Nonlinear effects in the extended Eady model: Nonzero ββ but zero mean PV gradient

An analytical solution in a small Rossby number approximation beyond quasigeostrophy is carried out for the extended Eady model with non-zero ββ, but zero mean potential vorticity gradient. The leading order solution exhibits symmetry in the geopotential height and potential temperature distributions standard for quasi-geostrophic approximation. In contrast, adding the ββ-effect into the first-order nonlinear dynamics results in a symmetry breaking. In addition to the high/low asymmetries, there also appears a meridional asymmetry resulting in an arching wave-train as a neutral mode at finite amplitude. These results may explain certain observed dynamical characteristics of synoptic and planetary-scale transients.