FDEPS 2012, Lecture 4

Polar vortex variability andstratosphere-troposphere
coupling
Ted Shepherd
Department of Meteorology
University of ReadingPolar temperatures at 30 hPa (approx 25 km)• Forcing of planetary Rossby waves is stronger in theNH than in the SH, so the Arctic winter is warmer andmore variable than the Antarctic (summer is quiet)
1979-1997

1955-2000

Yoden, Taguchi & Naito (2002 JMSJ)• Time series of polar temperatures exhibit ‘spiky’ behaviour• Rapid warming caused by a focusing of Rossby wave
drag at high latitudes, a highly nonlinear process
Shepherd (2003) Chem Rev.

• The most dramatic polar disturbances are Stratospheric
Sudden Warmings (SSWs); because the winds become
easterly, it can take the rest of the winter to recover

MLS
T@70N

Geosu@
70N
Manney et al. (2009 GRL)

The split ozone hole of 2002: a wave-2 sudden warming

TOMS data (smoothed), from NASA GSFC web site• More generally, NH polar vortex disturbances propagate
downwards, but there is only time for one oscillation in awinter30 dayrunningaveragepolar Tanomaly
Interannual
std dev ofmonthly
mean polarT
Kuroda & Kodera (2001 JGR)• In the SH, the variability is (usually) confined to springtime
and represents variability in the annual breakdown of thevortex
30 dayrunningaveragepolar Tanomaly
Interannual
std dev ofmonthly
mean polar
T
Kuroda & Kodera (2001 JGR)

• About half of all SSWs are short-lived, as in 2007-2008(left), while half have extended recovery periods, as in2008-2009 (right)– Figures show MLS polar-cap average temperatures– Note opposite response in the mesosphere2007-2008

2008-2009

Hitchcock, Shepherd & Manney (J Clim, in press)• Solving the zonal-mean
QG equations forced bywave drag from theCanadian Middle
Atmosphere Model(CMAM) shows that thelong timescales result
from:– Long radiative timescales
in the lower stratosphere– Continued descent frominduced radiative cooling– Absence of planetarywave forcing

Plots show polar T anomaly
Hitchcock & Shepherd(JAS, in press)• Only with long radiative damping timescales in the lower
stratosphere does a simplified GCM exhibit such events

Hitchcock, Shepherd, Yoden, Noguchi & Taguchi (JAS, submitted)

• In CMAM, this variability leads
to apparent decadal trends in
Arctic temperatures even for 3member ensembles (below)

Ph.D. thesis of Peter Hitchcock, University of Toronto (2012)• The oscillatory nature of NH polar vortex variability leads
to a see-saw relationship between early-winter and latewinter decadal variability (here in 30 hPa polar T)– There is a lot of power in the decadal variations, whichhave tended to be interpreted as trends
December

March

Updated from Labitzke & Kunze (2005 Meteor. Z.)

• At the equator, a spectrum of wave forcing genericallyleads to oscillating zonal winds (see e.g. Plumb 1977
JAS), which are super-rotating in their eastward phase• Manifested in the stratospheric Quasi-Biennial Oscillation(QBO)

Observations from Baldwin et al. (2001 Rev. Geophys.)• The hemispheric scale of the stratospheric surf zone
provides a mechanism through which changes intropical winds can directly affect the polar vortex
Off-line
isentropic
particleadvection atapprox. 35 km
altitude driven
by winds fromthe CMAM
Shepherd,
Koshyk & Ngan(2000 JGR)• The QBO affects polar vortex variability through theHolton-Tan effect (1981 JAS)– Qualitatively, results from meridional displacement ofregion of planetary wave breaking (stratospheric ‘surfzone’) in response to shifted subtropical critical layers
Easterly
QBO

WesterlyQBOAll

Is the origin of theobserved bimodality
in NH variability(here based on NAMindex at 20 hPa)
Years segregated by
FUB QBO index(shaded is easterly)

Christiansen (2010 J Clim)• The NH vortex is affected from late fall to early spring,
the SH vortex only in late spring– QBO phase defined here at 50 hPa in January for NH
and at 20 hPa in July for SH; which levels are causal?

