Overview of this week’s lectures:• Global circulation and transport• Satellite observations of stratospheric temperature and water vapor• Global upper troposphere - lower stratosphere (UTLS)• Circulation and transport near the tropical tropopause layer (TTL)• UTLS monsoon circulations and water vapor isotopes• Research seminar: tropical dynamics with GPS radio occultation dataLecture 1: Global atmospheric circulation and satellite observations• large-scale circulation of the troposphere and stratosphere

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Climatology and variability of the stratospherewave forcing of the zonal mean flow
Rossby waves: propagation and dissipation (critical layers)
Tropospheric baroclinic wave life cycles
large-scale tropical circulations
QBO and ENSO

• Large-scale transporttemperature
increase due
to ozone

Climatological temperatures and zonal winds in January

stratopause

warm

cold

tropopause

thermal windbalanceSeasonal cycle of zonal mean zonal winds

polar night jetstronger in SH
than in NH

climatological polar temperatureclimatological polar temperature
warmer andmuch larger
variability in NH

cold andquiet

stratosphericsudden warming•

Variability in NH winter stratospheretied to large-scale forcing from troposphere.

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Episodic forcing produces ‘stratospheric sudden
warming’ events.

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Largest observed stratosphere sudden
warming in January 2009strongwave forcing
from tropospherewinds reverse
at 10 hPaEP flux
JAS 2010

wave forcing
from tropospherepolar vortex
near 30 km

potential vorticity

split of polar vortex

solution to puzzle of stratospheric warmings

EP fluxes

overturning circulation

warmingcoolingGoverning equations for the zonal mean flow (Transformed Eulerian mean)

EP flux divergence (wave forcing)zonal momentum balancediabatic forcing

thermodynamic balance

continuity equation
geostrophic thermal wind

Andrews et al, 1987

EP flux divergence (wave forcing)

Eliassen-Palm fluxes:
momentum flux

climatology

components:
latitudinal
flux

vertical
flux

heat
flux

Important points:• DF quantifies zonal momentum forcing• F proportional to ‘wave activity’ flux
(DF shows sources and sinks of waves)• Ff and Fz indicate direction of wave propagation

height

Response of a balanced vortex to localized EP flux forcing (DF)latitudemomentum
forcing (DF)Circulation response depends on frequency of forcing:
fast
frequency(adiabatic)

Combine equations:time dependence

diabatic
heating
momentum
forcing (DF)In general both Q and DF drive the mean circulation.
These plots show the response to isolated forcing
from Rossby wave EP flux divergence. The lower cellbecomes more important for slower forcing.
Haynes et al 1991
Holton et al 1995

mediumfrequency~ annual cycle

zerofrequency‘downward
control’Wave – mean flow patterns from observations (reanalysis data)
global-scale overturning
circulation
regression onto
DF herewaves propagating
vertically fromtroposphere

Abalos et al, JAS 2014

Climatology of stratospheric
overturning circulation
gravitywaves

synoptic
waves

winter planetary waves
help drive the overturning
circulation

Plumb 2002

upper and lower branchesof the circulation
driven by Planetary
and Synoptic waves

The overturning circulation reverses between solstice seasonscirculation is stronger during NH winter, related tostronger wave forcing from troposphere

The stratospheric overturning circulation is often termed the Brewer-Dobson circulation(closely related to the Lagrangian or transport circulation)
deduced by Brewer (1949) studying stratospheric water vaporand Dobson (1956) studying stratospheric ozone

see recent review by Butchart 2014

ACP 2011

renewed appreciation that there areupper and lower branches of the BDC
Red: deep branch (slow)
Blue: shallow branch (fast)streamfunction

transit timealong constantstreamlines

Transit time is closely related to ‘mean age’ (time since air entered stratosphere)transit time

old

young

Air at any particular location is characterized by a distribution of transit times and ages
(so-called age spectrum)

Birner and Bönisch, 2011, Atmos. Chem. Phys.

