During 1997, the evolution of the surface zonal wind anomalies (in meters per second) along the equatorial Pacific Ocean. Red shading indicates anomalous westerly (from the west) winds, while blue shading denotes anomalous easterly winds. The left panel emphasizes winds that are relatively short-term (the filter isolates periods more than 5 days). The right panel emphasizes the “interannual” winds that are longer-term and more persistent (the filter isolates periods greater than 250 days). On average, the surface winds across the tropical Pacific are easterly, so westerly anomalies—departures from average conditions—can weaken the Walker Circulation. But only if they persist do they lead to El Niño. See Capotondi et al. (2018) for more details on the calculation and data. Figure provided by Antonietta Capotondi and modified by Climate.gov.