Anstey & Shepherd (QJRMS, submitted)

• The Holton-Tan effect has a sensitive seasonal dependence(seen here in ERA-40 W-E zonal wind differences)
Nov-Dec

Feb-Mar

C.I. = 2 m/s
30 hPa phasetransitions
betweenNovember andApril in theprevious year
Between Apriland Septemberin the previousyear

Anstey &Shepherd(2008 GRL)• The seasonality ofobserved QBO phasetransitions exhibits aninteresting decadalvariability• May explain why the QBOvortex coupling seems nonrobust• Non-robustness of QBOvortex coupling has been
attributed to solarvariability, but CMAM
shows the same behaviour
with no solar variability(Anstey, Shepherd &Scinocca 2010 JAS)
Anstey & Shepherd (2008 GRL)• Because SSWs disturb the NH vortex so strongly,
perturbations do not add (a second “trigger” is redundant)
Mean warmingof NH pole
(Feb-Mar over
10-50 hPa)
from different
combinations ofsolar and QBO
perturbations(based onNCEP/NCAR,1954-2005)
Camp & Tung (2007 JAS)• The only differences that seem robust in the data are
between QBO-W/SC-min and the other quadrants• However we have not sampled very much of phasespace in the observational record

Anstey & Shepherd (QJRMS, submitted)

• In both hemispheres, the stratospheric polar vortex
variability is connected to the troposphere (where itaffects the subtropical jet)Southern andNorthernHemisphere“annularmodes” (SAMand NAM),
based onhemispheric
EOFs
Thompson &Wallace (2000
J Clim)• Variations in the stratospheric polar vortex appear toinfluence tropospheric weather regimes for several months• Must be wave-mean flow interaction, but beyond that, themechanism has not yet been clearly elucidated
Composites ofNorthern
Annular Mode(NAM) indices

Baldwin &Dunkerton(2001 Science)• A negative NAM anomaly is preceded by anomalous
wave drag (left, colours), which warms the polar lower
stratosphere and leads to radiative cooling (right, colours)• Both stratospheric forcings weaken the surface zonal
wind through their induced meridional circulations (lines)

Thompson, Furtado & Shepherd (2006 JAS)FrictionCooling
Wave dragThe surface wind reduction
is initiated by the wave dragand maintained by thecooling, and damped bysurface friction
Surface wind anomalies arecomputed from the zonalmean QG response to theobserved forcings
Thompson, Furtado &Shepherd (2006 JAS)• The extended recoveries from SSWs (right) are associatedwith a strong suppression of planetary-wave fluxes (colour)
into the stratosphere — more than can be explained by theCharney-Drazin theorem (contours show zonal winds)
2007-2008

2008-2009

Vertical EP flux

Hitchcock, Shepherd & Manney (J Clim, in press)• Over Europe, the surface effects of stratosphericvariability are comparable to those from ENSO
• Provides a mechanism for effect of QBO on high-latitude
surface climate, through the Holton-Tan effect– Figure shows wintertime surface air temperature
differences (in K) between circulation regimes

Thompson, Baldwin & Wallace (2002 J. Clim.)• In the Arctic, the predicted wintertime surface response(here MSLP) to doubled CO2 depends sensitively on thesettings of the orographic gravity-wave drag scheme– Mechanism is effect on stratospheric planetary-wavedrag via effect of OGWD on climatological zonal flow

CMAM results from Sigmond & Scinocca (2010 J Clim)• Rather, whether the CMAM Arctic vortex strengthens orweakens under doubled CO2 depends on the mean state– So the sensitivity to orographic GWD is via its effect on
the climatological winds, which affect the planetarywave response (shown below) to doubled CO2Sigmond & Scinocca (2010 J. Clim.)• The ozone hole has been the primary driver of recent
trends in SH high-latitude summertime surface climate– Mechanism is not clear, but is presumably
analogous to the Baldwin-Dunkerton effectLinear trends
up to 2000

