In situ and satellite sea level data sets over 1980--1998are used to estimate interannual variations of the geostrophic zonal transportfrom the western Pacific Ocean into the Celebes Sea. Then the transport componentdue to the Pacific wind-driven and fully reflected equatorial waves is removed.Finally, the residual variations, named Indonesian throughflow correction(ITFC), are used to correct for the model closed western boundary. ITFC inflows/outflowsare leading the warm/cold events by a few months. They are then prescribedat the model western boundary over 1980--1998 to compare with closedboundary simulations. An ITFC inflow anomaly makes the eastern Pacific slightlywarmer and trade winds in the central Pacific slightly weaker. Indeed, thesequantities are simulated by both experiments in very good agreement with observations.More importantly, prescribing the ITFC greatly improves two characteristicsof the model that play a key role in the coupled simulations. The first isthe RMS variability of sea level versus longitude; the ITFC shifts to thewest the position of the minimum. The second is the basin average sea level;the ITFC largely amplifies its fluctuations, stimulating the charge and dischargeof the system. Consequently, the ITFC can have a very large impact on El Ni¿o-SouthernOscillation forecasts. For example, the 1982/1983 and 1997/1998 1-year leadforecasts which fail to predict the warm events with closed boundaries aresuccessful with the ITFC. The impact of the ITFC on coupled simulations isnot linearly dependent on the ITFC value itself: In addition to the fluctuatingconditions at the western boundary, results depend on how close to instabilitythe system is at each time step.