东亚夏季风降雨的可预报性研究

本文根据CMAP(The Climate Prediction Center Merged Analysis of Precipitation)观测资料,使用相关系数和均方根误差,对CHFP2(Coupled Historical Forecast Project, phase 2 )的2个模式对东亚夏季降雨的季节预报技巧作出评价。在完美模式的理论框架下,分别使用基于信噪比的潜在相关系数和基于信息熵的潜在可预报性指标,对该区域主要针对夏季降雨的可预报性作出评价。通过最可预报分量分析(PrCA),得到季节降雨的最可预报型。将最可预报型投影到海温场,得到了降水最可预报型对应的海温分布。研究发现:相关系数所反映的预报和观测的线性相关程度总体上是低纬度海洋区域比高纬度陆地区域高,而均方根误差反映的则是在海洋区域降雨预报偏离实际值的程度较陆地区域大,预报水平与目前降雨的季节预报水平相符。潜在可预报性估计表明,潜在可预报率存在空间上的变化,从低纬度向高纬度、从海洋到内陆,呈衰减趋势。同时,信号和噪音的分析表明,信号成分占主导作用,形成了潜在可预报率的空间分布格局,暗示了海洋外强迫的重要作用;中国大陆缺少像海洋区域那样明显的外强迫,因此降水季节预报技巧相比热带海洋区域非常有限。海温投影的分析表明海洋的外强迫是东亚降雨季节预报的重要来源。尽管厄尔尼诺本身的复杂性,它对东亚夏季风的重要影响及其与东亚降雨预报之间的遥相关揭示了它们内在的联系。

Predictability of the East-Asian summer monsoon rainfall in two coupled models

The skills of seasonal forecast of East-Asian precipitation, especially for summer seasons were studied. In the perfect model scenario, the potential predictability of East-Asian precipitation in seasonal time scale using signal-to-noise ratio and information-based indices were commented. The most predictable components and corresponding spatial patterns were also derived by PrCA analysis. The results show that the relationship of forecasts and observations displayed by correlation coefficient is characteristic of higher level in ocean region at low latitudes than in land region at high latitudes. While root mean square error skill shows that forecast diverging degree from observations in ocean region is much more than that in land region. Both agree with the current level for forecast. By estimating potential predictability, it shows that the potential predictability exists a spatial variation and it decays from low to high latitude and from ocean to land region. Further analysis of signal and noise components reveals the important role of marine forcing. The skill of seasonal rainfall is quite limited for lacking of apparent marine forcing in the mainland. By projecting the time series of the leading two modes onto the SSTA field, it shows that the forcing of ocean is the main source of potential predictability of East-Asian precipitation prediction in seasonal time scale. Despite the complexity of ENSO, it is clear that it has great influence on East-Asian monsoon, and the teleconnection between East-Asian precipitation prediction and ENSO may reveal the internal relations of them.