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)

(1) Max Planck Institute for Meteorology, Hamburg, Germany (2) Istituto di Geofisica e Vulcanologia and Centro Euro-

Mediterraneo per i Cambiamenti Climatici, Bologna, Italy

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”

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

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

AM-LOW

ECHAM5(T63L31)

AMIP2 SST+ice (1978-1999)

AM-HIGH

MAECHAM5(T63L47)

AMIP2 SST+ice (1978-1999)

CM-LOW

ECHAM5(T63L31)/MPIOM(GR1.5L40)

100 years (CTRL exp. for IPCC AR4)

CM-HIGH

MAECHAM5(T63L47)/MPIOM(GR1.5L40)

100 years

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

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)

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

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?

Coupled model

Annual mean temperature T (K)

Coupled model

Annual mean U (m/s)

Coupled model

Annual mean residual vertical velocity w* (mm/s)

Coupled model vs. uncoupled model

Annual mean temperature T (K)

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

Dynamical forcing terms in the stratosphere

dU/dt|dyn = dU/dt by Div. of EP-flux

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

END