Gillett &Thompson(2003 Science)• It follows that ozone recovery will weaken or possibly
even reverse the summertime SAM trends in the future• Plots show DJF zonal wind trends (in m/s/decade)over 2000-2050 for IPCC AR4 models with (left)
and without (right) prescribed ozone recovery

Son et al. (2008 Science)• Models show that the ozone hole has had a major effecton the width of the Hadley cell
• Reflected in differences between past and future
trendsLocation of max [u] Location of Hadley Cell

Son et al.
(2010 JGR)

at 850 hPa

boundary at 500 hPa(past) (future)(past) (future)• The ozone hole causes a poleward shift in upper
tropospheric eddy momentum flux convergence at
subpolar latitudes, which can explain the SAM trend– DJF trends at 250 hPa; colours show climatology (red is positive)

McLandress et al. (2011 J. Clim.)Summary• Variations in Rossby-wave forcing in the stratosphere leadto polar vortex variability– This variability is modulated by the QBO through the‘Holton-Tan effect’– Most dramatic events are ‘Stratospheric Sudden
Warmings’, about half of which exhibit extended
recoveries lasting up to two months– The extended recoveries result from the long radiative
timescales in the lower stratosphere, together with thesuppression of tropospheric wave forcing• Stratospheric polar vortex variability couples to the surface– Provides seasonal predictability; also effect of ozone
hole on summertime SH climate• The QBO affects surface climate, in both hemispheres(W-E differences)

Anstey & Shepherd (QJRMS, submitted)

References 1• Anstey, J. A., Shepherd T. G., Q. J. R. Meteorol. Soc., submitted• Anstey, J. A., Shepherd T. G., 2008: Response of the northern
stratospheric polar vortex to the seasonal alignment of QBO phasetransitions, Geophys. Res. Lett., 35, L22810.
• Anstey, J. A., Shepherd T. G., Scinocca J. F., 2010: Influence of theQuasi-Biennial Oscillation on the Extratropical Winter Stratosphere in
an Atmospheric General Circulation Model and in Reanalysis Data, J.
Atmos. Sci., 67, 1402-1419.
• Baldwin, M. P., Dunkerton, T. J., 2001: Stratospheric Harbingers ofAnomalous Weather Regimes, Science, 19, 581-584.
• Baldwin, M. P., Gray, L. J., Dunkerton, T. J., Hamilton, K., Haynes, P.
H., Randel, W. J., Holton, J. R., Alexander, M. J., Hirota, I., Horinouchi,T., Jones, D. B. A., Kinnersley, J. S., Marquardt, C., Sato, K.,
Takahashi, M., 2001: The quasi-biennial oscillation, Rev. Geophys.,
39, 179-229.

References 2• Camp, C. D., Tung, K.-K., 2007: The Influence of the Solar Cycle andQBO on the Late-Winter Stratospheric Polar Vortex, J. Atmos. Sci., 64,1267-1283.
• Christiansen, B., 2010: Stratospheric Bimodality: Can the Equatorial
QBO Explain the Regime Behavior of the NH Winter Vortex?, J. Clim.,
23, 3953-3966.
• Gillett, N. P., Thompson, D. W. J.. 2003: Simulation of Recent
Southern Hemisphere Climate Change, Science, 302, 273-275.
• Hitchcock, P., Shepherd, T. G.: Zonal mean dynamics of extended
recoveries from stratospheric sudden warmings, J. Atmos. Sci., inpress.
• Hitchcock, P., Shepherd, G. T., Manney, L. G.: Statisticalcharacterization of Arctic Polar-night Jet Oscillation Events, J. Clim., inpress.
• Hitchcock, P., Shepherd, G. T., Taguchi, M., Yoden, S., Noguchi, S.:
Lower-stratospheric Radiative Damping and Polar-night Jet Oscillation
Events, J. Atmos. Sci., in press.

