MECHANISM OF ZONAL INDEX EVOLUTION IN A 2-LAYER MODEL

A mechanism that drives a zonal jet to meander in the meridional direc tion is investigated with a two-layer multiwave quasigeostrophic beta- plane channel model. This model isolates the zonal index characteristi cs of a purely eddy-driven jet. Empirical orthogonal function analysis is used to characterize the northward- and southward-shifted states o f the jet, and the authors refer to these two states as ''high'' and ' 'low'' zonal indexes, respectively. Composite analysis is used to exam ine the evolution of the zonal-mean flow, eddy heat and momentum fluxe s, storm tracks, and energetics associated with both zonal indexes. It is found that the zonal index is the most prominent form of variabili ty over a broad range of meridional scales for the initially unstable region. As expected, the onset of either index is marked by an anomalo us momentum flux convergence-divergence pair on either side of the tim e-mean jet, and the high and low indexes are dynamically equivalent. T his eddy forcing of the zonal-mean flow takes place on a timescale muc h shorter than that for the persistence of the zonal index itself. Dur ing most of the zonal index persistence, the zonal wind anomaly decays slowly. From a qualitative viewpoint, the zonal index can be interpre ted as being impulsively forced by the eddies. Both the composite anal ysis and maps of instantaneous potential vorticity suggest that the zo nal index persistence is nor maintained by eddy-zonal-mean flow feedba ck. It is shown that the eddy forcing in normal-mode baroclinic life c ycles does not explain the onset to either zonal index; however, its r ole during the zonal index persistence is inconclusive. When two conse cutive persistent zonal index states are of opposite sign, the onset f or the latter state is typically characterized by merging of two distu rbances along two potential vorticity ''fronts.''