Evaluating Sudden Stratospheric Warmings and NAM Predictability in the NMME Phase-2 System

“The Northern Annular Mode (NAM) dominates variability of the Northern Hemisphere (NH) wintertime extratropical circulation in the troposphere and stratosphere. Changes in the tropospheric NAM directly alter NH mid-latitude temperature and precipitation patterns and increase chances for extreme winter weather in major population centers. Numerous studies have concluded that dynamical stratosphere-troposphere coupling constitutes an important forced component of NAM variability, with changes in the strength of the stratospheric polar vortex typically preceding large changes in the tropospheric NAM. These tropospheric NAM changes can then persist for 30 to 45 days following the initial stratospheric perturbation. Additionally, sudden stratospheric warmings (SSWs; i.e., weakenings of the stratospheric polar vortex) may also be predictable through documented anomalous tropospheric circulation patterns that precede them, possibly further extending the lead-times for tropospheric NAM predictions. However, evaluation of these stratosphere-troposphere coupling features in operational and coupled climate models yields mixed results. Generally, the models correctly simulate conditions that precede SSWs but incorrectly simulate the downward propagation of the anomalies into the troposphere. Therefore, tropospheric NAM predictability remains limited to one to two weeks.

Our proposed research will study the predictability of the wintertime tropospheric NAM in the hindcast simulations of the North American Multi-Model Ensemble Phase-2 (NMME-2) model suite. We will specifically evaluate SSWs and their relationship with the subsequent evolution of the tropospheric NAM. The research plan consists of three tasks. First, we will evaluate the general characteristics of the stratospheric and tropospheric NAM in the NMME-2 models. Metrics of model performance will include mean biases in the spatial pattern of the NAM, persistence of each phase of the NAM, and absolute errors in associated teleconnection patterns. Second, using NMME-identified SSW events, we will composite pre- and post-SSW circulation patterns in both the troposphere and stratosphere and compare the simulated patterns to those derived from reanalysis. We will also examine wave diagnostics to understand where models differ dynamically from reanalysis. The final task will examine lag-composites from the NMME-2 models based on reanalysis-identified (i.e., verified) SSW event dates and also involve scoring the forecast skill of the NMME-2 hindcasts to those verified events. In this way, we will quantify how well the models capture actual events and score absolute errors in meteorological fields and wave diagnostics to understand why models may have missed other events.

This proposal is submitted for consideration under the NOAA Modeling, Analysis, Predictions, and Projections Competition for North American Multi-Model Ensemble System Evaluation and Application. The proposed research aims to quantify stratosphere-troposphere coupling processes and NAM predictability within the NMME-2 system in order to identify model biases and subsequently improve sub-seasonal winter forecasts. Our work will document explicitly “the representation of climate phenomena underpinning known intraseasonal to interannual predictability sources and examine the linkages between those sources and prediction skill or lack thereof in the system.” explicitly sought by the call. Furthermore, because of the observed link between major SSWs and significant changes in tropospheric weather (including cold air outbreaks and winter storms), our study satisfies the interest of the competition to “evaluat[e] the prediction of large-scale, extended lead time conditions conducive to extremes.” Improving sub-seasonal forecasts from the models will heighten societal preparedness for potential extreme winter weather and therefore directly address the objectives set forth in NOAA’s long-term goal of “Weather-Ready Nation” outlined in NOAA’s Next-Generation Strategic Plan.”