Tropical tropopause dynamics observed from a
decade of GPS radio occultation dataBill Randel
Atmospheric Chemistry Division
NCAR Boulder, CO USA

Transport near the tropical tropopause layer (TTL)
TTL sets ‘boundary condition’ for global stratosphere
Region with complex balances:
two-way mixing
from baroclinic eddies
and monsoons

deep convection impacts
from below

Randel and Jensen, 2013, Nat. Geosci.

tropical upwelling influences
temps, ozone and stratospheric H2O

cirrus andclimate impacts

Dynamical forcing of tropical upwelling

midlatitude
baroclinic waves

subtropical convergencenear critical lines (U=c)
stratosphericplanetary waves

equatorial waves forced
by deep convection

Randel and Jensen, 2013, Nat. Geosci.

Deep and shallow branches ofBrewer-Dobson circulation

shallow branch fromsynoptic waves
deep branch tied
to planetary waves

Randel and Jensen, 2013,Nat. Geosci.

Plumb (2002); also Birner and Bonish, 20114

Brewer, 1949

Stratospheric H2O is controlled bytropical cold point temperatures
Interannual changes during 1992-2014

H2O
near-global mean (60o N-S)
water vapor at 82 hPafrom combined HALOE-MLS dataTCPcold-point tropical
tropopause temperatures

r=0.78black: radiosondes
red: GPS (after 2001)

GRL, 2013
temperaturecloud fraction

warm temps,
less clouds

What controls variability of the cold-point tropopause?
••

Convection or tropospheric temperatures?
Dynamically-forced upwelling?

heightMean
upwellingcold point
tropopause~17 kmRadiative
balancemain convective
outflow ~12 km
Lapse rate fromradiative-convective
equlibrium

T(z)GPS radio occultation

Basic measurement principle: Deduce atmospheric properties based on
precise measurement of phase delay
Occulting GPS

IonosphereUtility of GPS Radio Occultation:

Neutral atmosphereOcculting LEOEarth•Long-term stability•

All-weather operation•

High vertical resolution (< 1 km)•High accuracy: Averaged profiles to< 0.1 K

Using GPS data to understand variability of tropical temperature:•Construct a global, zonal average data set from GPS observations•

5-day (pentad) averages for 2001-2013 (over 12 complete years)
total > 6,200,000 occultations

Number of obs / pentad for 10o N-S
Example: 16 km, 10o N-S

Randel and Wu, 2014, J. Atmos. Sci.

Choose to analyze zonal averages because they are governedby a relatively simple equation:

TEM
thermodynamicbalanceapproximatebalance in tropics

linear damping
approximation(in stratosphere)Tropical variability for 10o N-S

QBOlarge annual cycle
in lower stratospheresmall annual cyclein troposphere

Randel and Wu, 2014, J. Atmos. Sci.

Amplitude of the tropical annual cycle in temperature

narrow vertical scale,peak in lower stratosphere
GPSradiosondes

tropopausevery small
annual cyclein troposphere‘raw’ time series

Randel and Wu, 2014, J. Atmos. Sci.

remove seasonal cycle

QBO is the large interannual signal in the stratosphere

Temp anomalies 10o N-S

Randel and Wu, 2014, J. Atmos. Sci.

Regression fits ofQBO and ENSO
2001-2013

T = a * ENSO + b1 * QBO1 + b2 * QBO2

QBO1 GPS temperature

proxytimeseries

QBO1 ERAi zonal wind+-.2-+--

tropopause

Randel and Wu, 2014, J. Atmos. Sci.

ENSO fitsENSO GPS temperature

ENSO ERAi zonal wind

ENSO-.2-.6.4

stratosphere coolingtied to enhanced upwellinge.g. Calvo et al 2010

Randel and Wu, 2014, J. Atmos. Sci.

wind maxima in subtropics,
extending into lower stratospheredeseasonalized

Randel and Wu,
2014, J. Atmos. Sci.

remove QBO and ENSO(‘residual’ variability)Components of zonal mean temperature varianceresidual dominates
deseasonalizedvariancenear tropopause

Randel and Wu, 2014, J. Atmos. Sci.

EOF analysis of residuals
deep stratosphere
mode

near tropopause
mode

Randel and Wu, 2014, J. Atmos. Sci.

