolution, i.e. the equatorial Pacific mean thermocline depth anomaly (D20) and the zonal wind stress anomaly (ZWS), the zonal current anomaly (ZCA) averaged in the CP is first extracted to construct a statistical model to predict the SSTA of the Niño4 region. The results show that this model has improved overall prediction skill and accuracy for several CP El Niño and La Niña events during 1980–2020, compared with the benchmark linear regression model based on D20 and ZWS. By further removing the components related to the equatorial Kelvin and first symmetric meridional () Rossby waves (namely, the principal part of the traditional ENSO mechanism) from the ZCA, the remainder, which contains higher-order Rossby waves and other nonlinear components and is called the zonal current anomaly residual (ZCA_RSD), is found to be the key part of the improvements in the prediction skill. This suggests that to better simulate and predict the complex ENSO events, more vertical and meridional modes of the tropical Pacific need to be included to obtain a realistic anomalous zonal current.