Lecture 4: circulation and transport in the TTL and tropical lower stratosphere• the large annual cycle in the TTL: temperature and ozone• observations: temperatures, circulation, trace species• thermodynamic and constituent budgets in the TTL• dynamical forcing of tropical upwelling1

Transport near the tropical tropopause layer (TTL)
TTL sets ‘boundary condition’ for global stratosphere
Region with complex balances:
two-waytransport bybaroclinic eddies

how does deep convection
impact composition?

Randel and Jensen, 2013, Nat. Geosci.

what controls tropical upwelling(temps, ozone and stratospheric H2O)?

cirrus andclimate impacts

Well-known: large annual cycle in temperature in tropical lower stratosphereJAS 1969

dynamically forced,
but exactly how?
extratropical stratosphere,
tropical waves, ??

narrow
maximum

almost zero annual cyclein tropical troposphereAmplitude of the tropical annual cycle in temperature
narrow vertical scale,peak in lower stratosphereGPSradiosondes

tropopausevery small
annual cyclein troposphere

What causes the annual cycle? Dynamically-forced upwellingsmall

Reed and Vicek, 1969, J. Atmos. Sci.

in this region, radiationacts as a damping term,
not forcing

There is also a large annual cycle in ozone above the tropical tropopause

Seasonal cycleat Nairobi

seasonal cycle at
7 SHADOZ ozonesondestations 10o N-Slarge annual cycle
at all stations, overnarrow vertical layer
17.5 km

Randel et al., 2007, J. Atmos. Sci.

Ozone annual cycle amplitude normalized by background i

HALOE satellite

narrow vertical layer~ 16-19 km

upper troposphere seasonal
cycle is different among stationsRandel et al., 2007, J. Atmos. Sci.

A1ln  A w1 A
zOzone seasonal cycle has similar vertical structure to temperatureozone

temperature

temps from SHADOZ stations
and zonal mean GPS data

Randel et al., 2007, J. Atmos. Sci.

Ozone seasonal cycle has similar vertical structure to temperatureozone

temperaturestrong backgroundvertical gradient

Climatological seasonal cycle

ozone and temp
respond toannual cyclein tropical
upwelling

long radiative
time scaleozone and temp
~ in phasevertical velocity (2 estimates)

Randel et al., 2007, J. Atmos. Sci.

Tracer transport equation similar to thermodynamic equation:

tracertemperatureidealized situation intropical lower stratospherevariations in upwelling
result in correlated
temperature and tracers* for tracers with strong
vertical gradients

There is a corresponding annual cycle in CO above the tropical tropopause(out of phase with temperature and ozone)

Climatological CO structurestrong gradientacross tropical
tropopause

Park et al., 2013, J. Geophys. Res. Atmos.

Annual cycle amplitude

Zonal mean temperature, ozone and CO averaged 18o N-S
Temps from ERAinterim, ozone and CO from MLS observations

CO inverted scale

Abalos et al., 2012, Atmos. Chem. Phys.

Complementary viewpoint: ozone annual cycle due to in-mixing
Isentropic calculations using CLaMS Lagrangian model

Konopka et al JGR 2009
Konopka et al ACP 2010
Ploeger et al JGR 2012

% tropical ozone due to in-mixing

50%

summer ozone maximum
due to Asian monsoon transportOzone annual cycle amplitude is reduced in isentropic coordinates
because of strong ozone-temperature correlation(forced by upwelling)
Konopka et al JGR 2009pressurecoordinates

isentropic
coordinates

Key points:•

Large annual cycle in temperature and ozone in tropical lower stratosphere• Also for other trace species with strong vertical gradients• Forcing by upwelling is a simple explanation• Possible importance of in-mixing linked to monsoon circulations

ACP 2012• Observational analysis of upwelling effect on tracers• MLS observations of ozone, CO 2004-2011• ERAinterim meteorology• 3 estimates of upwelling:w* (from reanalysis)w*Q (thermodynamic balance)

w*m (momentum balance)

3 estimates of tropical upwelling w* from observations:residual circulation from reanalysis w*

accurate radiative heating rate

thermodynamicbalance
wQ*

EP flux divergence

momentumbalancewm*Latitude structure of upwelling from 3 estimatesdifferent methodsof adjustment

Abalos et al., 2012, Atmos. Chem. Phys.

