Whitaker, J. S., and R. M. Dole, 1995:
Organization of storm tracks in zonally varying flows. J.
Atmos. Sci., 52, 1178-1191.
ABSTRACT
Organization of storm tracks in zonally varying flows
Jeffrey S. Whitaker
CIRES, University of Colorado, Boulder, CO
Randall M. Dole
NOAA/ERL/CDC, Boulder, CO
Abstract
A simple, two-layer quasi-geostrophic model is employed to
investigate the sensitivity of storm tracks to changes in an externally
imposed, zonally-varying large scale flow. Zonally asymmetric
temperature and horizontal deformation fields are varied
systematically in order to compare the effects of baroclinicity and
horizontal deformation on storm track dynamics. The sensitivity of
the storm tracks to uniform barotropic zonal flows is also examined.
The results show two competing processes for storm track
organization, one associated with a local maximum in baroclinicity
and the other with a local minimum in horizontal deformation. When the
equilibrium state consists of a zonally symmetric temperature field and a
barotropic stationary wave, the maximum in synoptic-scale transient eddy energy
(storm track) is located in the entrance region of the upper jet just downstream
of the point of minimum horizontal deformation. As zonal variations in
baroclinicity become large (keeping the upper layer horizontal deformation
constant), the storm track shifts to the jet exit region just downstream of the
point of maximum baroclinicity. For flows intermediate between the above cases,
that is, having weaker zonal variations in baroclinicity and the same uppper
layer deformation, two storm track maxima appear, one located in the jet
entrance and one located in the jet exit region.
The results also indicate that the storm tracks are sensitive to changes in a
uniform barotropic zonal flow. The presence of a uniform westerly flow extends
the storm track and strengthens eddy activity, while the addition of a uniform
easterly flow shortens the storm track and dramatically weakens eddy activity.
The changes in the magnitudes of eddy activity appear related to differences in
the efficiency of nonlinear barotropic decay processes in weakening the eddies
in the jet exit region.
Sensitivities of the location of the storm tracks to changes in large scale
flow patterns are well captured by linear calculations, although sensitivities
of the strength of the storm tracks are not. For sufficiently strong zonal
variations in baroclinicity, two coherent modes of low-frequency variability
develop. They are characterized by 1) a meridional shift, and 2) an
extension/contraction as well as a modulation in the strength of the upper layer
jet and storm track.