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

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How does the QBO affect the stratospheric polar vortex ?

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  1. How does the QBO affect the stratospheric polar vortex? Peter Watson, Lesley Gray Atmospheric, Oceanic and PlanetaryPhysics, Oxford University Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  2. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013 QBO influence on the vortex • Winter NH polar vortex is weakerduring QBO-E – the "Holton-Tanrelationship". ERA-40 QBO-E minus QBO-W ZMZW difference QBO-E minus QBO-W • Nov-Febzonalmeanzonalwindcompositedifferencein • ERA-40. Stippling indicates statistical significance of differences at the 95% level.

  3. 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). QBO-E QBO-W E W Vortex Vortex z W E Anomalous wave momentum flux Anomalous wave momentum flux Eq NP Eq NP Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  4. Observed QBO influence on EP flux QBO-E minus QBO-W EP flux differences • 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. Dec Nov Jan Feb 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. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  5. 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; Garfinkelet 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 (Grayet al. 2003, 2004; Naito et al. 2003). Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  6. QBO influence resembles the NAM • 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 (Ruzmaikinet al., 2005). QBO-E minus QBO-W EP flux Nov Dec Jan Feb NAM signature Nov Dec Jan Feb The QBO-E minus QBO-W EP flux and the acceleration associated with its divergence in ERA-40 (top) and the NAM signature (bottom). Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  7. Steady state response to forcing Palmer 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. Lorenz (1963) butterfly attractor • 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, Sassiet al. 2004, Labitzke 2005). • Therefore the response averaged over a long time does not generally allow discrimination between forcing mechanisms. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  8. 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 . Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  9. 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. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  10. Zonal wind transient response Days 1-2 Days 3-4 • Vortex weakening evident after ̴5 days. Days 5-6 Days 7-8 Transient zonal mean zonal wind response to nudging to QBO-E. Stippling indicates statistical significance at the 95% level. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  11. EP flux transient response Days 1-2 Days 3-4 • PolewardsEP 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. Days 5-6 Days 7-8 x10-8 m/s2 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. Peter Watson (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

  12. 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 (watson@atm.ox.ac.uk); DynVar/SNAP Workshop, Reading, 2013

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