Mean flow evolution of a baroclinically unstable potential vorticity front

Quasi-linear theory and numerical models are used to study the mean flow mo dification of a two-layer shallow water baroclinically unstable flow as a f unction of Rossby number. This Bow has an upper-layer potential vorticity f ront overlying a quiescent lower layer and is used as a simple representati on of the Gulf Stream. Quantities derived from an analytical expansion in the small meander amplit ude limit of the (quasi-linear) equations are found to compare quantitative ly well with numerical model simulations of the Bow in small amplitude and to pertain qualitatively even beyond the instability equilibration, where t he meander amplitude is as large as the meander wavelength. The baroclinic evolution is similar for all Rossby numbers, with differences arising from increased asymmetry of the Bow with increasing Rossby number. The equilibra tion of the instability is similar for all Rossby numbers and is due to the acceleration of a strong barotropic shear. This acceleration is predicted from the small amplitude analysis. Quasigeostrophic diagnostics are shown to be useful even for large Rossby n umber flows such as the Gulf Stream. One qualitative difference that appear s is that as the mean Row is modified, a lateral separation of the zonal me an potential vorticity front and the jet maximum appears, consistent with G ulf Stream observations. This feature is found only for finite Rossby numbe r Bows.