FDEPS 2012, Research Seminar

Strengthening of the BrewerDobson circulation under climatechange
Ted Shepherd
Department of Meteorology
University of Reading• The Brewer-Dobson circulation (BDC) controls thedistribution of chemical species within the stratosphere– The circulation is mechanically driven (a refrigerator)
by torques exerted by waves propagating up from thetroposphere (so-called “wave drag”)

March2004
ozone
fromOSIRIS

Shaw & Shepherd (2008 Nature Geosci.)• The Rossby wave dragcomes from breaking ofupward propagatingwaves, seen dramaticallyin the stratospheric “surfzone” (top panel, PV on
850 K isentropic surface,approx. 30 km altitude)• This behaviour was argued
to be that of a nonlinear
critical layer (bottom panel)
Critical layers occur wherethe wave phase speed
equals the flow speed
McIntyre & Palmer(1983 Nature)• The BDC raises and cools the tropical tropopause,and lowers and warms the extratropical tropopause

Positive forcing(anti-Rossbywaves)
Control case
Negative forcing(Rossby waves)

GCM calculations by Thuburn & Craig (2000 JAS)• The seasonal variation in the BDC leads to aseasonal variation in lower stratospheric temperature• Tropical temperatures are lowest in boreal winter,when the tropical upwelling is the strongest
MSU Channel 4temperature(approx 100hPa, ~ 17 km)
Compensation
between tropics
and extratropics
suggests theinfluence of theBDC
Yulaeva, Holton & Wallace (1994 JAS)• This causes a seasonal cycle in dehydration which isimprinted on the water vapour entering the stratosphere:
the “tropical tape recorder” (Mote et al. 1996 JGR)
• Tropical tropopause temperature also controls
stratospheric water vapour on interannual timescalesInterannual
anomalies in the“tropical taperecorder” as seen
in HALOE
measurementsfrom the UARS
satellite
Updated fromRandel et al. (2004JAS)

• Climate models consistently predict a strengthened
Brewer-Dobson circulation in response to climate change

Red is obs

Green is Cly

Wouldincreaseozone inNH midlat’s

There is acompensating
decreasein thetropics
CMAM simulations from Shepherd (2008 Atmos-Ocean)• The extent towhich the future
evolution ofozone will follow
the decline inESC varies
considerably
between regions• Shaded areas
show multi-modelensemble,including naturalvariability, basedon SPARC
CCMVal-2
simulations

Fahey and Hegglin (2011)

WMO 20 Q&A(2011)• This has implications for the flux of stratospheric ozone
into the troposphere, and clear-sky UV radiation– Over the extratropical NH the decrease in clear-sky
UV radiation would have adverse health implications
STE ozone flux

Change in clear-sky UV index
between 1960s and 2090s

CMAM results from Hegglin & Shepherd (2009 Nature Geosci.)• The strengthened tropical upwelling does not necessarily
imply strengthened polar downwelling– Here the focus is on the net mass overturning at 70
hPa, which is dominated by the extrapolar circulation

McLandress & Shepherd (2009 J. Clim.)• In principle, there are three ways to change the BDC:
– Change the amount of wave forcing generated
within the troposphere• No GCM study has identified such an effect,and several have explicitly found no detectablechange– Change the penetration of wave propagation into
the stratosphere• First suggested by Rind et al. (1990 JAS), andconfirmed by many subsequent GCM studies– Change the latitudinal distribution of wave dragwithin the stratosphere (Shepherd & Shaw 2004JAS)• Not really investigated• Several modelling studies have proposed increased
upwelling driven by drag from convectively forced quasistationary equatorial waves (in a warming climate) as theorigin of the strengthened BDC, but this upwelling isnecessarily confined within the tropics– Also it peaks in northern summer, not winterModelled change inTEM
streamfunction for
July-August
Deckert & Dameris(2008 GRL)• Using a highly idealized model, Eichelberger & Hartmann(2005) had found strengthened synoptic-scale wave
forcing from climate change due to increased baroclinicity– There is no evidence for this in CMAM

Shepherd &McLandress(2011 JAS)• Since the first BDC study (Rind et al. 1990 JAS),
attention has focused on the strengthened subtropicaljets that are a robust outcome of tropospheric warming– Several studies have shown that BDC changes result
from tropospheric warming, not stratospheric cooling• Some authors argue for effect of SST changes, but
this is the same as tropospheric warming!

