We study the effects of variations in jet width on the
downstream growth of baroclinic waves, using a simple quasi geostrophic
model with a vertically varying basic state and variable channel width,
as well as a simplified primitive equation model with a basic state that varies in
latitude and height. Our study is motivated by observations that in midwinter
in the Pacific the storm track is weaker and the jet is narrower during years
when the jet is strong.
Our linear models are able to reproduce the observed decrease of
spatial growth rate with shear, if we take into account the narrowing of the jet by
assuming it decreases the meridional wavelength of the perturbations,
thus hampering their growth. A common suggestion has been that perturbations
are weaker when the jet is strong because they move faster out of the unstable
storm track region. We find that we need to take into account that the jet
narrows when it strengthens, otherwise, the increase of growth rate
is strong enough to counteract the effect of increased advection speed.
We also find that when the model basic state is Eady-like (small or zero
meridional PV gradients in the troposphere) the short wave cutoff for
instability moves to large scale waves as shear is increased, due to the
accompanying increase in meridional wavenumber. This results in a
transition from a regime where upper level perturbations spin up a surface
circulation very rapidly and normal mode growth ensues, to a regime
where the initial perturbations take a very long time to excite growth.
Since waves slow down when a surface perturbation develops, this
can explain the observations that the storm track perturbations are
more ``upper level'' during strong jet years and their group
velocities increase faster than linearly with shear.