中国夏季降水异常EOF模态的时间稳定性分析

Analysis of Stability of EOF Modes in Summer Rainfall Anomalies in China

本文基于1980~2012年中国160个台站降水资料,利用滑动交叉检验等方法讨论了中国夏季降水距平和距平百分率EOF各模态的时间稳定性,在此基础上探讨了EOF方法在中国夏季降水短期气候预测中的应用条件和潜在能力。研究表明,随机剔除一年样本,中国夏季降水距平场前四个EOF模态表现出显著的稳定性。若时间系数完全预测准确,则潜在的可预测站点主要位于黄河以南地区,理想预测与原始降水的距平相关系数为0.6左右。相对而言,降水距平百分率各模态的时间稳定性易受极端降水事件的影响,当人为削弱这种影响后,随机剔除一年样本,其前三个模态的稳定性得到提高,潜在的可预测站点均匀分布,理想预测与原始降水的距平相关系数为0.48。但是,伴随着预报时效的增加,降水距平和距平百分率后三个EOF模态的时间稳定性下降,预示着EOF方法对未来两年以上降水的预测能力将会明显下降。

The stability of leading Empirical Orthogonal Function （EOF） modes in summer rainfall is evaluated by using the cross-validation and some other methods on the basis of 160 station-observed monthly precipitation values recorded in China during 1980-2012, and the potential advantage of EOF utilized in the seasonal forecasts of China summer rainfall is discussed. Our results suggest that when a one-year sample is randomly removed, the first four EOF modes suggest higher stability. The EOF-based potential forecast skill is mainly in South China, and the 33-year-averaged anomaly correlation coefficient （ACC） between the idealized forecast and the observed field is approximately 0.6. In contrast, the stability of the leading EOF modes for the precipitation anomaly percentage is greatly affected by extreme climate events. However, when we artificially removed this effect, the stability of the first three EOF modes improved, where the potential forecast skills appear in most parts of China, and the averaged ACC between the potential forecast and observed rainfall anomaly percentage is 0.48. The stability of the EOF mode exhibits an obvious decrease when the forecast period is extended, particularly for the last three EOF modes, which suggests that forecasting skill scores would decrease when the EOF method is applied to predict the future rainfall anomalies two years in advance.