tutorial: stratospheric dynamics & waves

Tutorial:Tutorial:

Stratospheric Dynamics & WavesStratospheric Dynamics & Waves

M. Joan AlexanderNWRA-CoRA Office, Boulder, CO

OutlineOutline

1. Diagnosing vertical motion1. Diagnosing vertical motion2. Brewer-Dobson Circulation2. Brewer-Dobson Circulation3. Wave driving of the zonal-mean circulation3. Wave driving of the zonal-mean circulation

Transformed Eulerian Mean (TEM) equationsTransformed Eulerian Mean (TEM) equationsEP flux and divergenceEP flux and divergence

4. Quasibiennial oscillation (QBO)4. Quasibiennial oscillation (QBO)5. Key current science questions5. Key current science questions

Vertical Motion: for slow timescale motions (synoptic and longer) vertical velocity is too small to measure directly. Instead, we infer it:

>

D/Dt = change“following the motion”

Slow Ascent in the TTL: Balance of Forces

● Wave-driven Ascent/Descent

● Radiative Heating and Cooling-Shortwave/Longwave-Clear/Cloudy Sky

● Latent Heating and Cooling

On slow time-scales,feedbacks among allare important.

Fueglistaler et al. 2009

Temperatureanomalies fromthe annual mean10N-10S

Big annual cyclein the lower stratosphere

Annual Cycle

The annualcycle has longbeen associatedwith the moreactive Rossbywave activity inthe NH winterdriving a stronger upwelling [e.g.Holton et al 1995].

Water vapor anomalies from the annual mean(in ppmv from

HALOE)

Zonal-mean verticalmotion can be inferred from the slopes of thetropical tape recorder signals.[Mote et al., 1996;Schoeberl et al. 2008]

The Brewer-Dobson Circulation

Describes the equatorto pole transport oftrace gases in thestratosphere.

● Upwelling in the tropical stratosphere.

● Downwelling in theextratropics.

A wave-driven circulation.

We use the Transformed Eulerian Mean (TEM) residual circulation [Andrews et al., 1987] to approximate the Brewer-Dobson circulation in dynamical models.


Describes the equatorto pole transport oftrace gases in thestratosphere.

Upwelling in the tropicalstratosphere (includingthe TTL).

Downwelling in theextratropics.

A wave-driven circulation.

We use the Transformed Eulerian Mean (TEM) residual circulation [Andrews et al., 1987] to approximate the Brewer-Dobson circulation in dynamical models.

Eulerian zonal means Wave terms(Stokes drift)

Plumb [2002]

Trace gases withsufficiently weak sources and sinkshave similar globaldistributions...

...these characteristicsare determined by transport, and thus by stratopsheric (and mesospheric) dynamics.

HFStratospheric sourceand tropospheric sink

CH4Tropospheric sourceand stratospheric sink


Plumb [2002]


Various types of waves drive this circulation:G: gravity wavesP: planetary wavesS: synoptic waves(Additional roles forinertia-gravity wavesin the stratosphereand equatorial wavesin the TTL.)

Without wave driving,the middle atmospherewould be in radiativeequilibrium with muchstronger zonal windsthan observed, and no residual circulation.

TURNAROUND LATITUDES

Quantifying the Brewer-Dobson Circulation in Climate Models(from a talk given by Charles McLandress at the BDC Workshop 2012)

● Use w* just above the tropical tropopause (i.e. 70 hPa) to deduce the net mass flux entering the stratosphere – a measure of the BDC strength.

● To quantify the overall strength of the BDC, it is important to consider the entire latitudinal region of upwelling, the net upwelling between the“turnaround latitudes”, where upwelling changes to downwelling.

Butchart et al. [2006]:

ERA-Interim upwelling too fast by ~40-50% [Dee et al. 2011; Ploeger et al. 2012-JGR]

Multi-modelmean

ERA-interim


● What waves drive the BDC upwelling at 70hPa?

● Use “downward control” to estimate contributions from different types ofwave drag, including both resolved and parameterized drag.