Shepherd &McLandress(2011 JAS)

Changes in transient EP flux,
EPFD, and zonal wind• The strengthened resolved wave drag results fromincreased penetration of Rossby waves into thesubtropical lower stratosphere (the critical region for
driving of the BDC) — as pointed out by many authors– But what is the mechanism?

Shepherd & McLandress (2011 JAS)• The key region for driving of the BDC is the subtropics• In CMAM the strengthened BDC results from a roughly
equal combination of transient planetary, transientsynoptic, and parameterized orographic gravity waves– The key region for driving of the BDC is the subtropics

Shepherd & McLandress (2011 JAS)• Transient waves are not often discussed in the context ofthe BDC, but their drag is strategically located close tothe turnaround latitudes• In CMAM, the stationary wave changes are largely
confined to the tropics and higher latitudes
Changes in vertical EP flux at 100 hPa

McLandress & Shepherd (2009 J. Clim.)• The importance of synoptic-scale Rossby waves to theBDC may seem surprising, but the drag from synopticscale waves extends continuously into the subtropicallower stratosphere
Annual meanobserved EPflux and flux
divergencefrom ERA-40
Tropopauseshown withthick line
Randel, Garcia & Wu(2008 JAS)

LC1

LC2

The subtropicalsynoptic-scale wave
drag is understood
to arise from thenonlinear breakingof baroclinic waves
Simmons & Hoskins(1978 JAS)
Edmon, Hoskins &McIntyre (1980 JAS)
Thorncroft, Hoskins& McIntyre (1993QJRMS)• The breaking of synoptic-scale Rossby waves occurs innonlinear critical layers– The subtropical critical layer in the upper troposphere,
and the midlatitude critical layer in the middletroposphere– Here for northern winter; northern summer is similar

Randel & Held (1991 JAS)• Rossby-wave drag responds to the strengthened
subtropical jet from climate change via the mechanismof critical-layer control– Plots show EP flux divergence and zonal wind

Shepherd & McLandress (2011 JAS)• The synoptic-scale wave drag shifts upward into thelower stratosphere, following the upward shift in thecritical layers– No apparent change in the amount of wave dragShepherd & McLandress (2011 JAS)• Critical-layer control is similarly seen to explain thechanges in planetary-scale wave drag in the NH
subtropical lower stratosphere– Critical layer control holds in the SH in DJF, and in
the NH in JJA for synoptic and planetary waves, butthe wave drag changes occur at higher latitudes

Shepherd & McLandress (2011 JAS)• For planetary-scale wave drag, the changes are notjust an upward shift, suggesting either meridional
redistribution or an increased forcing of planetary
waves from the troposphere (but still with criticallayer control) — needs further investigation

Shepherd & McLandress (2011 JAS)• The contribution of parameterized (mainly orographic)
gravity-wave drag (GWD) to the strengthened tropical
upwelling can be substantial, albeit with large
differences between models– These CCMVal-1 results are corroborated by themore recent CCMVal-2 comparison (Chapter 4 of
SPARC CCMVal Report, 2010)Black – total
Dark gray – resolvedLight gray – GWD

Butchart et al. (2010 J. Clim.)• Reanalyses corroborate the notion that something like
one-third of the climatological BDC upwelling may
come from unresolved (presumably gravity wave) dragEstimates of
total upwelling
Estimates ofupwelling induced
by resolved wave
dragRandel, Garcia &Wu (2008 JAS)• The orographic GWD contribution to the strengthened
BDC arises from an upward shift in gravity-wave
breaking levels induced by the strengthened lower
stratospheric westerlies (Li et al. 2008 J. Clim.)

