Zonal-index persistence in the two-layer model

The dominant mode of variability of the zonal-mean zonal wind consists of a meridional displacement of the extratropical jet about its mean position, which is often referred to as ‘the zonal index'. This variability is driven by changes in the eddy momentum flux but tends to occur on longer time scales than that of the forcing eddies. There is still not a clear answer to the question of what controls the persistence of the zonal index. Although the slower agents in the climate system might play a role in enhancing the persistence of the zonal index, idealized models without ocean or stratosphere tend in fact to be more persistent than the actual atmosphere. At least in those models, the enhanced persistence of the zonal index should thus be driven dynamically through a positive feedback mechanism. In this work we analyze the zonal-index variability in arguably the simplest model that can produce this type of variability: the two-layer quasigeostrophic model. We assess the relevance of ‘self-maintenance' for the enhanced persistence by considering the lagged relation between the zonal-mean barotropic and baroclinic anomalies and the characteristic lifecycles of these anomalies over different frequency ranges. We also study the sensitivity of persistence on the frictional and thermal forcing timescales and perform idealized experiments that underscore the dynamical mechanisms at work.