<p>Due to its potentially life—threatening and devastating economic impacts, variability in the East Asian Summer Monsoon needs better understanding, more accurate simulation and improved prediction. Much of the current ability of long-range summer monsoon forecasts stems from the link to the preceding winter El Niño Southern Oscillation, but the mechanisms behind this lagged impact are not fully understood. In this study, a simple new mechanism is proposed, based on migrating Atmospheric Angular Momentum anomalies. The polewards migration of atmospheric angular momentum associated with winter El Niño is shown to impact the northwest Pacific anticyclone responsible for much of the year-to-year variability in the monsoon. Model forecasts of the summer monsoon are shown to have increased accuracy when this new mechanism is active, with the increase in the success rate of forecasts due to migrating angular momentum anomalies shown to be as large as the effect of El Niño itself.</p>