Does an Unstable Baroclinic Wave Equilibrate/Decay Baroclinically or Barotropically?

Abstract

It is shown using a quasigeostrophic multilevel channel model that the stabilization of a zonal baroclinic flow by an unstable wave would be essentially baroclinic if the meridional domain is narrow. In contrast, an unstable baroclinic wave in a sufficiently wide domain would first propagate meridionally away from its source while intensifying and would then equilibrate/decay barotropically. The equilibration in this case occurs mostly by inducing a strong horizontal zonal shear and also partly by reducing the zonal baroclinicity. Since a strong impact of the lateral boundaries is generally a model artifact, barotropic equilibration is suggested to be more important in the atmosphere. The corresponding increase in the meridonal scale of the wave field associated with the meridional propagation of the unstable wave in a wide domain is self-limiting by the beta effect. This result can be understood in terms of a well-known aspect of geostrophic turbulence. When the model width is only comparable to the zonal scale of the unstable wave, reflection of the waves by the lateral boundaries would naturally prohibit the cascading process and would result in baroclinic stabilization.