Wave - Mean Flow Interaction: The Role of Continuous - Spectrum Disturbances

Traditionally, "eddy feeds zonal flow" in the atmosphere is considered as a result of decaying unstable waves. We show that disturbances made of non-modal solutions - the continuous - spectrum disturbances - can also effectively transport zonal angular momentum and interact with the zonal basic flow. These disturbances, though stable, eventually decay, losing their energy to strengthen the westerly jets in the atmosphere. Calculations with observational data illustrate that the atmospheric zonal flow is maintained primarily by continuous-spectrum disturbances rather than by unstable waves. Angular momentum transport by continuous-spectrum disturbances is one order of magnitude larger than that by all kinds of normal modes (referred as discrete-spectrum disturbances) including unstable waves.