| Summary: | Global meteorological analyses from an assimilation of operational and research observations spanning the ~0-90~km altitude range confirm earlier tentative suggestions that high-altitude winds throughout the upper mesosphere reversed a week before the major stratospheric sudden warming (SSW) of January 2009. Analyzed winds reveal descent of mean easterlies from the upper mesosphere to the lower stratosphere, followed by more easterly winds throughout the Arctic troposphere in the weeks after the SSW, indicating that these descending Northern Annular Mode (NAM) anomalies reached the surface.  Eliassen-Palm fluxes reveal that the mesospheric precursor to this event was driven by transient nonstationary wave-2 planetary waves that propagated rapidly from the troposphere into the upper mesosphere, where they dissipated and produced easterly mean-flow accelerations.  This early SSW phase was characterized by both descending mesospheric easterlies and poleward expansion of subtropical stratospheric easterlies, which eventually merged in the extratropical upper stratosphere. These wind structures may in turn have focused transient wave-2 PW activity emerging from the troposphere in ways that intensified the SSW.
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