Eliassen–Palm Flux Diagnosis Based on Isentropic Representation

Abstract

This paper demonstrates an isentropic representation of the Eliassen?Palm flux (EP flux) as a tool for diagnosis of the atmospheric zonal mean states. This is a full extension of the transformed Eulerian mean (TEM) to express lower boundary conditions and finite-amplitude effects of waves. The isentropic EP flux is diagnosed from the NCEP?NCAR reanalysis for 12 yr and compared with the TEM. In the free atmosphere, the two diagnoses show similarities in the mass streamfunction and in the EP flux except that the EP flux divergence is somewhat different near the polar vortex. The similarities reflect the equivalency between the two schemes under the quasigeostrophic assumptions. However, the lower boundary values are very different between the two schemes. In the TEM, the vertical EP flux inaccurately suggests a much larger momentum flux coming out from the surface. In the isentropic diagnosis, the lower boundary value of vertical EP flux, which expresses the momentum exchange with the earth's surface through the orographic form drag, is very small. It rapidly increases with potential temperature in the lower troposphere, as a result of intersections of isentropic surfaces with the boundary. In the context of wave?mean-flow interactions, the low-level EP flux divergence induces the equatorward flows through the geostrophic adjustment. Based on the isentropic diagnosis, the vertical EP flux is easily decomposed into adiabatic and diabatic terms. In the Tropics, the vertical EP flux is dominated by eddy diabatic mixing of momentum, while in the extratropics, it is dominated by the form drag over isentropic surfaces. A Fourier analysis of the EP flux shows that the wave activity is considerably different between the two winter hemispheres. In the NH winter troposphere, the EP flux is composed primarily of stationary ultralong waves (wavenumbers 1?3) and secondarily of transient long waves (wavenumbers 4?7). In the SH winter, the transient long waves dominate the EP flux. Most of the transient waves are considered to be baroclinic instability waves and their wave?mean-flow interactions drive the mean meridional circulation in the extratropical troposphere.