McLandress & Shepherd (2009 J. Clim.)• Radiosonde observations show a lower stratosphericcooling above the tropical warming from El Niño, alongwith midlatitude lower stratospheric warming — the latter
mainly in the SH
Regression of DJF
temperature onto
Nino 3.4 index
These stratosphericfeatures must be
dynamically driven
Free & Seidel (2009JGR)• The CMAM shows atemperatureresponse (top)consistent withobservations– DJF is shown• Symmetric between
El Niño and La Niña
• The stratosphericanomalies areassociated withchanges in
upwelling/downwelling (bottom)– Largest
response
Simpson,Shepherd&is in(JASthe SH
Sigmond2011)• The SH response isdriven mainly byanomalous drag fromresolved transient (i.e.
Rossby) waves,mainly of synoptic
scale (top)• The NH response isdriven mainly byanomalousparameterized
orographic gravity
wave drag (bottom)
Simpson, Shepherd &Sigmond (JAS 2011)

Summary• The strengthened BDC from climate change consistentlypredicted by models must be a result of increased wave
drag in the subtropical lower stratosphere– Increased wave drag within the tropics cannot increase
the mass flux out of the tropics• Many studies have appealed to “improved propagationconditions” from strengthened subtropical winds, but noclear mechanism has been previously identified– Role of stationary waves appears to be small• Critical-layer control of Rossby-wave breaking provides arobust mechanism for a strengthened BDC in CMAM
– Planetary and synoptic-scale waves both contribute– Same mechanism applies to orographic GWD– Explains why the effect is so robust among models

• The high-latitude stratospheric circulation response toclimate change is a completely separate issue– Driven by stationary planetary waves; not understood• Stratospheric response to ENSO may differ in significant
ways from response to GHG-induced troposphericwarming– While response also maximizes in DJF, the low-latitude
response is strongest in the SH rather than in the NH– OGWD response in NH subtropics appears to be drivenby the same dynamics as the OGWD response toclimate change (cf. Calvo et al. 2010 JAS)
– Synoptic Rossby-wave response in SH subtropics does
not seem explainable from critical-layer control• Rather it appears (in CMAM) to result from increased
wave forcing from the troposphere, with distinct
responses in the Pacific and elsewhereReference 1





















Butchart, N., Cionni, I., Eyring, V., Shepherd, T. G., Waugh, D. W., Akiyoshi, H., Austin, J., Bruhl,C.,Chipperfield, M. P., Cordero, E., Dameris, M., Deckert R., Dhomse, S., Frith, S. M., Garcia, R. R.,
Gettelman, A., Giorgetta, M. A., Kinnison, D. E., Li, F., Mancini, E., McLandress, C., Pawson, S., Pitari, G.,Plummer, D. A., Rozanov, E., Sassi, F., Scinocca, J. F., Shibata, K., Steil, B., Tian, W., 2010: Chemistry-Climate Model Simulations of Twenty-First Century Stratospheric Climate and Circulation Changes, J. Clim.,
23, 5349-5374
Calvo, N., Garcia, R. R., Randel, W. J. R., Marsh, D. R.,2010: Dynamical Mechanism for the Increase inTropical Upwelling in the Lowermost Tropical Stratosphere during Warm ENSO Events, J. Atmos. Sci., 67,2331-2340
Deckert, R., Dameris, M., 2008: Higher tropical SSTs strengthen the tropical upwelling via deep convection,Geophys. Res. Lett., 35, L10813, 5pp
Edmon, H. J., Hoskins, B. J., McIntyre, M. E., 1980: Eliassen-Palm cross sections for the troposphere, J.
Atmos. Sci., 37, 2600-2616
Eichelberger, S. J., Hartmann, D. L., 2005: Changes in the strength of the Brewer-Dobson circulation in asimple AGCM, Geophys. Res. Lett., 32, L15807, 5pp
Fahey, D.W., Hegglin, M.I., 2011: Twenty Questions and Answers About the Ozone Layer: 2010 Update,
World Meteorological Organization, 72pp
Free, M., Seidel, D. J., 2009: Observed El Nino Southern Oscillation temperature signal in the stratosphere,
J. Geophys. Res., 114, D23108
Hegglin, M. I., Shepherd, T. G., 2009: Large climate-induced changes in ultraviolet index and stratosphereto-troposphere ozone flux, Nature Geoscience, 2, 687-691
Li, F., Austin, J., Wilson, J., 2008: The Strength of the Brewer–Dobson Circulation in a Changing Climate:
Coupled Chemistry–Climate Model Simulations, J. Climate, 21, 40-57
McIntyre, M. E., Palmer, T. N., 1983: Breaking planetary waves in the stratosphere, Nature, 305, 593600
McLandress, C., Shepherd, T. G., 2009: Simulated anthropogenic changes in the Brewer–Dobson ccirculation, including its extension to high latitudes, J. Climate, 22, 1516–1540

