Impacts of Tropical Indian Ocean Decadal Variability on ENSO Transition Asymmetry

El Ni & ntilde;o is frequently followed by La Ni & ntilde;a, while La Ni & ntilde;a tends to sustain into the next year or even longer, exhibiting a notable phase transition asymmetry (TA) of El Ni & ntilde;o-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 Ni & ntilde;a, while bringing little effect on that of El Ni & ntilde;o. Physically, compared to the mid-1970s, TIO SSTA triggered westerly wind anomalies during the autumn and winter of the La Ni & ntilde;a development phase in the central equatorial Pacific in the late 1990s, which sped up the decay of La Ni & ntilde;a. It then regenerated westerly anomalies in the following winter, facilitating the development of El Ni & ntilde;o. 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 Ni & ntilde;a.