Extended Abstract (248K)6.1
Why is there Mid-Winter Minimum of Storm Tracks?
Yi Deng, Univ of Illinois, Urbana, IL; and M. Mak
The counter-intuitive statistical characteristic that the storm track over N. 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 the change in the isentropic potential vorticity gradient across the tropopause over N. Pacific. Such change gives 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 an instability analysis of a quasi-geostrophic model constructed under the guidance of the dynamical considerations alluded to above. A series of experiments illustrate that such background PV structure would lead to a reduction of the local growth rate, a shift of maximum eddy amplitude downstream of the jet core and a modulation of the shape and amplitude of the traveling wave packets. The overall dynamical process may be succinctly conceptualized as wave-packet-resonance associated with the barotropic and baroclinic components of an unstable wave packet. Additional insight of this process is deduced from the characteristics of their local energetics.
Session 6, Midlatitude Dynamics
Tuesday, 10 June 2003, 10:00 AM-11:30 AM
Next paper