Tropical cooling linked tostratospheric sudden warmings (SSW)-3

EOF1: deep stratosphere mode
PC1

spatial structureof temp anomalies-3

SH warming
Sept 2002

NH warming
Jan 2009

Randel and Wu, 2014, J. Atmos. Sci.

Tropical cooling linked tostratospheric sudden warmings (SSW)-3

EOF1: deep stratosphere mode
PC1

spatial structureof temp anomalies-3

SH warming
Sept 2002

NH warming
Jan 2009

Randel and Wu, 2014, J. Atmos. Sci.

Large stratospheric sudden warmingin January 2009

Polar stratosphere temperature

stratosphericsudden warming

high latitude
planetary waveforcing
Harada et al , 2010

polar vortex
near 30 km

potential vorticity

split of polar vortex

PC1

Regression of global temperatures and EP flux
onto PC1

cooling intropics

NH winterSH winterhigh latitude wave fluxes
and polar warming

Randel and Wu, 2014, J. Atmos. Sci.

Near-tropopause signal

EOF2: near-tropopause modetemperature regression onto PC2

PC2

maximum intropical lower
stratosphereanti-correlation
with tropical troposphere

Randel and Wu, 2014, J. Atmos. Sci.

Near-tropopause signal: correlation maps

regression onto PC2

Randel and Wu, 2014, J. Atmos. Sci.

correlation wrt 12 km

time series of tropicaltemperature anomalies

strong correlation
over narrow layer~16-19 km

weak anti-correlation
with troposphere

Randel and Wu, 2014, J. Atmos. Sci.

Spectrum analysis
Power spectra for zonal mean T

coherence between 12 – 18 km

99 %
95 %

30-60 days: MJO
Zonal mean MJO signal: Virts and Wallace, 2014

tropical coh2 with respect to 12 km

shaded regions~99% significant

low frequency
maximum inlower stratosphere(similar to ENSO)

maximum nearcold point.2
cold point
tropopausereference
at 12 km1.0

30-60 days: MJO

Randel and Wu, 2014, J. Atmos. Sci.

coh2 with respect to 12 km

coh2 with respect to the cold point.2.2

cold point

.2
1.0.5

small coherence forseasonal to interannual
variations

Randel and Wu, 2014, J. Atmos. Sci.

coherence with tropospherefor MJO time scales

Structure of zonal mean MJO (filtered 25-80 days bandpass).3

cooling neartropopause

MJO ERAi zonal winds

1.0

MJO GPS temperaturewarm
troposphere

Randel and Wu, 2014, J. Atmos. Sci.

zonal winds confinedto tropical troposphereExtreme near-tropopause event

PC2

3 factors contributing toanomalous tropical temps:
anomaly temps (K)
QBO
shear

4

deseasonalized anomalies over equator

tropospheric
La Nina

Randel and Wu, 2014, J. Atmos. Sci.

cold Arctic
stratosphere(weak BDC)

coldest Arctic
spring inobservational
record(1979-2014)

Arctic ‘ozone hole’in spring 2011
GPS Feb 2011

Randel and Wu, 2014, J. Atmos. Sci.

Deseasonalized anomalies

H2OTCP

r=0.78stratosphericwater vapor response
to warm tropopause

Links to tropical upwellingtwo estimatesof upwelling:w*m momentum balance
w*Q thermodynamic balance

20082009
Abalos et al, 2014, JAS3 estimates of tropical upwelling w* from observations:residual circulation from reanalysis w*

accurate radiative heating rate

thermodynamicbalancewQ*

momentumbalancewm*
EP flux divergencezonal wind tendencies

What forces transient tropical upwelling?
Regression of EP flux onto w*mw*mplanetarywaves

A

Modulation of climatological EP flux

C

subtropical EP fluxes

Randel and Wu, 2014, J. Atmos. Sci.

B

Abalos et al, 2014, JASRandel and Wu, 2014, J. Atmos. Sci.

Quantifying the relationship between w* and T:

harmonic expansion

temperature response
to upwelling:radiative damping
time scalefrequency

Spectrum analysis
Power spectra for T and w*mfrequency wheres~a

T sensitivity to w*mtheoryobservedenhanced responseat low frequencies(longer than 150 days)

Randel and Wu, 2014, J. Atmos. Sci.

note weak response
for MJO periods

Radiative damping time scales derived from:long damping time scales (~30 days)
in lower stratosphere

Randel and Wu, 2014, J. Atmos. Sci.

