Why is there midwinter minimum of storm track?

The counter-intuitive observation that the storm track over North Pacific has distinctly weaker intensity in mid-winter than in early- or late- winter remains largely unexplained. It is hypothesized that the phenomenon is a dynamical consequence of an increase in the isentropic potential vorticity (IPV) gradient across the tropopause over North Pacific during mid-winter. Such change would give rise to an increase in the barotropic-governor effect on local baroclinic instability. The observational basis of this hypothesis will be presented. The hypothesis is substantiated by a series of instability analyses with a quasi-geostrophic model that is constructed under the guidance of the dynamical considerations alluded to above. Sufficiently large downstream variation in the baroclincity and/or localized barotropic shear would give rise to baroclinic and/or barotropic localization of the unstable disturbances in the downstream region of the basic jet. The overall dynamical mechanism may be succinctly conceptualized as a mechanism of general wave-packet-resonance from the PV perspective. Additional insight of this process is deduced by diagnosing the local energetics of the unstable disturbances.