The Impact of Zonal Propagation and Seeding on the Eddy–Mean Flow Equilibrium of a Zonally Varying Two-Layer Model

Abstract

Abstract This paper investigates the role of zonal propagation for the equilibration of zonally varying flow. It is hypothesized that there exist two ideal limits, for very small or very large group speed. In the first limit the eddies equilibrate locally with the forcing, while in the second limit the equilibration can only be understood in a global sense. To understand these two limits and to assess which is more relevant for the extratropical troposphere, a series of idealized experiments that involves changing the magnitude of the uniform zonal wind is performed. The results of the idealized model experiments suggest that the actual troposphere is likely to be in a transition regime between the two limits, but perhaps closer to the global than the local limit. Near the global limit, both the eddy amplitude and local baroclinicity over the baroclinic zone are strongly affected by the amount of upstream seeding. When the propagation speed is reduced relative to the control run, the zonal mean eddy activity decreases because the residence time increases more over the stable part of the channel than along the baroclinic zone. With the decrease in upstream seeding, the local supercriticality along the baroclinic zone increases, although the increase is moderate. The decrease in seeding and increase in baroclinicity partially offset the effects of each other, leading to only small changes in the maximum eddy amplitude downstream of the baroclinic zone. Changes in upstream seeding can also be achieved by enhanced damping. When the eddies are locally damped, the baroclinicity is also enhanced downstream of the damping region due to reduced eddy fluxes. For typical parameters, the recovery of the eddy amplitude occurs over very long distances. Based on these idealized results, it is argued that the coexistence of enhanced baroclinicity and weaker eddy amplitude over the Pacific storm track, as compared to the Atlantic storm track, is consistent with the effects of strong eddy damping over Asia.