•Lower stratosphere temps
especially sensitive tolow frequency forcing•

Cause of enhanced annual cycleand large T variance inlower stratosphereKey points:• Novel high vertical resolution temperature record from GPS• Strong, coherent QBO, ENSO, SSW and MJO signals in GPS data
• 2 modes of stratospheric variability: deep, shallow branches of BDC• Cold point T variability tied to tropopause-level upwelling- anti-correlated with troposphere for MJO variations- no correlation with troposphere for seasonal to interannual time scales• Lower stratosphere T most sensitive to low frequency forcing

GPS EOF patterns

Deep and shallow branches ofBrewer-Dobson circulation

deep branch tied
to planetary waves
shallow branch fromsynoptic waves

Plumb (2002); also Birner and Bonish, 2011

42

ENSO and MJO temperature signals from GPS

MJO

ENSO
ENSO

-.2-.6-.2.4.3

Similar spatial structure, but different vertical structure near tropopause. Why?

Randel and Wu, 2014, J. Atmos. Sci.

Why is the stratospheric upwelling signature of ENSO ‘deeper’ than the MJO?

Zonal wind anomalies linked
to ENSO and MJO
ENSO

For ENSO, zonal winds (and influence on wave driving)extends into lower stratospherewell above cold point tropopause

Randel and Wu, 2014, J. Atmos. Sci.

MJO

MJO confinedto troposphere

PNAS 2013in a global model,
stratospheric H2O increases
with tropospheric temperatureThank youThank you for inviting me to FDEPS!

extremely high correlations
during NH winter-spring

Randel and Jensen, 2013

weaker correlations
during NH summer-fallTime series of tropical temperature residuals

deseasonalized

Randel and Wu, 2014, J. Atmos. Sci.

also remove QBO, ENSO

Dependence on the reference altitude for w*m

20 hPaupper branch ofBrewer-Dobson
circulation

50 hPalower branch of BDC

100 hPa

Abalos et al, 2014, JASdT/dt and circulation

EP fluxes

high latitudestratosphere
forcing

20 hPa50 hPa100 hPa

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

subtropicalupper troposphereforcing

Reference1•
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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.
Birner, T. and Bönisch, H., 2011: Residual circulation trajectories and transit times into
the extratropical lowermost stratosphere, Atmos. Chem. Phys., 11, 817-827.
Brewer, A.W., 1949: Evidence for a world circulation provided by the measurementsof helium and water vapour distribution in the stratosphere, Q.J.R. Meteorol. Soc., 75,351-363.
Calvo, N., Garcia, R.R., Randel, W.J. and Marsh, D.R., 2010: Dynamical mechanism
for the Increase in tropical upwelling in the lowermost tropical stratosphere during
warm ENSO events, J. Atmos. Sci., 67, 2331-2340.
Davis, S.M., Liang, C.K. and Rosenlof, K.H., 2013: Interannual variability of tropical
tropopause layer clouds, Geophys. Res. Lett., 40, 2862-2866.
Dessler, A.E., Schoeberl, M.R., Wang, T., Davis, S.M. and Rosenlof, K.H., 2013:
Stratospheric water vapor feedback, Proc. Natl. Acad. Sci. U.S.A., 110, 18087-18091.
Harada, Y., Goto, A., Hasegawa, H., Fujikawa, N., Naoe, H. and Hirooka, T., 2010: Amajor stratospheric sudden warming event in January 2009, J. Atmos. Sci., 67, 20522069.
Plumb, R.A, 2002: Stratospheric transport, J. Meteorol. Soc. Japan, 80, 793-801.
Randel, W.J. and Jensen, E.J., 2013: Physical processes in the tropical tropopause
layer and their role in a changing climate, Nature Geoscience, 6, 169-176.

Reference2••

Randel, W.J. and Wu, F., 2014: Variability of zonal mean tropical temperaturesderived from a decade of GPS radio occultation data, J. Atmos. Sci., accepted.
Virts, K.S. and Wallace, J.M., 2014: Observations of temperature, wind, cirrus, andtrace gases in the tropical tropopause transition layer during the MJO, J. Atmos. Sci.,
71, 1143-1157.