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M. A. Giorgetta (1) , E. Manzini (2) , M. Esch (1) and E. Roeckner (1)

Role of the Stratosphere in Climate Modelling: The Connection Between the Hadley and the Brewer-Dobson Circulation. M. A. Giorgetta (1) , E. Manzini (2) , M. Esch (1) and E. Roeckner (1)

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M. A. Giorgetta (1) , E. Manzini (2) , M. Esch (1) and E. Roeckner (1)

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  1. Role of the Stratosphere in Climate Modelling: The Connection Betweenthe Hadley and the Brewer-Dobson Circulation M. A. Giorgetta(1), E. Manzini(2), M. Esch(1) and E. Roeckner(1) (1) Max Planck Institute for Meteorology, Hamburg, Germany(2) Istituto di Geofisica e Vulcanologia and Centro Euro-Mediterraneo per i Cambiamenti Climatici, Bologna, Italy

  2. Motivation • The tropospheric mean circulation in a GCM depends on the representation of the stratosphere [Boville 1984] • Tropospheric weather is sensitive to the state of the stratosphere [Baldwin et al. 2003] • Are climate change projections sensitive to the stratospheric representation? (Most AOGCMs used for AR4 do not fully resolve the stratosphere) • Investigate and demonstrate effects of different models of the stratosphere on the tropospheric climate in GCM experiments • Contribute to SPARC DynVar “Top”

  3. Aims of this work • Explore effects of “Low Top” vs. “High Top” GCMs • Low top atmosphere: troposphere + lower stratospherelower stratosphere = upper boundary region of AGCM • High top atmosphere: trop. + strat. + lower mesosphere • Use coupled atmosphere ocean GCM to explore effects of different stratospheric representations on the tropospheric climate • Use atmospheric GCMs with prescribed lower boundary conditions

  4. Experimental design MPI-M AGCMs and AOGCMs: • ECHAM5 Low Top atmosphere, ptop= 10 hPa (Roeckner et al. 2006) • MAECHAM5 High Top atmosphere, ptop = 0.01 hPa (Manzini et al. 2006) • ECHAM5/MPIOM Low top atmosphere / ocean (Jungclaus et al. 2006) • MAECHAM5/MPIOM High top atmosphere / ocean

  5. Experimental design Common features of all 4 experiments • Horizontal atmospheric resolution T63 / ~1.9°x1.9° • Troposph. vertical grid: 26 levels in [surface, 110hPa] • Dynamics and processes in troposphere • Ocean model: ~1.5° resolution, 40 levels Differences • Vertical resolutions from ~110 hPa to 0 hPa • Low top: 31 levels, 5 levels in ]110,10] hPa • High top: 47 levels, 9 levels in ]110, 10] hPa +12 levels in ]10, 0.01] hPa

  6. Experimental design Differences (cont.) • Horizontal diffusion: dx/dt = -(-1)q∙Kx∙∇2qx, 2q=8 • Low top: • To avoid spurious wave reflection at the upper boundary,the order of hyper-diffusion is reduced in the stratosphere: 2q=(6,4,2,2,2) at (90, 70, 50, 30, 10 hPa) • Acts on waves, incl. large scale waves, and zonal mean • High top: • Equal order of hyper-diffusion 2q=8 at all levels • Gravity wave drag parameterization • Low top: • Orographic GWD (Lott and Miller, 1999) • High top: • Orographic GWD (Lott and Miller, 1999) • GWD from a spectrum of gravity wave with atmospheric sources. (Hines, 1997)

  7. Coupled experiments • Low top:CM31 is a ~500 year control experiment for CMIP3 • High top:CM47 is started from an ocean state of the CM31 simulation, the atmosphere is initialized at the new vertical resolution • Initial drift of CM47 over ~60 years • Compare years 61 to 160 of CM47 with a 100 year period of CM31

  8. Questions • Is the tropospheric climate different between the Low Top and High Top CM simulations? • What differences occur if the lower boundary conditions (SST+ice) are prescribed – and how much do these changes correspond to changes in the coupled system? • Which mechanisms induce these changes?

  9. Coupled model Annual mean temperature T (K)

  10. Coupled model Annual mean U (m/s)

  11. Coupled model Annual mean residual vertical velocity w* (mm/s)

  12. Coupled model vs. uncoupled model Annual mean temperature T (K)

  13. Common in coupled and AMIP experiments • T is significantly changed in the stratosphere and upper tropical troposphere • Hadley circulation stronger in high top model • Brewer-Dobson circulation stronger in high top model • Differences • AMIP: dT in troposphere is ~0 below 300 hPa • Coupled: dT = ~0.5 K in troposphere below 200 hPa • Differences between coupled experiments must be explained by different stratospheric forcing terms and resolution effects

  14. Dynamical forcing terms in the stratosphere dU/dt|dyn = dU/dt by Div. of EP-flux

  15. Conclusions • A low and high top AGCM has been used for uncoupled and coupled experiments to explore effects of different models of the stratosphere on the troposphere • Using identical resolution in the troposphere and the same tropospheric parameterizations, the tropospheric climate changes under the influence of the stratospheric dynamics. • The analysis of dynamical forcing terms shows: • Horizontal diffusion acting on large scale waves becomes visible as a strong difference in EP flux divergence at 50 hPa • Horizontal diffusion is also non-negligible in the zonal mean • Between 50 hPa and 10 hPa, the change in Div.F drives the changes in the Brewer-Dobson circulation and thereafter the Hadley circulation changes • Stratospheric representation matters for tropospheric climate • N.B.: Resolution effects would be much larger for better resolution. Use of MAECHAM5 with ~90 layers would generate QBO Amplification of interannual variability • See also poster of Shaw and Shepherd

  16. END

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