This research was supported by the National Science Foundation Grants ATM-1016095 and ATM-1118429. The ERA-Interim data were downloaded from the Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory (NCAR CISL). We thank three anonymous reviewers for their constructive comments.
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Some of the 22 storms have a wave pouch present at either 850 or 925 hPa at genesis. They are excluded because the wave evolution is examined following the 700-hPa pouch track.
Note that Fig. 1a only shows northern waves that have a closed circulation in the wave’s comoving frame of reference. Not all northern waves have a pouch.
The OW parameter is a measure of the “shape preserving” component of a vortical flow (Rozoff et al. 2006; Dunkerton et al. 2009) and is defined by OW = ζ2 −