A simple coupled model is used to examine decadal variations in El Nino/Southern Oscillation (ENSO) prediction skill and predictability. Without any external forcing, the coupled model produces regular ENSO-like variability with a five year period. Superimposed on the five year oscillation is a relatively weak decadal amplitude modulation with a 20 year period. External uncoupled atmospheric "weather noise: that is determined from observations is introduces into the coupled model. The weather noise produces irregularity in the ENSO events, shifts the dominant period to four years and amplifies the decadal signal. The decadal signal results without any external prescribed changes to the mean climate of the model. Using the coupled simulation with weather noise as initial conditions for verification, a large ensembe of prediction experiments were made. The forecast skill and predictability was examined and shouwn to have a strong decadal dependence. During decades when th eamplitude of the interannual variability is large, the forecastskill is relatively short. During decades when the predictability is high, the delayed oscillator mechanism drives the sea surface temperature anomaly (SSTA), and during decades when the predictability is low, the atmospheric noise strongly influences the SSTA and the delayed oscillator is determined by decadal variations in the mean east-west thermocline slope along the equator. When the slope is strong (weak) the delyed oscillator mechanism is also strong (weak).
Complete copies of this report are available from:
Center for Ocean-Land-Atmosphere Studies
last update: 13 June 1997
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