Maintenance of Multiple Jets in a Baroclinic Flow

This article describes the generation and maintenance of persistent zonal jets in a two-layer quasigeostrophic, b-plane channel model, focusing on the transition from a one to a two jet state. For weak and moderate values of surface friction and supercriticality, the transition occurs abruptly as the width of the baroclinically unstable region of the initial flow is gradually increased. Across the transition point, the persistent two jet state is characterized by a smaller value of eddy energy than that for the one jet state. This reduction in eddy energy is due to increased barotropic energy conversion from the eddies to the zonal mean flow. Consistent with this result, the abrupt emergence of two persistent jets is accompanied by sharply defined eddy momentum flux divergence maxima at the critical latitudes between the two jet maxima. Essentially the same behavior is found in the transition from two to three, and three to four jets. When two or more jets are present, baroclinically growing waves are found to exist along inter-jet minima, which are referred to as ‘‘inter-jet disturbances.’’ More importantly, the momentum fluxes of the interjet disturbances diverge at the interjet minimum, further decelerating the jets. Unstable normal modes similar to the inter-jet disturbances are also found. It is argued that the systematic wave absorption at the critical latitudes and the momentum flux divergence by the interjet disturbances may play a central role in the persistence of the