References 3• Kuroda, Y., Kodera, K., 2001: Variability of the polar night jet in thenorthern and southern hemispheres, J. Geophys. Res., 106, 20,70320,713.
• Labitzke, K., Kunze, M., 2005: Stratospheric temperatures over theArctic: Comparison of three data sets, Meteorol. Z., 14, 65-74.
• Manney, G. L., Schwartz, M. J., Krüger, K., Santee, M. L., Pawson, S.,
Lee, J. N., Daffer, W. H., Fuller, R. A., Livesey, N. J., 2009: Aura
Microwave Limb Sounder observations of dynamics and transport
during the record-breaking 2009 Arctic stratospheric major warming,Geophy. Res. Let., 36, L12815, 5pp.
• McLandress, C., Shepherd, T. G., Scinocca, J. F., Plummer, D. A.,
Sigmond, M., Jonsson, A. I., Reader, M. C., 2011: Separating theDynamical Effects of Climate Change and Ozone Depletion. Part II:
Southern Hemisphere Troposphere, J. Clim., 2011, 24, 1850-1868.
• NASA GSFC web site
http://archive.org/details/antarctica_ept_2002267.

References 4• Hitchcock, A. P., 2012: The Arctic Polar-night Jet Oscillation, Ph.D.
thesis of University of Toronto, 161pp.
• Plumb, R. A., 1977: The Interaction of Two Internal Waves with theMean Flow: Implications for the Theory of the Quasi-Biennial
Oscillation, J. Atmos. Sci., 34, 1847-1858.
• Shepherd, T. G., Koshyk, J. N., Ngan, K.,2000: On the nature of largescale mixing in the stratosphere and mesospher, J. Geophys. Res.,
105, 12433–12446.
• Shepherd, T. G., 2003: Large-Scale Atmospheric Dynamics forAtmospheric Chemists, Chem. Rev., 103, 4509-4531.
• Sigmond, M., Scinocca, J. F., 2010: The Influence of the Basic Stateon the Northern Hemisphere Circulation Response to Climate Change,
J. Climate, 23, 1434–1446.
• Son, S.-W., Polvani, L. M., Waugh, D. W., Akiyoshi, H., Garcia, R.,
Kinnison, D., Pawson, S., Rozanov, E., Shepherd, T. G., Shibata, K.,
2008: The Impact of Stratospheric Ozone Recovery on the SouthernHemisphere Westerly Jet, Science, 320, 1486-1489.

References 5• Son, S.‐W., Gerber, E. P., Perlwitz, J., Polvani, L. M., Gillett, N. P.,Seo, K.-H., Eyring, V., Shepherd, T. G., Waugh, D., Akiyoshi, H.,Austin, J., Baumgaertner, A., Bekki, S., Braesicke, P., Brühl, C.,
Butchart, N., Chipperfield, M. P., Cugnet, D., Dameris, M., Dhomse,S., Frith, S., Garny, H., Garcia, R., Hardiman, S. C., Jöckel, P.,
Lamarque, J. F., Mancini, E., Marchand, M., Michou, M., Nakamura,T., Morgenstern, O., Pitari, G., Plummer, D. A., Pyle, J., Rozanov, E.,
Scinocca, J. F., Shibata, K., Smale, D., Teyssèdre, H., Tian, W.,
Yamashita, Y., 2010: Impact of stratospheric ozone on SouthernHemisphere circulation change: A multimodel assessment, J.
Geophys. Res., 115, D00M07.
• Thompson, D. W. J., Wallace J. M., 2000: Annular Modes in the
Extratropical Circulation. Part I: Month-to-Month Variability, J. Climate,13, 1000–1016.
• Thompson, D. W. J., Baldwin, M. P., Wallace, J. M., 2002:
Stratospheric Connection to Northern Hemisphere Wintertime
Weather: Implications for Prediction, J. Climate, 15, 1421–1428.

References 6• Thompson, D. W. J., Furtado, J. C., Shepherd, T. G., 2006: On theTropospheric Response to Anomalous Stratospheric Wave Drag andRadiative Heating, J. Atmos. Sci., 63, 2616–2629.
• Yoden, S., Taguchi, M., Naito, Y., 2002: Numerical studies on time
variations of the troposphere-stratosphere coupled system, J. Meteor.
Soc. Japan., 80, 811-830.