A new ocean and atmosphere fully coupled Climate Forecast System (CFS) has been developed and made operational at the National Centers for Environmental Prediction. A study on evolution of tropical El Nino/Southern Oscillation (ENSO) phenomena reveals that, although the model simulates the ENSO sea surface temperature (SST) variability with realistic frequency and comparable amplitudes as the observed, the model ENSO events occur more regularly, and the life span of an average event is longer. A massive set of retrospective forecasts by this CFS, out to 9 months and covering 24 years, is employed to investigate the factor which causes the above discrepancies.
A few months ahead of an onset or a phase transition of an ENSO event, a clear precursor in sea level height (SLH) anomaly, which has long been building up in the warm pool region and is an excellent proxy of the subsurface heat content anomaly, makes an eastward spread followed by a shift toward the central and eastern equatorial Pacific. The eastward spread and shift of the SLH anomaly manifest itself as the observed SST anomaly in the central and eastern equatorial Pacific. This study reveals that the CFS predictions on eastward spread and shift of SLH anomaly encounter great difficulties. This weakness on SLH prediction a few months ahead of an ENSO onset leads to a vital difficulty in SST prediction on an onset or a phase transition of an ENSO event.
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