how does the qbo affect the stratospheric polar vortex ?

How does the QBO affect the stratospheric

polar vortex?Peter Watson, Lesley Gray

Atmospheric, Oceanic and Planetary Physics, Oxford University

Peter Watson ([email protected]); DynVar/SNAP Workshop, Reading, 2013

Winter NH polar vortex is weaker during QBO-E – the "Holton-Tan relationship".

QBO influence on the vortex

QBO-E minus QBO-W Nov-Feb zonal mean zonal wind composite difference in ERA-40. Stippling indicates statistical significance of differences at the 95% level.


ERA-40 QBO-E minus QBO-W ZMZW difference

Holton-Tan mechanism• Holton and Tan (1980) speculated that reflection of stationary

planetary wave zonal momentum flux from a critical surface where the ZMZW is zero causes vortex weakening c.f. Tung (1979), Killworth and McIntyre (1985).


W

E

Anomalous wave momentum flux

Vortex

Eq NP

E

W Vortex

Eq NP

QBO-W QBO-E

Anomalous wave momentum flux

z

Observed QBO influence on EP flux• Under QBO-E there

is greater upwards Eliassen-Palm flux in early winter but not in late winter.

• No evidence of anomalous equator-to-pole flux in QBO-E.


QBO-E minus QBO-W EP flux differences (arrows) and differences in the acceleration associated with its divergence (colours) in ERA-40. Black arrows and stippling respectively indicate where the EP flux difference and the divergence are statistically significant at the 95% level.

QBO-E minus QBO-W EP flux differencesNov Dec

Jan Feb

Alternatives to the HT mechanism


• Recent analyses have concluded that observations and GCM data are inconsistent with the HT mechanism and that the tropical lower stratospheric wind plays a smaller role than the QBO meridional circulation (Naoe and Shibata 2010; Garfinkel et al. 2012) or the tropical upper stratospheric wind (Yamashita et al. 2011).

• But previous experiments in primitive equation models indicate that having more easterly winds in the tropical lower stratosphere alone gives a substantially weaker vortex (Gray et al. 2003, 2004; Naito et al. 2003).

QBO influence resembles the NAM


The QBO-E minus QBO-W EP flux and the acceleration associated with its divergence in ERA-40 (top) and the NAM signature (bottom).

• The QBO composite difference of EP flux closely resembles the signature of the Northern Annular Mode (NAM).

• Difference of zonal wind is also NAM-like (Ruzmaikin et al., 2005).QBO-E minus QBO-W EP flux

Nov Dec Jan Feb

NAM signatureNov Dec Jan Feb

Steady state response to forcingPalmer and Weisheimer (2011) – time-averaged response of mean state of Lorenz 1963 system to a constant forcing is very nearly parallel to the leading EOF, for all forcings in the xy plane.

• Tropospheric models also display this behaviour (Ring and Plumb 2008, Branstator and Selten 2009).

• Consistent with this, the stratospheric response to the solar cycle, ENSO and volcanic eruptions, as well as to the QBO, resembles the NAM (e.g. Dunkerton and Baldwin 1991, Kodera 1995, Sassi et al. 2004, Labitzke 2005).

• Therefore the response averaged over a long time does not generally allow discrimination between forcing mechanisms.


Lorenz (1963) butterfly attractor

Transient response to forcing

• If constant then the short-term transient response closely resembles the forcing.

• If then the transient response may show the forcing’s evolution up to about a dynamical time scale.

• Dynamical time scales in the polar mid-stratosphere are 2–3 weeks. ̴

• Use branching of model runs to study .


Method to investigate transient response to QBO-E forcing

• Use HadGEM2, a stratosphere-resolving GCM.

• Control run – equatorial stratospheric winds nudged to ERA-Interim monthly-mean climatology.

• 120 branch runs with winds nudged to 3xQBO-E state (derived from observed anomalies), initialised at Jan 1 or Feb 1.

• Examine short-term transient evolution of mean differences between the runs.


Zonal wind transient response• Vortex

weakening evident after 5 days. ̴


Transient zonal mean zonal wind response to nudging to QBO-E. Stippling indicates statistical significance at the 95% level.

Days 1-2 Days 3-4

Days 5-6 Days 7-8

EP flux transient response• Polewards EP flux

anomalies are present from tropics to high latitudes after 5 days in the ̴lower stratosphere, indicative of reflection of eddy momentum flux.

• Agrees with HT mechanism.

• Does not match predictions of alternative mechanisms.


Transient response of EP flux and the acceleration associated with its divergence to nudging to QBO-E. Flux response is plotted where it is statistically significant. Stippling indicates statistical significance of the divergence response at the 95% level.

Days 1-2 Days 3-4

Days 5-6 Days 7-8

x10-8 m/s2

Summary• The mechanism behind the Holton-Tan relationship is not

well understood.• QBO-E minus QBO-W composite differences of quantities in

the extratropical stratosphere closely resemble the Northern Annular Mode signature.

• The short-term transient response to QBO forcing is likely to be more informative than the response averaged over a long time period.

• GCM experiments indicate that forcing by QBO-E results in anomalous reflection of EP flux from the tropical lower stratosphere that converges in the polar stratosphere, in agreement with the HT mechanism.– Consistent with studies in primitive equation models.

• Ref: Watson and Gray, 2013, J. Atmos. Sci., submitted.Peter Watson ([email protected]); DynVar/SNAP Workshop, Reading, 2013

how does the qbo affect the stratospheric polar vortex ?

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