Formation of extratropical jets in baroclinic atmospheres

It is commonly held that multiple extratropical jets in baroclinic atmospheres form as a result of an inverse cascade of eddy kinetic energy to large scales, with the jet separation scale given by the Rhines scale. Here we show that multiple jets in baroclinic atmospheres can form when the nonlinear eddy-eddy interactions that give rise to an inverse energy cascade are weak or entirely absent. In simulations with an idealized GCM, multiple extratropical jets form when the Rossby radius is smaller than the planetary radius, for example, when the planetary radius or rotation rate are sufficiently large or the static stability of the atmosphere is sufficiently small. The meridional jet scale is similar to the Rossby radius and, if the flow is sufficiently baroclinic, is also similar to the Rhines scale (otherwise the Rhines scale is smaller). When nonlinear eddy-eddy interactions are eliminated in the GCM, multiple jets continue to form, with meridional scales that are similar to, albeit somewhat smaller than, those in the fully nonlinear GCM simulations. This demonstrates that eddy-mean flow interactions alone can account for the formation of multiple jets in baroclinic atmospheres and that eddy-eddy interaction are not essential; however, eddy-eddy interactions lead to isotropization of eddies and modify meridional jet scales by O(1) factors.