Subseasonal predictability of the North Atlantic Oscillation
Subseasonal predictability of the North Atlantic Oscillation
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DOI：
10.1088/1748-9326/abe781
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发表时间：
2021-04-01
影响因子：
6.7
通讯作者：
Newman, Matthew
中科院分区：
文献类型：
--
作者：
Albers, John R.;Newman, Matthew
在“次季节”（数周至不到一个季节）时间尺度上巧妙地预测北大西洋涛动 (NAO) 和密切相关的北环模 (NAM) 是业务预报中心的首要任务，因为 NAO 与高影响天气事件相关，尤其是在冬季。不幸的是，主导 NAO 总变化的相对快速的、与天气相关的过程在大约两周后是不可预测的。在较长的时间尺度上，热带对流层和平流层提供了一定的可预测性，但它们对 NAO 总方差的贡献相对较小。此外，次季节预测只是偶尔有技巧，这表明实际需要在预测时识别较少的潜在可预测事件。在这里，我们构建了一个基于观测的线性逆模型 (LIM)，用于预测次季节 NAO 预测何时最为有效并诊断其原因。我们使用 LIM 来识别那些动态模式，尽管这些动态模式仅捕获整体 NAO 变异性的一小部分，但它们在很大程度上负责扩展范围的 NAO 技能。可预测的 NAO 事件源于这些模式的线性叠加，这些模式代表热带海面温度和平流层下层的联合变率加上捕获平流层上层向下传播的单一模式。我们的方法具有广泛的适用性，因为 LIM 和最先进的欧洲中期天气预报中心综合预报系统（IFS）对于同一组预测的高技能预报事件都具有更高（且可比）的技能，这表明 LIM 识别的低维可预测子空间与现实世界次季节 NAO 预测相关。
Skillfully predicting the North Atlantic Oscillation (NAO), and the closely related northern annular mode (NAM), on 'subseasonal' (weeks to less than a season) timescales is a high priority for operational forecasting centers, because of the NAO's association with high-impact weather events, particularly during winter. Unfortunately, the relatively fast, weather-related processes dominating total NAO variability are unpredictable beyond about two weeks. On longer timescales, the tropical troposphere and the stratosphere provide some predictability, but they contribute relatively little to total NAO variance. Moreover, subseasonal forecasts are only sporadically skillful, suggesting the practical need to identify the fewer potentially predictable events at the time of forecast. Here we construct an observationally based linear inverse model (LIM) that predicts when, and diagnoses why, subseasonal NAO forecasts will be most skillful. We use the LIM to identify those dynamical modes that, despite capturing only a fraction of overall NAO variability, are largely responsible for extended-range NAO skill. Predictable NAO events stem from the linear superposition of these modes, which represent joint tropical sea-surface temperature-lower stratosphere variability plus a single mode capturing downward propagation from the upper stratosphere. Our method has broad applicability because both the LIM and the state-of-the-art European Centre for Medium-Range Weather Forecasts Integrated Forecast System (IFS) have higher (and comparable) skill for the same set of predicted high skill forecast events, suggesting that the low-dimensional predictable subspace identified by the LIM is relevant to real-world subseasonal NAO predictions.