The slow eastward phase propagation of covarying sea surface temperature (SST) and sea level pressure (SLP) anomalies in the Antarctic Circumpolar Wave (ACW) persisted in the midlatitude Southern Ocean from 1950 to 2001. Its northern extent reached into the subtropical South Indian and South Pacific oceans, where it influenced the magnitude and phase of El Ni¿o in the eastern equatorial Pacific Ocean. This is corroborated by the observation that multidecadal changes in the ACW can also explain multidecadal changes in the way El Ni¿o evolved over the last half of the twentieth century. Before 1977, El Ni¿o evolved from the slow eastward phase propagation of a coupled SST/SLP wave across the subtropical South Pacific Ocean to South America, then equatorward along the eastern boundary to the equator. During this epoch, the ACW expanded equatorward into a warmer subtropical South Pacific Ocean, accounting for the subtropical coupled SST/SLP wave that initiated El Ni¿o. After 1977, El Ni¿o evolved from the slow eastward phase propagation of a coupled SST/SLP wave across the tropical Pacific Ocean. During this epoch, the ACW receded from a colder subtropical South Pacific Ocean but expanded into a warmer subtropical South Indian Ocean. There it spawned a coupled SST/SLP wave directed equatorward into the Warm Pool north of Australia, thereafter propagating eastward into the eastern equatorial Pacific Ocean to initiate El Ni¿o. If this pattern of global warming persists into the 21st century, then the ACW will continue to influence El Ni¿o through the South Indian Ocean.