Eddy-Zonal Flow Feedback in the Southern and Northern Hemispheres A Review of Two Papers by D. J. Lorenz and D. L. Hartmann Stephanie Sydorko AT750

Baroclinic Mechanism for the Eddy Feedback on the Zonal Index (Robinson 2000) Eddy momentum flux forces a region of anomalously strong westerlies Wind is deflected equatorward by the Coriolis force, leading to areas of convergence and divergence The sinking and rising air is warmed and cooled adiabatically, generating areas of high and low pressure at the surface, and enhancing the temperature gradient Air moving from high to low pressure is deflected eastward by the Coriolis force Baroclinic eddies are generated more vigorously in the resulting enhanced baroclinic region Surface drag reinforces region of enhanced baroclinicity in a region of anomalously strong westerlies Eq.Pole HL Conv.Div. F co deflects wind equatorward F co deflects wind eastward EP Flux divergence forces westerly winds Enhanced temperature gradient

Enhancement of Baroclinicity in Source Region by Baroclinic Eddies Baroclinic eddies propogate upward from source region Break up and dissipate, resulting in convergence of EP flux equatorward (and poleward) Momentum flux in opposite direction of EP flux convergence forces zonal wind anomalies above baroclinic source region Fig. 4, Robinson

Synoptic-Eddy Zonal-Mean Wind Feedback Synoptic eddy forcing pattern agrees overall with the zonal wind anomaly pattern (Figs. 2b and 8a) Reinforces the wind anomalies at 40 o and 60 o S

EP Flux Vectors, Anomalous Synoptic-Eddy Momentum Flux Regressed on PC1 EP flux vectors show enhanced eddy temperature flux at low levels in the region of positive wind anomalies (Fig. 8b) Baroclinic wave generation follows N/S movement of baroclinicity, which corresponds with the position of the upper level jet (Fig. 8c) Equatorward propogation yields maximum forcing poleward of 40 o S Fig. 8b

Residual-Eddy Zonal-Mean Wind Feedback Residual eddy forcing out of phase with synoptic eddy forcing Fig. 9b shows that residual eddies are mostly external Rossby waves Fig. 9c shows a net propogation of Rossby waves into the jet, and momentum out of the jet Residual eddy fluxes damp positive zonal wind anomalies

Total Eddy Momentum Forcing Associated with EOF1 Reinforces zonal wind anomalies, but not as strongly as synoptic eddies Also implies poleward propogation of zonal wind anomalies Fig. 10 Fig. 11

Comparison of EOF1 and EOF2 in the Southern Hemisphere EOF2 corresponds to strengthening and weakening of the midlatitude jet Cross correlation implies that a positive zonal-wind eddy feedback does not exist for EOF2 PC1 larger than PC2 due to feedback for EOF1 –A positive zonal wind-eddy feedback on PC1, most important at low frequencies –Synoptic eddies account for differences in total eddy feedback Fig. 13

EOF1 and EOF2 Eddy Responses in the SH Following physical arguments applied to EOF1 case, we would expect the same response from EOF2 as from EOF1 EOF2 horizontal shear coincident with horizontal shear of the midlatitude jet –Changes in strength of barotropic shear on flanks of jet suppress baroclinic wave growth –Opposes the positive feedback seen for EOF1 –Leads to a poleward propagation of the wind anomalies

Synoptic-Eddy Forcing in the NH Anomalies tend to reinforce the zonal-mean wind anomalies associated with EOF1 Changes in baroclinicity are consistent with changes in the vertical EP flux In NH EP flux anomalies stronger on poleward side of zonal wind anomaly –More symmetric when the AMI is used –Atlantic variability more like zonal- mean variability of the SH Fig. 9 Fig. 10

Residual-Eddy Forcing in the Northern Hemisphere Residual eddies again identified as external Rossby waves Eddies act to damp the zonal wind anomalies by removing momentum from the jet Fig. 11

Quasi-Stationary Eddy Forcing in the NH Index of refraction anomaly at high latitudes has largest effect on quasi-stationary forcing –Low index leads to a positive refractive index anomaly wave sink which decelerates zonal winds –High latitude eddy forcing reinforces zonal wind anomalies and shifts them poleward Stationary wave mountain torque has a damping effect on zonal wind anomalies at high latitudes Fig. 12 a, b

EOF1 and EOF2 Eddy Responses in the NH EOF1 anomalies are more persistent than EOF2 anomalies (Fig. 5b) Feedback responsible for EOF1 leading over EOF2 (Fig. 7) Synoptic and quasi- stationary eddies cause a poleward drift in zonal wind anomalies (Fig. 16) Fig. 16 Fig. 7

Comparison between the Northern and Southern Hemisphere Feedbacks SH has positive feedback for EOF1 only, while NH has positive feedback for both EOFs and poleward propogating feedback for EOF1 and EOF2 –Could be due to jet differences between Pacific and Atlantic jets in NH –EOF1 explains most of the variance, EOF2 picks up the rest of the feedback Synoptic response in NH is not symmetric –Due to wave suppression in Pacific during low index –Pacific jet making a transition from midlatitude to subtropical jet, with different forcing