Abstract
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.
