Eddy feedbacks and zonal index persistence in the two-layer model

The leading mode of extratropical zonal wind variability represents a shift of the jet about its mean position. Although it has been known for some time that a positive eddy feedback makes this mode more persistent than might be expected based on its damping scale, the dynamics of this feedback is still subject to debate. Recent studies using stirred barotropic models have shown that this type of models can reproduce some aspects of the observed variability but it is unclear whether baroclinic dynamics might also play a role.

In this work we study the dynamics of zonal index variability in the two-layer QG model, which is the simplest model that can produce a baroclinic feedback. We show that zonal index variability is very strongly correlated with baroclinicity variability, especially at low frequency. Consequently, the barotropic (zonal index) and baroclinic zonal-mean zonal wind exhibit similar decorrelation rates at long lags, a result that remains robust as the model's external parameters are varied. This implies that baroclinicity damping may also affect the persistence of jet shifts, although this is only observed in the model with unrealistically strong damping. For realistic parameters, the strength of the positive feedback and zonal index persistence are most sensitive to changes in barotropic friction.