St. Yang et al., MULTIPLE WEATHER REGIMES AND BAROCLINICALLY FORCED SPHERICAL RESONANCE, Journal of the atmospheric sciences, 54(11), 1997, pp. 1397-1409
Systematically recurrent, geographically fixed weather regimes forced
by a single isolated mountain in a two-layer, high-resolution, quasige
ostrophic model modified for the sphere are found to be robust phenome
na. While the climatological stationary wave is often confined to (or
has maximum amplitude in) the region just downstream of the orography,
giving the appearance of a wave train propagating into the Tropics, t
he regional maximum centers of low-frequency variance appear around th
e hemisphere, giving the appearance of zonal resonance or some type of
zonally confined propagation. This result is not anticipated in light
of Rossby wave dispersion theory on the sphere. On the other hand, ba
roclinic disturbances developing on a meridional temperature gradient
of finite extent force subtropical and polar easterlies as well as a s
harpened midlatitude westerly jet, which provides a zonal waveguide (b
y refraction and/or reflection) for the Rossby waves. These conditions
are favorable for the establishment of multiple weather regimes. The
baroclinicity of the atmosphere is then continuously forcing a mean st
ate that favors forced zonal propagation, counteracting the meridional
dispersion generated by the spherical geometry alone. These ideas sug
gest that the multiple-equilibria theories may be more applicable to t
he atmosphere than originally suggested by linear dispersion theory on
the sphere. It may also help explain why channel models work as well
as they do even for the largest scares.