Eddy kinetic energy redistribution within idealized extratropical cyclones using a two‐layer quasi‐geostrophic model

Abstract

Eddy kinetic energy (EKE) redistribution within midlatitude cyclones is investigated using a two‐layer quasi‐geostrophic model. The flow is composed of synoptic localized cyclones in both layers interacting with a baroclinic zonal basic flow. The effects of the planetary vorticity gradient β and the relative vorticity gradient of the westerly basic flow are to reduce the upstream ageostrophic geopotential fluxes in the lower layer and to intensify the downstream fluxes in the upper layer. Furthermore, they act to reduce the downward ageostrophic geopotential fluxes and to intensify the upward ageostrophic fluxes. When cyclones are embedded in a cyclonic shear, EKE rapidly accumulates on the southwestern flank of the lower‐layer cyclone before being cyclonically redistributed by the rearward cyclonically oriented ageostrophic fluxes and nonlinear advection. In contrast, when cyclones are embedded in an anticyclonic shear, the lower‐layer ageostrophic fluxes are mainly westward‐oriented and the pressure work combines perfectly with nonlinear advection to maintain the EKE increase at the same place upstream or downstream of the lower‐layer cyclone, depending on the value of β. Finally, a simulation showing the crossing of a westerly jet by cyclones condenses all the above effects. At the early stages, when cyclones lie on the anticyclonic side of the jet, a strong EKE increase occurs downstream of the lower‐layer cyclone. Just after the jet crossing, the EKE is rapidly rearward and cyclonically redistributed by the ageostrophic fluxes and nonlinear advection, leading to the formation of a lower‐layer jet to the south of the cyclone.