Different prediction skill for the East Asian winter monsoon in the early and late winter season

The prediction skill for the East Asian winter monsoon (EAWM) in early (November-December, ND) and late winter (January-February, JF) is investigated based on the NCEP's Climate Forecast System, version 2 (CFSv2), at different lead months. Results show that the prediction skill of ND-EAWM is higher than that of JF-EAWM. ND-EAWM can be reasonably reproduced by CFSv2 with initial conditions from the previous May to November, whereas with initial conditions from the previous July to January it has little forecasting skill for predicting JF-EAWM. The difference in the impacts of ENSO between early and late winter may influence the ENSO-EAWM relationship and cause the subseasonal change. As the seasonality of the ENSO cycle changes from early winter to late winter, the anomalous tropical east-west Walker circulation system shifts from a double-cell to a single-cell regime. The double-cell regime in early winter is usually accompanied by intense tropical convection localized in the Maritime Continent. ENSO-related anomalous convection strongly influences the EAWM through a systematic wave train pattern. However, ENSO cannot easily influence the EAWM in late winter through this process when the turnabout of ENSO commences in late winter. CFSv2 can successfully reproduce this physical process of ENSO affecting the EAWM in early winter. It is also found that the year-to-year increment approach may enhance the skill of CFSv2 in predicting the EAWM in late winter.