Can we trust the simulated gravitywave response to climate change

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Can we trust the simulated gravity-wave response
to climate change?
NCAR TIIMES Gravity-Wave Retreat, 2006

• Ted Shepherd
• Department of Physics
• University of Toronto

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• GW parameterizations are highly tuned to
  reproduce current climate
• So why should we trust their response to climate
  change?
• There are two issues
• Changes in source characteristics
• Changes in propagation and dissipation
• This talk will only address the latter
• Will focus primarily on polar vortex

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Impact of parameterized mesospheric GW drag on
downwelling and temperature over the winter pole
in a zonal mean model
Dashed line is without GW drag, solid line is
with GW drag
From Garcia Boville (1994 JAS)
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Cumulative contribution of resolved and
parameterized wave drag at various altitudes on
polar downwelling at 10 hPa in CMAM with only
orographic GW drag
Parameterized
Resolved
Total
From Beagley et al. (1997 Atmos.-Ocean)
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Wave driving vs polar temperature in the Antarctic
Heat flux at 100 hPa estimates the (resolved)
wave activity entering the stratosphere More wave
forcing implies more polar downwelling and a
warmer pole Differences reflect GW drag
From Austin et al. (2003 ACP)
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• Holton (1983 JAS) explained the mesospheric
  cooling observed above stratospheric sudden
  warmings as due to a GW feedback
• Filtering of GW momentum fluxes leads to a
  positive wave drag anomaly
• Same reasoning applies to climate perturbations,
  e.g. to the ozone hole
• How robust is this effect?
• Shaw Shepherd (JAS, in press)

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• Response of downwelling over SH polar cap to
  combined effects of climate change and ozone
  depletion
• Solid line shows October, dashed shows November
• Left is total downwelling, right only from
  resolved EPFD

From Manzini et al. (2003 JGR)
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• There is a strong constraint from (angular)
  momentum conservation
• In the steady limit, downwelling is constrained
  by downward control (Haynes et al. 1991 JAS) F
  is force/unit mass
• For GWs, this simplifies to

(assuming no flux of momentum to space)
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• Thus the downwelling at a given height is
  independent of exactly where the waves break
  above that height
• What goes up must come down
• But what happens at the model lid?
• If any momentum flux remaining at the model lid
  is thrown away, then
• which now depends on model lid height

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• To conserve momentum, any remaining momentum flux
  at the model lid must be deposited as a drag,
  e.g. in the top few levels of the model
• This maintains the integrity of the downward
  control relation
• Throwing away momentum flux is equivalent to
  imposing an opposite drag above the model lid
• Also, there must be no Rayleigh drag or zonal
  mean sponge layer (Shepherd et al. 1996 JGR)

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GW feedbacks to a radiative perturbation
Rayleigh drag (violates momentum constraint)
Physically consistent
From Shepherd Shaw (2004 JAS)
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• Difference between 80-km and 96-km lids with
  Hines GWD in Met Office UM (letting momentum flux
  at model lid escape to space)
• Influence extends to low altitudes
• From Lawrence (1997 JGR)

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• Top ratio of downwelling in 96-km model from
  below 80 km to below 96 km
• Bottom ratio of downwelling in 80-km model to
  that in 96-km model
• From Lawrence (1997 JGR)

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• The effect of a background jet on an
  anti-symmetric source spectrum is to create a
  dipole of negative drag above positive drag,
  hence polar downwelling

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• Imposing a polar cooling shifts each part of the
  drag dipole, so the difference drag is composed
  of two dipoles, driving two circulation cells
  (left)

Rayleigh drag gives a single-signed response
(unphysical)
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Circulation response to polar cooling at 15 km

• Enforcing momentum conservation can improve the
  robustness of GWD feedback to polar cooling

Non-MC AD99
AD99
MC AD99 RD
MC AD99 Low lid
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Vertical profile of downwelling in response to
polar cooling around 15 km, with AD99 GWD scheme

• Dashed is 80N, solid is 85N
• acontrol
• cf. Garcia Boville (JAS, 1994)
• cMC AD99
• Physical response is significant
• enon-MC AD99 (50 km lid)
• Spurious response is also significant

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• Sensitivity of AD99 and H97 induced downwelling
  to model lid height

Tropospheric circulation
Downwelling at 25 km, 85N
MC
Actual (solid)
Inferred from downward control (dashed)
non-MC (solid)
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Sensitivity to the source spectrum
Anti-symmetric
Asymmetric
Resting state
With polar cooling
Difference
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• Conclusion GW induced warming above an imposed
  polar cooling is robust to
• Model lid height
• Source spectrum
• Breaking criterion
• Background flow
• if any only if momentum is conserved

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Zonal mean wind at SH midlatitudes in CMAM and in
observations

• GW drag doesnt just slow the mesospheric jet,
  it reverses it above about 90 km altitude (so
  isnt really a drag)
• Requires non-zero GW phase speeds

From Beagley et al. (2000 GRL)
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• Doubled CO2 simulations with the CMAM (note no
  heterogeneous chemistry in these runs)
• We separate the effect of doubled CO2 from that
  of the associated change in SSTs (taking SSTs
  from CCCma coupled atmosphere-ocean run)
• The combined response is surprisingly linear
• Figure shows temperature change in January
• (blue is 99 significant, purple 90)

Total response From 2xCO2 From ?SSTs
Fomichev et al. (JC, 2006)
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• There is a robust dynamical temperature response
  at the summer mesopause
• Tropospherically induced dynamical changes negate
  the CO2-induced cooling
• From gravity-wave drag
• Consistent with the lack of a cooling trend in
  observations
• Fomichev et al. (JC, 2006)

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Summary

• There are some robust aspects to the GW response
  to climate change (assuming fixed source spectra)
• Based on filtering of GW fluxes
• Robustness depends on enforcing momentum
  conservation
• Without momentum conservation, model
  intercomparisons will be ill-posed

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• However there is a robust response in the lower
  tropical stratosphere
• Tropospherically induced changes now augment the
  CO2-induced cooling
• Increased upwelling from stratospheric wave drag
  (in both NH and SH)
• Fomichev et al. (JC, in revision)

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• The annual cycle of tropical and extratropical 50
  hPa temperature (global mean is subtracted)
  points to a strengthened diabatic circulation

2xCO2SST
Extratropics
Control
Tropics
2xCO2SST
Fomichev et al. (JC, in revision)
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• Changes to tropical upwelling at 70 hPa
• Black from resolved EPFD, gray total
• Half these models use Rayleigh drag

From Butchart et al. (CD, in press)