The Full Extent of El Niño’s Precipitation Influence on the United States and the Americas: The Sub-Optimality of the Niño 3.4 SST Index

Key features of El Niño’s influence on North American winter precipitation are well-known but we show that this influence has, hitherto, been sub-optimally characterized in winter, and especially in fall and spring. The sub-optimality has arisen from the historical over-reliance on regressions on the Niño 3.4 SST index – a widely used marker of El Niño variability. We show that El Niño’s full influence is obtained from assembling the regressions on the spatiotemporal modes constituting El Niño variability, rather than from regressions on an SST index keyed just to its mature phase (Niño 3.4 index). The notably different influence of central and eastern Pacific El Niños on Eastern Seaboard is documented. The full influence of El Niño on North American precipitation is shown to be substantially larger than previously recognized and pronounced not just in northern winter but even in fall and spring – enhancing prospects for year-round seasonal prediction.