Ekman Friction and the Formation of Upper Tropospheric Zonal Flows

The influence of the Ekman friction on the dynamics of zonal flows (ZFs) has been studied within the framework of a quasigeostrophic model of the atmosphere with two horizontal boundaries (the underlying surface and the tropopause). It is assumed that these flows have zero potential vorticity and are caused by specified buoyancy distributions at the boundaries. It is shown that, in the case of periodic distributions, the oppositely directed vertical velocity profile of ZFs transforms into a unidirectional profile with a maximum velocity at the upper boundary and zero velocity at the lower boundary. During this transformation, the velocity at the upper boundary increases; i.e., the upper tropospheric ZFs intensify due to the Ekman friction. A similar intensification occurs also in the case of initial distributions of buoyancy of the frontal type, which induce a system of two oppositely directed jet flows located in the upper and lower halves of the atmospheric layer. Due to the Ekman friction, the axial velocity of the lower flow drops to zero and the velocity of the upper flow, gradually covering the entire troposphere, doubles. The resulting flow is a jet pressed against the upper boundary, which may be considered a prototype of a western upper tropospheric jet flow. The important structural features of such a jet, which are established within the framework of a complete nongeostrophic model, are associated with horizontal jet asymmetry and the formation of fronts (discontinuity surfaces) adjacent to the upper boundary.