Key points:• Asymmetry in winter stratosphere circulations: more disturbed in the NH,
cold and quiet in the SH• The stratosphere is forced by waves from the tropopshere (stronger forcing in NH;
episodic stratospheric sudden warmings)• Dynamical response of balanced vortex to wave forcing (non-local temperature
and wind changes)• Eliassen-Palm (EP) fluxes quantify wave forcing• Brewer-Dobson transport circulation (deep and shallow branches)

Rossby waves

Rossby wave (potential vorticity wave)(+) positive PV anomaly=> cyclonic circ.

(-) negative PV anomaly=> anticyclonic circ.

High PV

t=0

Low PVPV
gradient

t=1
Newly displaced parcels
by induced circulation
Wave propagation
Vallis 2006 (Atmospheric and Oceanic Fluid Dynamics, 2nd Ed.)

Rossby wave propagation: quasi-geostrophic linearized PV equation

eddy PVbackground PV gradientwave solution:

wave equation:
propagation for
ns2 > 0refractive index

JGR 1961~ cos (lat) + U terms

2 key points:• ns2 proportional to ~cos (lat) (Rossby wave refraction towards low latitudes)• vertical propagation for U > 0 and small zonal wavenumbers(planetary waves propagate to stratosphere only during winter)no propagation insummer easterliesRefractive index squared (Matsuno 1970)

zonal wave 1 and 2 can propagate
vertically in climatological basic statecalculated ns2 andobserved EP fluxes

qj
f2
s2
n =
- 2
2 2
au 4H Na cos 2 j2ssmaller scales
are trapped

thanks to Joowan Kim

Rossby wave dispersion on the sphere: response to isolated topography

X

Rossby waves cannot propagate into regions where U < 0 (i.e. across equator)

Breaking Rossby waves: overturning of PV contours

Postel and Hitchman 1999
Homeyer et al 2013

Two types of wave breaking, depending on shear of background winds

poleward breaking (cyclonic)

equatorward breaking (anticyclonic)
Gabriel and Peters 2008

Rossby wave critical layer interactions (critical layer: U = c)nonlinear overturning at critical layer(irreversible transport and mixing)

Warn and Warn 1978

Example of a large-scale breaking Rossby wave

overturning
PV contours

fast, synoptic flow mainly along isentropes:

tropopause

340 K isentropespans tropical troposphere
to extratropical lower
stratosphereRossby waves during January-May at 340 K

extra-tropicallower stratospheretropicaltroposphereKey points:• Rossby waves: conserve PV• General refraction of Rossby waves towards low latitudes• Latitudinal or vertical propagation for U > 0 (more generally U > c)
• Rossby wave breaking near critical lines (U = c)
• Poleward or equatorward breaking depending on background U shear• Key mechanism for dissipation, transporting PV or trace species

Climatological EP fluxes in the tropospheremain regions ofwave dissipation

Garcia and Randel, 2008

Extratropical EP flux patterns are related to baroclinic wave life cycles
initialday 5idealized
zonal wave 6
baroclinic eddy
life cycle

energybaroclinic
growthday 8life cycle averagetime (days)

barotropic
decay

Simmons and Hoskins 1980
Edmon et al 1980

synoptic viewsof developing
baroclinic
wave

500 hPasurfacepoleward
heat flux

phase tilt:
equatorward
momentum flux

warm
‘conveyor belt’LC2 poleward breaking

LC1 equatorward breaking

Idealized baroclinic wave life cycles
equatorward propagation (LC1)

Thorncroft et al., 1993, Q.J.R. Meteorol. Soc.

poleward propagation (LC2)LC1

Climatology

LC2

Garcia and Randel, 2008, J. Atmos. Sci.

Thorncroft et al., 1993, Q.J.R. Meteorol. Soc.

