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Evaluating the relationship between sudden stratospheric warmings and tropospheric weather regimes in the NMME phase-2 models

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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.

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Acknowledgements

The authors would like to thank B. Kirtman and other members of the NMME modeling group for their assistance in acquiring the necessary model output, particularly the stratospheric model output, to perform the analyses in this manuscript. All NMME Phase-2 hindcasts are publicly available for download from the Climate Data Gateway hosted by NCAR (https://www.earthsystemgrid.org/search.html?Project=NMME). The NMME project and data dissemination is supported by the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), the National Aeronautic and Space Administration (NASA), and the Department of Energy (DOE). ERA-Interim data are available for download (upon free registration) from https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim. This work was funded by the NOAA Climate Prediction Office (CPO) Modeling, Analysis, Predictions, and Projections (MAPP) division (Grant #NA15OAR4310077). The authors would also like to thank three anonymous reviewers for their thorough and constructive reviews of our manuscript and their help in improving its content and writing.

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Correspondence to Jason C. Furtado.

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Furtado, J.C., Cohen, J., Becker, E.J. et al. Evaluating the relationship between sudden stratospheric warmings and tropospheric weather regimes in the NMME phase-2 models. Clim Dyn 56, 2321–2338 (2021). https://doi.org/10.1007/s00382-020-05591-x

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  • DOI: https://doi.org/10.1007/s00382-020-05591-x

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