Effect of asymmetric air-sea feedback on ENSO decay: the role of the zonal propagation of wind stress anomalies

The zonal wind stress ( τ^x ) over the equatorial Pacific plays a vital role in the air-sea interaction associated with the El Niño-Southern Oscillation (ENSO). The equatorial westerly wind stress (w τ^x ) anomalies during the El Niño (EN) decay phase propagate eastward and decrease rapidly, while the equatorial easterly wind stress (e τ^x ) anomalies during the La Niña (LN) decay phase do not display obvious zonal movement and remain near the dateline. The cause of the asymmetry in τ_x anomalies and its contribution to ENSO asymmetric decays are investigated in this study. The difference in τ^x anomalies is due to the interaction between the atmospheric response to sea surface temperature anomalies (SSTAs) and the forcing of the wind field on subsurface temperature. The results of numerical experiments demonstrate that the e τ^x anomalies during the LN decay phase have a strong and prolonged suppressive effect on the zonal advective feedback and thermocline feedback, resulting in the weakness of the delayed negative feedback. However, due to the eastward propagation and rapid decay of w τ^x anomalies, the suppressive effect during the EN decay phase is weak and favors its rapid decay. Previous studies have extensively analyzed the asymmetric response of the atmosphere to SSTAs. The difference in zonal displacement of air-sea couplings during ENSO decay phases leads to the decay asymmetry between EN and LN through weakening the negative air-sea feedback during LN, in which our results highlight that the zonal propagation asymmetry of τ^x anomalies plays a crucial role.