Mechanisms that control the latitude of jet streams and surface westerlies Gang Chen (Princeton University) Isaac Held (GFDL) August 1,

Outline GFDL climate models: The responses of the jet streams and surface westerlies to ENSO and global warming in austral summer. Changes in the eddy momentum flux spectrum. An idealized dry model: Case I: the jet shift to a decrease in surface friction. Case II: the jet response to prescribed zonal forcing.

Motivations La Ni ñ a is associated with a poleward shift in the SH surface westerlies (e.g. Seager et al. 2003, L ’ Heureux and Thompson 2006). There is a positive annular mode trend in the observations in recent decades, which has been attributed in part to stratospheric ozone depletion (e.g. Thompson and Solomon 2002, Gillett and Thompson 2003). The midlatitude storm tracks are displaced poleward under global warming (e.g. Yin 2005).

Part I: Diagnoses from GFDL climate models

GFDL models GFDL AM2.1( , 10 realizations): atmosphere model, with observed SST, sea ice and radiative forcings. CM2.1( ): coupled model, IPCC A2 scenario. Red: ERA-40 Cyan: model surface-wind-max latitude (SH DJFM)

Methodology Austral summer (DJFM seasonal mean) weak Stratosphere-Troposphere Coupling weak stationary waves 1.La Ni ñ a (regression onto the inverted Cold Tongue Index in AM2.1) 2.Internal variability (regression onto the internal variability index after removing the ensemble mean in AM2.1) 3.Global warming ( minus in CM2.1)

La Ni ñ a C Internal variability Zonal mean zonal wind --- La Ni ñ a Tropopause Climatology

W C Global warming Climatology Internal variability Zonal mean zonal wind --- global warming Tropopause

The eddy momentum flux spectrum In the zonal and seasonal average, the zonal mean zonal momentum balance is s -  (vu)/  y dp ¼ surface stress The eddy momentum flux, vu, can be decomposed as vu = ss vu(k,  ) dkd  = ss vu(k,c) dkdc where k is wavenumber,  is frequency, c=  /k is phase speed. (Randel and Held 1991)

Latitude/phase speed spectrum – climatology Red: convergence; Blue: divergence U/cos  at 250 hPa U/cos  at surface Eddy momentum flux convergence

Latitude/phase speed spectrum – La Ni ñ a Climatology: shading; La Ni ñ a: contours The subtropical divergence shifts poleward due to negative subtropical wind anomalies. Equator S. Pole

Latitude/phase speed spectrum – global warming Equator S. Pole Climatology: shading; global warming: contours The midlatitude eddies get faster, which can be thought of the result of increased stratospheric winds

wavenumber/phase speed spectrum La Ni ñ a global warming Eddy momentum flux (45S--55S) Climatology: shading; Response: contours Red: equatorward flux; Blue: poleward flux

Summary for Part I In the austral summer, the zonal mean zonal wind displays a poleward shift in the tropospheric jet in response to La Ni ñ a and global warming. The differences from the internal variability pattern can be roughly related to thermal forcings. In the phase speed spectrum, the response to La Ni ñ a is consistent with the poleward shift in the critical latitudes due to subtropical wind anomalies, and the global warming trend is associated with an increase in eddy phase speed, which can be related to increased stratospheric winds.

Part II: Studies with an idealized dry model

An idealized dry model GFDL atmospheric spectral dynamical core Held-Suarez Physics (Held and Suarez, 1994) Equinox, No topography Two case studies: 1) Troposphere only: T42, Z20 (equally spaced sigma levels) surface friction (Robinson 1997) 2) Troposphere + Stratosphere: T42, Z40 (enhanced stratosphere resolution) prescribed zonal forcing (Song and Robinson 2004; Ring and Plumb 2007)

Case I: surface friction barotropic governor (see James 1987)

eddy drag vs. mean drag The zonal flow and jet latitude are controlled by mean drag. U (  =0.875): eddy drag U (  =0.875): mean drag U (  =0.875): total drag

Transient response (30 realizations) Fast adjustment 0-15: abrupt decrease of K E Slow adjustment : slow poleward jet shift : slow increase of K M U (  =0.875) Energy -  (vu)/  y (  =0.275)

Slow adjustment The equilibrated response = initial rapid barotropic acceleration at the jet latitude + the following slow poleward jet shift. Fast adjustment

Eddy momentum flux convergence spectra (1.5d)-(0.5d),  =0.275 (1.5d),  =0.275(0.5d),  =0.275

A shallow water model Thermal relaxation to equilibrium thickness (H 2eq ) A stochastic stirring that mimics baroclinic instability

Difference Fast eddiesSlow eddies Eddy momentum flux convergence spectra

The mechanism for the critical latitude movement 1) Rossby waves prefer to propagate equatorward in the atmosphere. 2) The eddy phase speeds can be modified by the strength of midlatitude jet.

Case II: prescribed zonal forcing Zonal mean zonal wind climatologyannular mode

Tropospheric forcing Zonal wind (Full model) (Zonally symmetric model) Torque

Torque (  =0.85,  =30)Torque (  =0.85,  =50) Equator N. Pole Eddy momentum flux convergence spectra Responses Control

Stratospheric forcing Zonal wind EP vectors Torque

Poleward shift Equatorward shift Torque position and jet movement Jet

Summary for Part II Case I (surface friction) shows that the increased eddy phase speed, due to the accelerated westerly jet, can lead to the poleward shift in the critical latitudes of midlatitude eddies. Case II (zonal forcing) shows that the direction of the jet shift depends on the position of the zonal wind acceleration.

A schematic summary phase speeds subtropical winds

Acknowledgement Advisor: Isaac Held General and thesis committee: Isidoro Orlanski, Geoff Vallis, Steve Garner, Lorenzo Polvani (Columbia) Walter Robinson (UIUC), Paul Kushner (U. Toronto), Gabriel Lau and other AOS Faculty members Postdocs: Jian Lu, Pablo Zurita-Gotor and others Fellow students: Dargan Frierson, Edwin Gerber and others

40-year trend C Climatology Internal variability Zonal mean zonal wind --- Trend Tropopause

Latitude/phase speed spectra – Trend Equator S. Pole Climatology: shading; 40-year trend: contours The midlatitude eddies get faster, which can be thought of the result of increased stratospheric winds

Downward control (Haynes et al. 1991) Torque Zonal windMeridional circulation