Impacts of the annual cycle of background SST in the tropical Pacific on the phase and amplitude of ENSO

The dominant annual cycle of sea surface temperature (SST) in the tropical Pacific exhibits an antisymmetric mode, which explains 83.4% total variance, and serves as a background of El Niño–Southern Oscillation (ENSO). However, there is no consensus yet on its anomalous impacts on the phase and amplitude of ENSO. Based on data during 1982–2022, our results show that anomalies of the antisymmetric mode can affect the evolution of ENSO on the interannual scale via Bjerknes feedback, in which the positive (negative) phase of the antisymmetric mode can strengthen El Niño (La Niña) in boreal winter via an earlier (delayed) seasonal cycle transition and larger (smaller) annual mean. The magnitude of the SST anomalies in the equatorial eastern Pacific can reach more than ±0.3℃, regulated by the changes in the antisymmetric mode based on random sensitivity analysis. Our results reveal the spatial pattern of the annual cycle associated with the seasonal phase-locking of ENSO evolution and provide new insight into the impact of the annual cycle of background SST on ENSO, which possibly carries important implications for forecasting ENSO.