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