The TEM Momentum Equation:

F = Eliassen-Palm (EP) Flux [e.g. Andrews et al., 1987] Divergence of EP-Flux describes the resolved wave force on the circulation.

Xp = Parameterized Gravity Wave Drag [e.g. McLandress, 1998].

Describes the force due to unresolved gravity waves.






The TEM Momentum Equation:

EP-Flux vector:Calculation requires spectral (x,t)->(k,f)decomposition of 3D, time-dependentwinds and geopotential.

Compute covariances in spectral space, then can integrate over the spectrum.

Downward Control – Haynes et al. [1991]

(again from the McLandress talk at the BDC Workshop)

Can compute thecomponent of theforce F due to waves of different types, then estimate theircontributions todriving the BDC.

Watanabe et al. 2008b

EP-Flux in a “Gravity Wave-Resolving” Climate Model

● Special high-resolution climatesimulation: - T213 horizontal resolution, - 300m vertical resolution, - Model top at the mesopause, –> Realistic middle atmospherecirculation with no parameterizedgravity wave drag!

● Diagnosis of EP-fluxes and zonal forcing of the circulation.

EP-flux vectors Color = Zonal forcing


Zonal Wn = 1-3 only


Horiz. Wavelengths < 1000km

Butchart et al. 2010

● Mean upwellingtropical mass fluxat 70 hPa showspositive trends inmodels.

annual (black)DJF (dark gray)JJA (light gray)

● Trends appear inboth past and future.

Note: No trenddiscernable yet in observations.

Predicted Trends in the Brewer-Dobson Circulation


Predicted Trends in the Brewer-Dobson Circulation

The size of the trendvaries among differentmodels, but none predict a slowing trend.

The cause is wave driving, but the types of waves responsible for the trendvaries from model to model.


Total (black)EP flux div (dark gray)Orographic GWD (med gray)Nonorographic GWD (light gray)

Diagnosis of Brewer-Dobson Circulation Forcing

● Many of the models showan important role for smallscale gravity waves.

Mechanism for Brewer-Dobson Circulation Trend

Shepherd and McLandress [2011]

Tropospheric warming andstratospheric cooling lead tostronger subtropical jets inthe lower stratosphere.

Rossby wave and orographic gravitywave dissipation occurs at higher altitude, giving a stronger, deeper Brewer-Dobson circulation.

Past windFuture wind

at 30N

Figure showssynoptic waveEP-flux div.moves upwith thecritical line.

The Quasibiennial Oscillation (QBO)Baldwin et al. [2001]

● THE dominant cycle in tropical zonal wind inthe stratosphere.

● Driven by abroad spectrumof equatorial waves andgravity waves.

Gravity Wave Driving of the QBO Lindzen and Holton [1968]

Their modelassumedcritical levelinteractionsbetween the mean flowand aspectrum ofwaves.

Waves wouldactually breakbelow critical levels butthis is onlya smallcorrectionto the basicresult.

Equatorial Wave Driving of the QBO

Holton and Lindzen [1972]:

Plumb's [1984] 1-D model schematic● Updated model forces QBOwith radiative damping ofequatorial waves: Kelvin andmixed Rossby-Gravity waves.● Doesn't require critical levelsand explains why equatorial.

Haynes [1998]● Confinement of the QBO tothe tropics is a consequenceof latitudinal variation in the Coriolis force, and the type ofresponse at low frequenciesto a zonal force at different latitudes: Equator –> zonal acceleration Pole –> meridional motion due to Coriolis torque

Modern understanding is that amixture of equatorial waves andgravity waves are needed.

Plumb and Bell 1982

Temperature and circulation anomalies associated with the QBO.

● These explain the different easterly and westerly shear descent rates observed in the QBO.

● Warm phase – diabatic cooling – descent ● Cold phase – diabatic heating – ascent

● These anomalies also effect tracer distributions.

Eastward shear

Westward shear

QBO and BDC Interactions

QBO Response to Climate Change (doubled CO2)

Kawatani et al. [2012]

Model study: Finds QBO oscillation in the futureis slower by 1-3 months.

