A Multiple-Regime Approach to Atmospheric Zonal-Flow Vacillation

Zonalow vacillation in an idealized two-layer, global, primitive-equation model is studied in the context of multiple regimes. The spatial structure of vertically and zonally averaged zonalow variability resembles that observed in the Southern Hemisphere, with dipolar anomalies centered at 40 and 60. The probability density function (PDF) of the model's zonal ow is studied in the subspace of its two leading principal components. The PDF exhibits multiple regimes consisting of a pronounced central peak with two distinct shoulders attached to it. All three features are statistically signi cant at the 95% con dence level against the null hypothesis of a two-dimensional Markov process of order one. Flow composites for the two shoulders show meteorologically signi cant di erences from climatology and similarities with observed Southern Hemisphere ow patterns. In the low-latitude regime the narrow jet's maximum lies equatorward of the climatological mean position, while in the high-latitude regime the broad jet is sometimes bimodal and has its maximumpoleward of the climatological position. The residence times in the highand low-latitude regimes are typically 2{3 times as long as in the central-peak regime. Both regimes' onsets are found to be abrupt, but the regime breaks exhibit a signi cant asymmetry: the low-latitude regime decays more slowly, while the high-latitude regime breaks rather abruptly. Eddy momentum forcing acts to maintain the zonalow regimes against surface drag. The changes in low-level eddy heat ux precede those of the eddy momentum forcing that drive both the regime onset and break. This phase di erence between heat and momentum uxes suggest that baroclinic processes play an important role in the regime transitions. The dependence of the preferred regimes' zonal-jet latitudes on bottom friction shows that multiple regimes coexist in a wide range of the parameter values.