A 10-15-Yr Modulation Cycle of ENSO Intensity

This study examines the slow modulation of El Nino-Southern Oscillation (ENSO) intensity and its underlying mechanism. A 10-15-yr ENSO intensity modulation cycle is identified from historical and paleoclimate data by calculating the envelope function of boreal winter Nino-3.4 and Nino-3 sea surface temperature (SST) indices. Composite analyses reveal interesting spatial asymmetries between El Nino and La Nina events within the modulation cycle. In the enhanced intensity periods of the cycle, El Nino is located in the eastern tropical Pacific and La Nina in the central tropical Pacific. The asymmetry is reversed in the weakened intensity periods: El Nino centers in the central Pacific and La Nina in the eastern Pacific. El Nino and La Nina centered in the eastern Pacific are accompanied with basin-scale surface wind and thermocline anomalies, whereas those centered in the central Pacific are accompanied with local wind and thermocline anomalies. The El Nino-La Nina asymmetries provide a possible mechanism for ENSO to exert a nonzero residual effect that could lead to slow changes in the Pacific mean state. The mean state changes are characterized by an SST dipole pattern between the eastern and central tropical Pacific, which appears as one leading EOF mode of tropical Pacific decadal variability. The Pacific Walker circulation migrates zonally in association with this decadal mode and also changes the mean surface wind and thermocline patterns along the equator. Although the causality has not been established, it is speculated that the mean state changes in turn favor the alternative spatial patterns of El Nino and La Nina that manifest as the reversed ENSO asymmetries. Using these findings, an ENSO-Pacific climate interaction mechanism is hypothesized to explain the decadal ENSO intensity modulation cycle.