The interactions between the tropical Indian Ocean and Pacific Ocean via the Walker circulation and the Indonesian Throughflow (ITF) play an important role in the inter-basin exchange and climate variations. The effects of the eastern Indian Ocean subsurface temperature conditions during the 2019 Indian Ocean dipole (IOD) event on the development of the 2020/2021 La Niña event are investigated of the National Marine Environmental Forecasting Center of China. Results of pacemaker experiments comparing the effects of the atmospheric bridge and the oceanic channel suggest that the subsurface temperature anomalies in the tropical Indian Ocean dominates the inter-basin forcing leading to the outburst of the 2020/2021 La Niña event. The strong subsurface cooling is found to propagate to the eastern equatorial Pacific Ocean through the Indonesian seas and induce a strong La Niña event at the end of 2020, showing the long memory of the oceanic channel dynamics and the amplification by the ocean-atmospheric coupling. In comparison, experiments forced by surface temperature anomalies over the tropical Indian Ocean show that the atmospheric bridge alone is unable to induce the onset of the 2020 La Niña. The results suggest the importance of the Indo-Pacific oceanic channel dynamics in ENSO forecasts.