Variability in seasonal forecast skill of Northern Hemisphere winters over the twentieth century
Variability in seasonal forecast skill of Northern Hemisphere winters over the twentieth century
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二十世纪北半球冬季季节预报技巧的变化
DOI：
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发表时间：
2017
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影响因子：
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通讯作者：
T. Woollings
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作者：
C. O’Reilly;James Heatley;D. MacLeod;A. Weisheimer;T. Palmer;N. Schaller;T. Woollings
使用规定的海面温度 (SST) 进行的季节性后报实验对 1900 年至 2010 年的北半球冬季进行了分析。集合平均太平洋/北美指数 (PNA) 技能变化很大，在 20 世纪中叶降至零，北大西洋涛动 (NAO) 后报技能也有类似的变化。 PNA 技能密切关注观测到的 PNA 指数与热带太平洋海温异常之间的相关性。在本世纪中叶，PNA 和 NAO 的后报误差密切相关。 PNA 可预测性的下降是由于本世纪中叶的 PNA 负事件造成的，这些事件并不是由热带太平洋海温异常以可预测的方式强制发生的。总体而言，负 PNA 事件的可预测性较差，而且与正 PNA 事件相比，负 PNA 事件似乎更多地是由内部大气变化引起的。我们的结果表明，在近 30 年期间评估的季节性预报系统在诸如二十世纪中叶等热带太平洋海温异常强迫相对较弱的时期可能不太熟练。
Seasonal hindcast experiments, using prescribed sea surface temperatures (SSTs), are analyzed for Northern Hemisphere winters from 1900 to 2010. Ensemble mean Pacific/North American index (PNA) skill varies dramatically, dropping toward zero during the mid‐twentieth century, with similar variability in North Atlantic Oscillation (NAO) hindcast skill. The PNA skill closely follows the correlation between the observed PNA index and tropical Pacific SST anomalies. During the mid‐century period the PNA and NAO hindcast errors are closely related. The drop in PNA predictability is due to mid‐century negative PNA events, which were not forced in a predictable manner by tropical Pacific SST anomalies. Overall, negative PNA events are less predictable and seem likely to arise more from internal atmospheric variability than positive PNA events. Our results suggest that seasonal forecasting systems assessed over the recent 30 year period may be less skillful in periods, such as the mid‐twentieth century, with relatively weak forcing from tropical Pacific SST anomalies.