Extraordinarily strong El Niño events, such as those of 1982/1983 and 1997/1998, have been poorly predicted by operational seasonal forecasts made before boreal spring, despite significant advances in understanding, improved models, and enhanced observational networks. The Equatorial Atlantic Zonal Mode—a phenomenon similar to El Niño but much weaker and peaking in boreal summer—impacts winds over the Pacific, and hence affects El Niño, and also potentially its predictability. Here we use a climate model to perform a suite of seasonal predictions with and without sea surface temperature (SST) in the Atlantic restored to observations. We show for the first time that knowledge of equatorial Atlantic SST significantly improves the prediction across boreal spring of major El Niño events and also weaker variability. This is because Atlantic SST acts to modulate El Niño variability, rather than triggering events. Our results suggest that better prediction of major El Niño events might be achieved through model improvement in the equatorial Atlantic.