资源环境科技发展态势分析平台

The ensemble spread of seasonal prediction is investigated in this study to understand its role in the predictability of El Nino/Southern Oscillation (ENSO) based on the results of SINTEX-F2, a coupled ocean-atmosphere general circulation model. In the SINTEX-F2 seasonal prediction system, the first ENSO precursor appears as a cyclonic wind anomaly over the central north Pacific in boreal winter (January). This is followed by warm SST, positive rainfall and cross-equatorial southerly wind anomalies in the northern hemisphere during spring (particularly in April). These anomalies in April are accompanied by westerly wind anomaly in the western equatorial Pacific. Finally, El Nino-like conditions with warm SST and positive rainfall anomalies become dominant in the ensemble standard deviation after boreal summer. The 500hPa geopotential height suggests that stochastic atmospheric variability excites El Nino-like spread through air-sea interaction. The oceanic response in the form of upper heat content (in the top 150m) appears to result from the equatorial wind forcing during boreal spring and summer. These model results suggest that air-sea interaction related to the seasonal footprinting mechanism (SFM) is important for ENSO spread and the spring predictability barrier. The dependence of ENSO spread on the background ensemble-mean state is also investigated.