Time Scale and Feedback of Zonal Mean Flow Variability

The physical processes which determine the time scale of zonal mean flow variability are examined with an idealized numerical model that has zonally symmetric lower boundary (Son and Lee, 2005). In the part of the parameter space where the time-mean zonal flow is characterized by a single (double) jet, the dominant form of zonal mean flow variability is the zonal index (poleward propagation), and the time-mean potential vorticity gradient is found to be strong and sharp (weak and broad). The e-folding time scale of the zonal index is found to be close to 55 days, much longer than the observed 10-day time scale. The e-folding time scale of the poleward propagation is about 40 days.

The long e-folding time scales for the zonal index are found to be consistent with an unrealistically strong and persistent eddy-zonal mean flow feedback. A calculation of the refractive index indicates that the background flow supports eddies that are trapped within midlatitudes, undergoing relatively little meridional propagation.

Additional model runs are performed with an idealized mountain to investigate whether zonal asymmetry can disrupt the eddy feedback. The time scale in these runs is reduced to about 30 days except in very high mountain run. It is found that the stationary eddies excited by the mountain weaken the time-mean potential vorticity gradient over a broad region, and lead to the formation of critical latitudes on the equatorward side of the jet. As a result, eddies propagate equatorward, and the zonal index events are replaced by shorter time scale poleward propagation. It is these changes to the time-mean flow that disrupt the eddy feedback and substantially reduce the time scale of the zonal index.