Mechanism of Zonal Index Evolution in a Two-Layer Model

Abstract

A mechanism that drives a zonal jet to meander in the meridional direction is investigated with a two-layer multiwave quasigeostrophic β-plant channel model. This model isolates the zonal index characteristics of a purely eddy-driven jet. Empirical orthogonal function analysis is used to characterize the northward- and southward-shifted states of the jet, and the authors refer to these two states as “high” and “low” zonal indexes, respectively. Composite analysis is used to examine the evolution of the zonal-mean flow, eddy heat and momentum fluxes, storm tracks, and energetics associated with both zonal indexes. It is found that the zonal index is the most prominent form of variability over a broad range of meridional scales for the initially unstable region. As expected, the onset of either index is marked by an anomalous momentum flux convergence-divergence pair on either side of the time-mean jet, and the high and low indexes are dynamically equivalent. This eddy forcing of the zonal-mean flow takes place on a timescale much shorter than that for the persistence of the zonal index itself. During most of the zonal index persistence, the zonal wind anomaly decays slowly. From a qualitative viewpoint, the zonal index can be interpreted as being impulsively forced by the eddies. Both the composite analysis and maps of instantaneous potential vorticity suggest that the zonal index persistence is not maintained by eddy-zonal-mean flow feedback. It is shown that the eddy forcing in normal-mode baroclinic life cycles does not explain the onset to either zonal index; however, its role during the zonal index persistence is inconclusive. When two consecutive persistent zonal index states are of opposite sign, the onset for the latter state is typically characterized by merging of two disturbances along two potential vorticity “fronts.”