Daily variations in upwelling 18o N-S
Averageseasonal cycle

Large annual cycles and significant sub-seasonal variability

Abalos et al., 2012, Atmos. Chem. Phys.

Thermodynamic balance+ eddy term

radiative
heatingupwellingwQ*
annual cycle follows upwelling

Result: upwelling ~ radiative heating
Abalos et al., 2012, Atmos. Chem. Phys.

eddy transport

Ozone budget
Residual = unresolved eddy effects +imbalances from resolved terms

mean advection

chemistry

residual

P-L

upwelling

annual cycle follows upwelling

Result: upwelling ~ photochemical production+ residual (eddy effects?)

Abalos et al., 2012, Atmos. Chem. Phys.

CO budget

upwelling

Annual cycle
follows upwelling

P-L

Result: upwelling ~ photochemical loss

Abalos et al., 2012, Atmos. Chem. Phys.

Summary from budgets calculated from observations:• Upwelling is the dominant forcing for temp, ozone and CO• Relatively large residual for ozone budget; are these due
to unresolved eddy effects?

What are the detailed balances in afree-running climate model (WACCM) ?
• Archive and analyze daily output of astandard WACCM simulation
Abalos et al, 2013, ACPHow realistic is the near-tropopause structure in WACCM?
temp

tropopause is slightly
higher in WACCM

ozone

CO

Accurate simulation of CO in WACCM
Observed ACE-FTS

WACCM

Park et al, JGR 2013

Tropical seasonal variations at 68 hPa (19 km)
free running WACCM

ozone
MLS

CO

Abalos et al., 2013, Atmos. Chem. Phys.

Coherent WACCM variations of T, ozone and CO* similar to observations *CO inverted scale

Abalos et al., 2013, Atmos. Chem. Phys.

Amplitude of annual cycle
tempozone

Realistic amplitudes,
but slightly higher
altitude in WACCM

Abalos et al., 2013, Atmos. Chem. Phys.

COWACCM thermodynamic balance:
eddy fluxes are typically small

radiative
heating

eddy term w’T’

upwelling

Note very small residuals (not always easy with model results)

Abalos et al., 2013, Atmos. Chem. Phys.

WACCM ozone budget:
eddy fluxes

P-L

eddy
fluxesupwelling

Note: explicitly resolved eddy fluxes are similar to observational ‘residuals’

Abalos et al., 2013, Atmos. Chem. Phys.

Contribution of terms to forcing ozone annual cycle

upwelling

tropopause
in-mixing

Abalos et al., 2013, Atmos. Chem. Phys.

•upwelling is dominant
in lower stratosphere•in-mixing is relatively
large near and below
tropopause

Ozone budget in isentropic coordinates

P-L

eddy
fluxes

Cross-isentropictransport(upwelling)

Balances are similar to pressure coordinates, but reduced ozone amplitude
Abalos et al., 2013, Atmos. Chem. Phys.

WACCM ozone budget
verticalozone
gradientozone
tendency
in % per day

meanadvection

eddytransport
latitudinalozone
gradient

Abalos et al., 2013, Atmos. Chem. Phys.

eddy ozone transport
from midlatitudes intotropics (downgradient)Ozone••Formed in stratosphere (stratospheric source gas)
Strong gradients across tropopause

Ozone column density, DU/km
Satellite climatology in January

eddy
mixingstratosphere-troposphereexchangemean meridionalcirculation

WACCM eddy flux tendencies

d/dy (v’O3’)

Winter-spring
maximum inboth hemispheres

summermaximum

Abalos et al., 2013, Atmos. Chem. Phys.

lower
stratosphereneartropopauseNH summer eddy transport from Asian monsoon anticyclone

WACCM

Phase-speed vs. latitude spectra for eddy fluxes (v’O3’)
following
Randel and Held 1991

NH wintereddy ozone
transport
at 440 K•

eddy fluxes intothe tropics
due to transient
Rossby waves•Eddy fluxes ‘see’
critical lines !
(u=c)

convergencezonalmeanwind

Abalos et al., 2013, Atmos. Chem. Phys.

