Journal article

A 10-15-Yr modulation cycle of ENSO intensity

Sun F, Yu J ...see all

Journal of Climate, vol. 22, issue 7 (2009) pp. 1718-1735

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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.

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  • Fengpeng Sun

  • Jin Yi Yu

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