Abstract The evolution of linear Rossby waves on representative zonally varying upper-tropospheric flows is examined. Structures associated with maximum perturbation energy growth over both short and long time intervals are obtained. Sensitivity is determined with respect to the background flow, the manner in which the background flow is maintained, and vertical level. The background flows considered are a 26 winter 250-mb climatology obtained from the National Meteorological Center, 13-winter 300-, 250-, and 150-mb climatologies obtained from the European Centre for Medium-Range Weather Forecasts, flows representative of warm and cold ENSO events, and nearby free steady solutions of the nonlinear barotropic vorticity equation. The principal finding is that no matter which of the above background flows is used, no matter how the perturbation problem is formulated, no matter whether modal or nonmodal evolution is considered, the growth of free perturbations in a linear barotropic model is too weak to expla...