The Northern Annular Mode (NAM) dominates variability of the Northern Hemisphere (NH) wintertime extratropical circulation in both the troposphere and stratosphere. Changes in the tropospheric NAM (i.e., changes in the position and strength of the polar jet stream) directly alter NH mid-latitude temperature and precipitation patterns, making forecasting these changes a significant priority for subseasonal-to-seasonal (S2S) forecasts during boreal winter. This study examines fundamental characteristics of the wintertime tropospheric circulation pattern in the hindcast simulations of the North American Multi-Model Ensemble (NMME) Phase-2 model suite through examining how the models capture sudden stratospheric warming (SSW) events, known to precede large changes in the tropospheric NAM by 2–6 weeks. Findings indicate that the NMME Phase-2 models have an overall mixed performance in capturing the characteristics of the NAM and its teleconnections. Biases are apparent in the dominant nodes of the tropospheric NAM pattern, storm tracks and associated wave fluxes in the Atlantic, and a systematic underestimation of intraseasonal variability of the NH stratospheric polar vortex in the models (i.e., the stratospheric NAM). We then investigate the ability of the models to simulate the life cycle of model-identified SSW events, including pre- and post-SSW circulation patterns and sensible weather conditions. Specific model biases include inconsistent geopotential height precursor fields, weaker-than-observed vertical wave propagation prior to SSW events, and incorrect surface temperature regimes following the events. Together, the results suggest potential pathways forward for improving subseasonal winter weather forecasts associated with the NAM using the NMME Phase-2 models.

在对流层和平流层中，北半球环形模式（NAM）主导着北半球（NH）冬季温带环流的变化。对流层NAM的变化（即极地急流的位置和强度的变化）直接改变了NH中纬度的温度和降水模式，因此预测这些变化是北半球季节至季节（S2S）预报的重要优先事项冬季。这项研究通过检查模型如何捕获已知的发生在大变化之前的平流层突然变暖（SSW）事件，研究了北美多模式合奏（NMME）第二阶段模型套件的后预报模拟中冬季对流层循环模式的基本特征。在对流层NAM中需要2-6周。研究结果表明，NMME Phase-2模型在捕获NAM及其远程连接的特性方面总体上表现良好。在对流层NAM模式的主要节点，风暴轨迹和大西洋的相关波通量中存在明显的偏差，并且对模型中NH平流层极涡的季节内变化（即平流层NAM）进行了系统的低估。然后，我们调查了这些模型模拟模型识别的SSW事件的生命周期的能力，包括SSW前后的循环模式和合理的天气条件。具体的模型偏差包括不一致的地势高度前兆场，SSW事件之前的垂直波传播弱于观察到的事件以及事件之后的不正确的表面温度范围。一起，