We study zonal-flow vacillation in an idealized two-layer global primitive equations model using the nonlinear paradigm of multiple regimes. The dry-atmosphere, flat bottom spectral model with T21 resolution was run subject to perpetual-winter forcing with zonally symmetric boundary conditions. The spatial structure of the model's zonal-flow variability resembles that observed in the Southern Hemisphere, with equivalent-barotropic dipolar anomalies centered at 40o and 60o. The angular probability density function (PDF) of the model's zonal flow--computed in the subspace of the three leading EOFs--is found to exhibit clear bimodality. Flow composites reveal two distinct anomalous zonal-jet states that persist for typically 10-15 days, with the jet displaced equatorward or poleward from its climatological mean position.
In order to examine the time evolution of the vacillation between the two PDF modes, regime composites were made relative to regime onset and break, which are defined by the first and last day of persistent events, respectively. Thermal wind balance is well maintained throughout the evolution of the zonal-flow vacillation. We find evidence that high-frequency baroclinic eddies act to maintain the zonal-flow regimes via weakening or strengthening of the poleward transfer of westerly momentum from lower latitudes; this suggests a possible feedback between the zonal flow and high-frequency eddies. A few days before the regime break, an outburst of eddy heat flux occurs, while precursors of regime onset are hard to find.
The effect of surface friction on the zonal-flow vacillation is investigated by changing the Ekman drag time scale. The dependence of the preferred regimes' jet latitudes on bottom drag takes the form of back-to-back saddle-node bifurcations, with zonal-flow vacillation occurring only for an intermediate range of bottom-friction values.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics