Impacts of Tropical Indian Ocean Decadal Variability on ENSO Transition Asymmetry

El Niño is frequently followed by La Niña, while La Niña tends to sustain into the next year or even longer, exhibiting a notable phase transition asymmetry (TA) of El Niño-Southern Oscillation (ENSO). This study explores the potential influences of tropical Indian Ocean (TIO) decadal variability on TA based on a comparative analysis of the relationship between the TIO sea surface temperature anomalies (SSTA) and ENSO during different periods. Generally, the TIO SSTA strengthened TA before the 1980s, corresponding to a highly positive relationship between the whole TIO SSTA and ENSO. However, the weakening effect was exhibited after the 1980s when the correlation diminished. After the late 1990s, ENSO was only positively correlated with western TIO, with the westerly exhibit of the SSTA center leading to smaller impacts on TA. Moreover, TIO SSTA tends to weaken TA by promoting the transition efficiency of La Niña, while bringing little effect on that of El Niño. Physically, compared to the mid-1970s, TIO SSTA triggered westerly wind anomalies during the autumn and winter of the La Niña development phase in the central equatorial Pacific in the late 1990s, which sped up the decay of La Niña. It then regenerated westerly anomalies in the following winter, facilitating the development of El Niño. This study quantifies the impact of the TIO SSTA on TA in seasonal signals and investigates the decadal variability of such influence, aiming to further understand phase transition asymmetry and offer valuable insights for the prediction of multi-year La Niña.