A modulating effect of Tropical Instability Wave (TIW)‐induced surface wind feedback in a hybrid coupled model of the tropical Pacific

Abstract

AbstractTropical Instability Waves (TIWs) and the El Niño‐Southern Oscillation (ENSO) are two air‐sea coupling phenomena that are prominent in the tropical Pacific, occurring at vastly different space‐time scales. It has been challenging to adequately represent both of these processes within a large‐scale coupled climate model, which has led to a poor understanding of the interactions between TIW‐induced feedback and ENSO. In this study, a novel modeling system was developed that allows representation of TIW‐scale air‐sea coupling and its interaction with ENSO. Satellite data were first used to derive an empirical model for TIW‐induced sea surface wind stress perturbations (τTIW). The model was then embedded in a basin‐wide hybrid‐coupled model (HCM) of the tropical Pacific. Because τTIW were internally determined from TIW‐scale sea surface temperatures (SSTTIW) simulated in the ocean model, the wind‐SST coupling at TIW scales was interactively represented within the large‐scale coupled model. Because the τTIW–SSTTIW coupling part of the model can be turned on or off in the HCM simulations, the related TIW wind feedback effects can be isolated and examined in a straightforward way. Then, the TIW‐scale wind feedback effects on the large‐scale mean ocean state and interannual variability in the tropical Pacific were investigated based on this embedded system. The interactively represented TIW‐scale wind forcing exerted an asymmetric influence on SSTs in the HCM, characterized by a mean‐state cooling and by a positive feedback on interannual variability, acting to enhance ENSO amplitude. Roughly speaking, the feedback tends to increase interannual SST variability by approximately 9%. Additionally, there is a tendency for TIW wind to have an effect on the phase transition during ENSO evolution, with slightly shortened interannual oscillation periods. Additional sensitivity experiments were performed to elucidate the details of TIW wind effects on SST evolution during ENSO cycles.