● The change is primarily associated with increasedsea surface temperatures, not cooling of thestratosphere.

● The slower oscillation is primarily caused by anincrease in tropical upwelling of 30-40%, which slows the descent of the QBO shears.

● Changes in tropical wave EP-fluxes are relativelyweak. Instead the increased upwelling is primarilyassociated with increases in subtropical EP-fluxdivergence in the stratosphere.

Key Current Science Questions

● How will the forces that give rise to TTL upwelling respond tochanging climate? (How will T and H2O respond?)

● Are current climate models correctly predicting acceleration of theBrewer-Dobson circulation in the 21st century?

(Do climate models include sufficiently realistic treatments of future wave responses to climate change, wave driving, and dynamical feedbacks?)

● What measurements are needed for improved estimates of trends inthe Brewer-Dobson circulation?

● Many climate models still have difficulty simulating the QBO.(A symptom of the poor representation of tropical wave processes?)

● How do we improve the treatments of convection, precipitation variability,and equatorial waves in global models?

References:Andrews D. G. and Holton, J. R. and Leovy, C. B., 1987: Middle Atmosphere Dynamics, Academic Press, Orlando, 489pp.

Butchart, et. al., 2006: Simulations of anthropogenic change in the strength of the Brewer-Dobson circulation, Clim. Dyn.,

27, 727-741.

Butchart et al., 2010: Chemistry-climate model simulations of 21st century stratospheric climate and circulation changes,

J. Clim., 23, 5349-5374.

Dee et al., 2011: The ERA-Interim reanalysis: configuration and performance of the data assimilation system, Q. J. Roy.

Meteorol. Soc., 137, Issue 656, pages 553–597

Haynes, P.H., C.J. Marks, M.E. McIntyre, T.G. Shepherd, and K.P. Shine, 1991: On the "downward control" of extratropical

diabatic circulations by eddy-induced mean zonal forces, J. Atmos. Sci., 48, 651-678.

Holton, J.R.,P.H. Haynes M.E. Mcintyre, A.R. Douglass, R.B. Rood, and L. Pfister, 1995: Stratosphere-Troposphere

Exchange, Rev. Geophys., 33, 403-439.

Holton, J.R. and R.S. Lindzen, 1972: An updated theory for the quasibiennial cycle of the tropical stratosphere, J. Atmos. Sci.,

29, 1076-1080.

Kawatani, Y., K. Hamilton, and A. Noda, 2012: The Effects of Changes in Sea Surface Temperature and CO2 Concentration

on the Quasi-Biennial Oscillation, J. Atmos. Sci., 69, 1734-1749.

Lindzen, R.S. and J.R. Holton, 1968: A theory of the quasibiennial oscillation, J. Atmos. Sci., 25, 1095-1107.

Mote et al., 1996: An atmospheric tape recorder: The imprint of tropical tropopause temperatures on stratospheric water

vapor, J. Geophys. Res., 101, 3989-4006.

Ploeger et al., 2012:

Plumb, R.A., 1984: The quasi-biennial oscillation, Dynamics of the Middle Atmosphere, edited by J.R. Holton and T. Matsuno,

pp. 217-251, Terra Sci., Tokyo.

Plumb, R.A., 2002: Stratospheric transport, J. Meteorol. Soc. Japan, 80, 793--809

Plumb, R.A. and R.C. Bell, 1982: A model of the quasi-biennial oscillation on an equatorial beta-plane, Q. J. R. Meteorol. Soc.,

108, 335-352.

Schoeberl, M.R., A.R. Douglass, R.S. Stolarski, S. Pawson, S.E. Strahan, and W. Read, 2008: Comparison of lower

stratospheric tropical mean vertical velocities, J. Geophys. Res., 113, D24109, doi:10.1029/2008JD010221.

Watanabe, S., Y. Kawatani, Y. Tomikawa, K. Miyazaki, M. Takahashi, and K. Sato, 2008: General aspects of a T213L256 middle atmosphere general circulation model, J. Geophys. Res., 113, D12110, doi:10.1029/2008JD010026.

tutorial: stratospheric dynamics & waves

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