WACCM CO budget

CO tendency
in % per day

meanadvection

relatively small
contribution in
the tropics for CO

eddytransport

Abalos et al., 2013, Atmos. Chem. Phys.

CO eddy fluxes at 400 K(v’CO’)

NH wintereddy COtransport
at 400 K•

eddy fluxes out of
the tropics•

Eddy fluxes ‘see’
critical lines !
(u=c)

Key points:• WACCM results for temp, ozone and CO are very similar to observations• Upwelling is a dominant term in all balances, and primarily responsible
for the coupled seasonal variations in T, ozone and CO
in the tropical lower stratosphere• Eddy transport into the tropics is important for ozone* summertime maximum near tropopause (Asian monsoon)* transient Rossby waves in winter lower stratosphere* evidence for critical-layer behavior in phase-speed spectraWhat drives the annual cycle in tropical upwelling?

• Extratropical stratospheric planetary waves

Yulaeva et al, 1994, Ueyama and Wallace 2010, Ueyama et al 2013
• Equatorial waves
Kerr-Munslow and Norton, 2006, Ortland and Alexander, 2013
• Subtropics (baroclinic eddies from midlatitudes)
Randel et al 2008, Taguchi 2009, Chen and Sun 2011,
Jucker et al 2013, others

Seasonal mean
EP fluxes

Evaluating upwelling fromsubtropical EP fluxes

upwelling

w*m evaluated fromsubtropical EP fluxes

both extratropicaland tropical waves
contribute to
subtropical forcing

Randel, Garcia and Wu, 2008

wave forcing extends
deeper during DJF

Ortland and Alexander, 2013:
Equatorial waves respond to variations in background tropical zonal winds,
driving stronger tropical upwelling for westerlies (boreal winter)

alternative: tropics driven completely by extratropics (e.g. Jucker et al, 2013)This is still an active topic of researchDynamics of sub-seasonal variability

w*m and w*Q

remove annual cycle

use regressions onto w*m to identify circulation anddynamical forcing of transient upwelling

Abalos et al., 2014, J. Atmos. Sci.

ERAi reanalysis 1979-2011

Regressions onto w*m : residual circulation and dT/dt
Boreal winter
DJFM

K/day

Abalos et al., 2014, J. Atmos. Sci.

Coherent signals in ozone tendencies

dT/dt
MLS ozone

dO3/dtK/day

ppbv/day

Abalos et al., 2014, J. Atmos. Sci.

Regressions onto w*mDF
both termsare importantdu/dt

DF – du/dt

Abalos et al., 2014, J. Atmos. Sci.

EP flux ‘centers of action’ for forcing transient tropical upwelling:AC

Abalos et al., 2014, J. Atmos. Sci.

B

Composites of extreme 5% w*m events

Top 5% w*m

Difference

Bottom 5% w*m

Abalos et al., 2014, J. Atmos. Sci.

regressions

extreme event composites

DifferenceAC

B

Abalos et al., 2014, J. Atmos. Sci.