Using phase speed spectra to diagnose critical layer interactions
wave flux
co-spectra as a functionof zonal wavenumber
and phase speed

traditionalwavenumber vs.
frequency

Randel and Held 1991

wavenumber vs. frequency

wavenumber vs. phase speed

Rossby waves move eastward
at ~5-15 m/s

Integrate over wavenumber to derive eddy flux phase speed spectra
eddy momentum flux u’v’ 300 hPa

red arrows:
EP fluxes

zonal
wind

Randel and Held 1991•

EP fluxes: propagation to near
critical lines (c = U)•evidence for critical layer behavior

EP flux divergence phase speed spectra

EP flux divergencepoleward of critical lines

zonal
wind

Randel and Held 1991

Subtropical critical layers for Rossby waves with phase speeds ~ 5-15 m/sforced mean circulation
Randel and Jensen, 2013, Nat. Geosci.

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criticallayers15
5Key points:• Baroclinic wave life cycles: baroclinic growth and barotropic decay• Two idealized types of life cycles: equatorward and poleward wave breaking(LC1 and LC2)
• Consistent with tropospheric EP flux circulation statistics• Phase speed spectra: clear evidence for critical layers in subtropics(important influence of extratropical waves on tropical circulations)Large-scale tropical circulations are forced by latent heating from deep convection

OLR climatology January

Outgoing Longwave Radiation (OLR)
is a useful proxy for deep convection

high clouds ~deep convection

OLR climatology July

Seasonal variations in tropical overturning circulation (Hadley cell)
NH winterNH summer

Dima etal 2005

zonal winds

streamfunction

deep
convection

deep
convection

Dynamical response to low frequency convective forcing

low level
convergence

Rossbygyres

Kelvin wave
heating

low level
circulation

vertical structure

Gill, 1980

Dynamical response to heating centered north of equator
heating north of equator

low level
convergencestronger circulation
close to heating

low level
circulation

Gill, 1980

vertical structure: out of phase between
lower troposphere and upper troposphere
QJRMS 1998

Walker circulation(east-west)

deep convection / heatingTropical heating produces subtropical anticyclones in the UTLSJJA geopotential height and winds 100 hPa

anticyclones

Convection (heating)Park et al 2007

heating

Matsuno-Gill Solution
53

Key points:• Organized deep convection (latent heating) drives large-scale tropical circulations• Seasonal movement between solstices (SH – NH subtropics)• Hadley and Walker overturning circulations• Matsuno-Gill dynamical response to local heating: subtropical Rossby waves
and equatorial Kelvin waves• Subtropical anticyclones in UTLS (especially Asian monsoon during NH summer)

The Quasi-Biennial Oscillation (QBO) and El Nino Southern Oscillation (ENSO)
are important modes of circulation in the stratosphere, troposphere and UTLS
• QBO: approximate 28-month oscillation in tropical stratosphere, forcedby vertically propagating waves interacting with mean flow
• ENSO: ~2-6 year time scale oscillations of convection, oceanic and atmosphericcirculations. Wave effects extend into high latitudes and into tropicallower stratosphere.
• Both QBO and ENSO influence temperature, circulation and transport in UTLS

Brief observational characteristics of QBO

easterly descent is regular;
sometimes the westerly phase ‘stalls’

Zonal winds at Singapore (1o N) - standard QBO referenceMeridional circulations and temperatures linked with the QBO

extension to
middle latitudes

equatorial temp anomalies
cold

warm

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temp anomalies at equator up to +/- 4 K
extend downward to near tropopause

Plumb and Bell, 1982b, Q.J.R.
Meteorol. Soc.

