Dynamics of Annular Modes Dennis L. Hartmann Department of Atmospheric Sciences University of Washington Seattle, Washington USA AGU San Francisco 13:30, December 10, 2002 A22D-01

Lorenz & Hartmann (2001)

Observational Facts In Southern Hemisphere, Annular variability is maintained by interaction between zonal flow and transient synoptic-scale waves (Hartmann and Lo 1998), and quite easy to simulate (Yu and Hartmann 1993).

Dominant EOF is shift in latitude of eddy-driven jet and is easy to model and understand. Dipole Pattern Yu & Hartmann (1993) Very Red Spectrum

Power spectrum of PC-1 Hartmann & Lo (1998)

Eddy Structure changes with zonal index.

Does Eddy, Zonal-Flow Interaction contribute to the persistence of annular modes? • Feldstein and Lee (1998) showed that the eddy momentum fluxes associated with high frequency (synoptic) eddies are persistent and feed back positively on the zonal flow anomalies. Net transient eddy feedback is smaller than high-frequency feedback effect. • Lorenz and Hartmann (2001SH, 2002NH) used a simple linear model to estimate quantitatively how much the covariation of the zonal flow and the synoptic eddy forcing contributed to the redness of the spectrum of the annular mode.

Northern Hemisphere DJFM Zonal Wind Vertically Integrated Eddy Momentum Forcing

EOF-1 EOFs of Zonal Wind in Northern Hemisphere EOF-2

Zonal Index Tendency = Eddy forcing - Drag

Divide eddy forcing, m, into Stationary (t > 40 days) Synoptic (t < 15 days) Residual By filtering u and v. Calculate cross-covariance with Zonal index (PC-1)

Northern Hemisphere Annular Mode High-frequency (t < 15 days) eddies are more important for persistence than stationary eddies. Even after residual eddies are added, transient eddies are more important for persistence for lags larger than two weeks. Lorenz and Hartmann (2002)

Lagged Regression of u’v’ and EP Flux onto EOF-1 averaged over lags 8-30 days, with eddy forcing lagging zonal wind anomaly.

Co-Spectrum between Z and M suggests, Positive lagged correlation of M following Z suggests, Where bj is a feedback parameter Without this feedback, Lorenz and Hartmann (2002) N.Hemisphere case

First EOF of zonal wind has strong coupling to eddy forcing at very low frequencies.

Conclusions • Synoptic baroclinic eddies increase persistence of first EOF of zonal wind. Jet is source for eddies, eddies transport momentum into source region as they propagate away. • Intermediate frequency eddies damp zonal flow anomalies. These are quasi-barotropic eddies, perhaps resulting from end of baroclinic eddy life cycle, which are refracted back into the jet. • Stationary eddies also feed back positively on zonal wind anomalies. Could be similar mechanism as for synoptic eddies. Westerlies give wave source, as waves propagate away they flux momentum into jet. • Positive feedback increases low-frequency (t > 2 weeks) variance of EOF-1 by factor of two. No feedback for EOF-2. David Lorenz will explain why no feedback for EOF-2 in later talk.

Questions: How do transient and eddy feedbacks response to forcing? Do they give a coherent feedback to forced changes? ENSO Stratospheric Warmings Stratospheric Ozone Depletion Greenhouse gas changes - global warming