Eddy-zonal flow feedback in the Southern Hemisphere

The eddy- zonal flow feedback in Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence timescale of the leading principal components (PCs) of the zonal mean zonal flow shows substantial seasonal variations. In SH summer, the persistence timescale of PC1 is significantly longer than that of PC2, while the persistence timescales of the two PCs are quite similar in SH winter. A storm track modeling approach is applied to demonstrate that seasonal variations of eddy-zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence timescale. The eddy feedback timescale estimated from a storm track model simulation and a wave response model diagnostic shows that PC1 in JJA and DJF, and PC2 in JJA, have significant positive eddy-mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback timescales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence timescale.

Eliassen-Palm flux diagnostics have been performed to demonstrate the dynamics governing the positive feedback between eddies and anomalous zonal flow. The mechanism of the positive feedback, for PC1 in JJA and DJF, and PC2 in JJA, is as follows: enhanced baroclinic wave source (heat fluxes) at low level in the region of positive wind anomalies, propagate upward and then equatorward from the wave source, thus giving momentum fluxes that reinforce the wind anomalies. The difference of PC2 between DJF and JJA is because of the zonal asymmetry of the climatological flow in JJA. For PC2 in DJF, wind anomalies reinforce the climatological jet, thus increasing the barotropic shear of the jet flow. The “barotropic governor” plays an important role in suppressing eddy generations for PC2 in DJF and thus inhibiting the positive eddy-zonal flow feedback.