Meridional structure and global influences of the QBO
Baldwin et al 1998

influence on high latitudes
via subtropical critical lines••

waves propagate vertically until reaching critical level (U=c)
wave absorption produces zonal acceleration (towards phase speed of wave)

critical
layer

zonal
wind

eastward and westward propagating waves forced from troposphere

ENSO – large-scale shifts in tropical convection, winds, temperatures

Standard time series for ENSO

Stratospheric tracer transport: evidence from satellite observations•

N2O is a ‘tropospheric source gas’•destroyed by photolysis (radiation)
in upper stratosphere•

Source of reactive nitrogen (NO, NO2)
in upper stratosphere; importantfor stratospheric ozone•

Behavior reflects Brewer-Dobson
circulation and eddy mixing

N2O mixing ratio

Methane CH4 is another tropospheric source gas, oxidized to H2O in upper stratosphereObserved ozone and Brewer-Dobson circulation•ozone is made in the tropical stratosphere••

Short lifetime in upper stratosphereLong lifetime in lower stratosphere•transport causes high latitude maximum
during winter / springSeasonal cycle of column ozone reflects Brewer-Dobson circulation
winter-spring polar maximum

Bowman and Krueger, 1982

QJRMS, 1949
The stratosphere is
extremely dry because
air is dehydrated
passing the cold
tropical tropopause

Tropical tape recorder observed by MLS 2004-2012

cold point
tropopause

2005

2006

2007

2008

2009

2010

2011

2012

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annual cycle in tropopause temperature imparts annual cycle in H2O•

upward propagation with Brewer-Dobson circulation

tracer zonal mean transport budget
mean advection

eddy transportmodel N2O (ppbv)

these terms
generally balanceeach othermean advection

eddy transport

contours:
N2O
tendency(ppbv/100 days)

Randel et al 1994

Tracer transport equation similar to thermodynamic equation:

tracertemperatureThis is why temperature and tracers are sometimes highly correlated:for example,
T, O3 and CO
in tropicalstratosphere(Abalos et al 2012)

Examples of stratospheric wave mixing

model

N2O near 35 km from CLAES instrument on UARS

observations

Randel et al 1993

Mixing across tropopause linked to Rossby wave breaking

potential vorticity
ozone derived from AIRS

Rossby wave variability reflected in ozone near tropopause

JGR 2000

Estimates of mixing based on stretching of
PV contours in trajectory calculations

important points:• mixing on flanks of jet (near critical lines for c ~10 m/s)
• small mixing across jet core (jet cores are mixing barriers)

Key points:• Stratospheric transport: Brewer-Dobson circulation and wave mixing(clear behavior for tropospheric source gases)• Stratrospheric ozone: produced in tropical stratosphere, transported tohigh latitudes (reflects seasonal Brewer-Dobson circulation)• Stratospheric water vapor: dehydration near tropical cold point,strong seasonal cycle (‘tape recorder’)• Tracer budgets: mean advection and eddy transports (tied to Rossby waves
and critical layers)

Extra slides

Model simulation of gravity waves
forced by deep convection

Alexander and Holton, 2000

Gravity waves interacting with a critical levelGravity waves interacting with a critical level

criticallevel

Alexander and Holton, 2000, J. Atmos. Sci.

Latitudinal propagation depends
on background zonal winds:• Rossby wave propagation
through westerly wind ‘ducts’
westerly winds

climatology of Rossby wave breaking events

anticyclonic (equatorward)
breaking much more common

total

Homeyer and Bowman 2013

observations vs. modelshading: deep convection

upper troposphere
geopotential height
and winds

annual meanobservations

nonlinearshallow watermodel
imposed heating

Dima et al 2005

Seasonal variation of tropical waves

tropical zonal mean
momentum balance:

eddy momentum
fluxes balance
Hadley flow
Dima et al 2005

Hadley cell interactions with extratropical eddies: (a complicated subject)
Idealized model simulations:angular momentum contours

zonally symmetric dynamics

heating

midlatitude‘Ferrel cell’ linked
to midlatitude eddies

extension of Hadleycell to higher latitudes

include extratropical eddies

Schneider 2006

tracer zonal mean transport budget

eddy transport

tropical ozone budget
from WACCM
mean advection
eddy transportozone
tendency
in % per day

Abalos et al 2013

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