Transcript Can we trust the simulated gravity

NCAR TIIMES Gravity-Wave Retreat, 2006
Can we trust the simulated
gravity-wave response to
climate change?
Ted Shepherd
Department of Physics
University of Toronto
• 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
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)
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)
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)
• 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)
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)
• 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]
1

w 
a  cos 
*


z
F cos
dz
2sin 
• For GWs, this simplifies to

1  cos u' w' 
w 


acos   2sin  
*
(assuming no flux of momentum to space)
• 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

*
1   cos u' w'  cos u' w' 
w 

 

acos   2sin  z  2sin  z
top





which now depends on model lid height
• 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)
GW feedbacks to a radiative perturbation
Rayleigh drag (violates
Physically
momentum constraint)
consistent
From Shepherd & Shaw (2004 JAS)
• 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)
• Top: ratio of
downwelling in 96km model from
below 80 km to
below 96 km
• Bottom: ratio of
downwelling in 80km model to that in
96-km model
From Lawrence
(1997 JGR)
• 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
• 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)
Circulation response to polar cooling at ~15 km
AD99
MC AD99
Low lid
Non-MC
AD99
MC AD99
+ RD
• Enforcing
momentum
conservation
can improve the
robustness of
GWD feedback
to polar cooling
Vertical profile of downwelling in • Dashed is 80°N,
solid is 85°N
response to polar cooling around
15 km, with AD99 GWD scheme • a=control
– cf. Garcia & Boville
(JAS, 1994)
• c=MC AD99
– Physical response is
significant
• e=non-MC AD99 (50
km lid)
– Spurious response is
also significant
• Sensitivity of AD99 and H97 induced
downwelling to model lid height
MC
Downwelling
at 25 km,
85°N
Tropospheric
circulation
Actual
(solid)
non-MC
(solid)
Inferred from
downward
control
(dashed)
Anti-symmetric
Asymmetric
Sensitivity to the
source spectrum
Resting state
With polar cooling
Difference
• 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
Zonal mean wind at SH midlatitudes in CMAM and in observations
• GW drag doesn’t just slow the mesospheric jet, it reverses
it above about 90 km altitude (so isn’t really a “drag”)
• Requires non-zero GW phase speeds
From Beagley et al. (2000 GRL)
• 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)
• 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)
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
• 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)
• The annual cycle of tropical and
extratropical 50 hPa temperature (global
mean is subtracted) points to a
strengthened diabatic circulation
2xCO2+SST
Control
Extratropics
Tropics
2xCO2+SST
Fomichev et al. (JC, in revision)
• 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)