Reference 2

























Mote, P. W., Rosenlof, K. H., McIntyre, M. E., Carr, E. S., Gille, J. C., Holton, J. R., Kinnersley, J. S.,Pumphrey, H. C., Russell, J. M. III., Waters, J. W. 1996: An atmospheric tape recorder: The imprint of tropical
tropopause temperatures on stratospheric water vapor, J. Geophys. Res., 101, D2, 3989-4006
Randel, W. J., Garcia, R., Wu, F., 2008: Dynamical Balances and Tropical Stratospheric Upwelling, J. Atmos.
Sci., 65, 3584–3595
Randel, W. J., Held, I. M., 1991: Phase Speed Spectra of Transient Eddy Fluxes and Critical Layer
Absorption, J. Atmos. Sci., 48, 688–697
Randel, W. J., Wu, F., Oltmans, S. J., Rosenlof, K., Nedoluha G. E., 2004: Interannual Changes ofStratospheric Water Vapor and Correlations with Tropical Tropopause Temperatures, J. Atmos. Sci., 61,2133–2148
Rind, D., Suozzo, R., Balachandran, N. K., Prather, M. J., 1990: Climate Change and the Middle Atmosphere.
Part I: The Doubled CO2 Climate, J. Atmos. Sci., 47, 475–494
Shaw, T. A., Shepherd, T. G., 2008: Atmospheric science: Raising the roof, Nature Geoscience, 1, 12—13
Shepherd, T. G., 2008: Dynamics, stratospheric ozone, and climate change, Atmosphere-OceanShepherd, T. G., McLandress, C., 2011: A Robust Mechanism for Strengthening of the Brewer–Dobson
Circulation in Response to Climate Change: Critical-Layer Control of Subtropical Wave Breaking., J. Atmos.
Sci., 68, 784–797
Shepherd, T. G., Shaw, T. A., 2004: The Angular Momentum Constraint on Climate Sensitivity and Downward
Influence in the Middle Atmosphere, J. Atmos. Sci., 61, 2899–2908
Simmons, A., Hoskins, B., 1978: The life-cycles of some nonlinear baroclinic waves. J. Atmos. Sci., 35, 414432
Simpson, I. R., Shepherd, T. G., Sigmond, M., 2011: Dynamics of the Lower Stratospheric Circulation
Response to ENSO., J. Atmos. Sci., 68, 2537–2556
Thorncroft, C. D., Hoskins, B. J., McIntyre, M. E., 1993: Two paradigms of baroclinic-wave life-cycle behaviour,
Q. J. R. Meteorol. Soc., 119, 17–55
Thuburn, J., Craig, G. C., 2000: Stratospheric Influence on Tropopause Height: The Radiative Constraint, J.
Atmos. Sci., 57, 17–28
Yulaeva, E., Holton, J. R., 1994: On the cause of the annual cycle in tropical lower-stratospheric temperatures,J. Atmos. Sci., 51, 169-174