How does remote forcing influence tropical upwelling?
High latitude stratosphere

Correlation reference

DFdu/dt

forcedcirculation

DF – du/dtAbalos et al., 2014, J. Atmos. Sci.

theory (Holton et al, 1995)(response to extratropical EP flux divergence)

response to high latitude forcing
response
extends totropics

Abalos et al., 2014, J. Atmos. Sci.

deep barotropic
wind response

Subtropical upper troposphere
Correlation reference

DFDF – du/dt

Abalos et al., 2014, J. Atmos. Sci.

du/dt

forcedcirculation

response to subtropical forcing

Abalos et al., 2014, J. Atmos. Sci.

response to subtropical forcing

Abalos et al., 2014, J. Atmos. Sci.

response to subtropical forcing

Theory (Haynes, Holton)

response extends
above forcing

Net result: subtropical EP flux
effective at forcing transient
upwelling across tropopause

Abalos et al., 2014, J. Atmos. Sci.

Dependence on the reference altitude for w*m

20 hPaupper branch of
Brewer-Dobson
circulation

50 hPalower branch of BDC

100 hPa

Abalos et al., 2014, J. Atmos. Sci.

dT/dt and circulation

EP fluxes

high latitudestratosphere
forcing

20 hPa50 hPasubtropicalupper troposphere
forcing

100 hPa

Abalos et al., 2014, J. Atmos. Sci.

Lower branch of the BDC is primarily related to subtropical wave forcing

Residual circulationnote coherent tropospheric effects

Abalos et al., 2014, J. Atmos. Sci.

EP fluxes

subtropical EP fluxes
drive lower branch of BDC

Is stratospheric forcing correlated with subtropical forcing?

AA

C

B
Abalos et al., 2014, J. Atmos. Sci.

BIs subtropical forcing related between the two hemispheres?

CAC

B
Abalos et al., 2014, J. Atmos. Sci.

BKey points:•

Transience in tropical Brewer-Dobson circulation linked to remote wave forcing
 high latitude winter stratosphere, subtropics of both hemispheres•

Zonal wind changes are an important component of the remote response•

Clear identification of upper/lower branches of BDC: Deep branch tied to high latitude stratosphere forcing
 Shallow branch linked to subtropical wave dissipation
Plumb 2002

AC

B

Abalos et al., 2014, J. Atmos. Sci.

Extra slides

boreal winter
DJFM

boreal summer
JJAS

Abalos et al., 2014, J. Atmos. Sci.

Regressions onto w*m
SH winter JJAS
DFdu/dt

DF – du/dt

Abalos et al., 2014, J. Atmos. Sci.

DFNote contribution from (transient) equatorial waves

Abalos et al., 2014, J. Atmos. Sci.

Composites of extreme 5% w*m events: du/dt

Top 5% w*mDifference

Bottom 5% w*m

Abalos et al., 2014, J. Atmos. Sci.

Reference1••••••••

Abalos, M., Randel, W.J. and Serrano, E., 2012: Variability in upwelling across thetropical tropopause and correlations with tracers in the lower stratosphere, Atmos.
Chem. Phys., 12, 11505-11517.
Abalos, M., Randel, W.J. and Serrano, E., 2014: Dynamical forcing of sub-seasonalvariability in the tropical Brewer-Dobson circulation, J. Atmos. Sci., 71, 3439-3453.
Abalos, M., Randel, W.J., Kinnison, D.E. and Serrano, E., 2013: Quantifying tracer
transport in the tropical lower stratosphere using WACCM, Atmos. Chem. Phys., 13,10591-10607.
Chen, G. and Sun, L., 2011: Mechanisms of the tropical upwelling branch of theBrewer-Dobson circulation: The role of extratropical waves, J. Atmos. Sci., 68, 28782892.
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Jucker, M., Fueglistaler, S. and Vallis, G.K., 2013: Maintenance of the stratosphericstructure in an idealized general circulation model, J. Atmos. Sci., 70, 3341-3358.
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Konopka, P., Grooß, J.-U., Gunther, G., Plöeger, F., Pommrich, R., Müller, R. andLivesey, N., 2010: Annual cycle of ozone at and above the tropical tropopause:
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Reference2••••••••

Konopka, P., Grooß, J.-U., Plöger, F. and Mülller, R., 2009: Annual cycle of horizontal
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