1. Decadal Variation of the Holton-Tan Effect Hua Lu, Thomas Bracegirdle, Tony Phillips, Andrew Bushell DynVar/SNAP Workshops, 22-26 April, 2013, Reading, UK

2. QBO – Polar Vortex Relationship - the HT effect Composite differences (wQBO  eQBO) update based on ERA-40+Interim: 1958-2011 stronger (weaker) polar vortex under wQBO (eQBO) the signal is weaker during later winter UU TT Oct-Dec Jan-Mar where the phase of the QBO is defined

3. Decadal change of the QBO-Polar Vortex yet to be Explained update based on ERA-40+Interim: 1958-2011 JFM polar mean U & T DJF QBO The HT effect in late winter is substantially weaker during 1977-1997 But why? year

4. The Objective, Data and Methods  Objective: gain insight into the dynamic processes that either originate or modify the HT-effect planetary waves forcing residual mean circulation subtropical critical line  Data & Methods ERA-40 (1958-2001) + Interim (1979-2011) Composite differences and significant tests

5. QBO Signature in EP fluxes and Divergence significant signals in EP-flux divergence are found mainly at ~5-20 hPa where the easterly wind anomalies exist at the EQ. the magnitude of the signal in the lower stratosphere is much weaker anomalous divergence (convergence) at the high (low) lats Equatorward and upward EP- flux anomalies at mid-lat. mid stratosphere Poleward EP-flux anomalies at subtropics (a)(b) (c)(d) OND JFM Climatology wQBO  eQBO

6. QBO Signal in EP flux Div & Mean Residual Circulation Time-height cross-section anomalies have the opposite sign to the signals in EP flux divergence clearer and larger magnitude signal in meridional circulation than in EP flux divergence at low latitude lower stratosphere wQBO  eQBO 25-45  N55-75  N

7. A schematic of the Dynamics of the HT Effect Modified from Yamashita et al. (2011) EQ Pole tropopause stratospause W E W QBO induced residual circulation play an important role in the lower stratosphere consistent with Yamashita et al. (2011) and Garfinkel et al. (2012)

8. What processes has made the polar response weaker during 1977-1997? Timing of the QBO phase transition?

9. Decadal-scale Variation of the QBO Phase Transition A significantly stronger meridional circulation during 1977-1997 winter QBO phase transition occurred primarily during 1977-1997!

10. Effect of the QBO phase transition on EP fluxes & Divergence ONDJFM 1977-1997  1958-1976 JFM - wQBO only stronger wave forcing from high latitude troposphere during winter transition years the effect is similar but stronger in later winter than in early winter similar effect is obtained from the composite difference between 1977-1997 and 1958-1976 under wQBO Winter  Summer transitions

11. Effect of Timing of QBO Phase Transition on Mean Residual Circulation Time-height cross-section of composite differences Winter  Summer An anomalously stronger meridional circulation in the lower and upper stratospheres An apparently opposite effect in the upper troposphere Winter  Summer trans.

12. The Mechanism Behind the Decadal Change of HT Effect EQ Pole tropopause stratospause W E W Cancellation of the QBO induced residual circulation occurs when there is excess planetary wave forcing from high latitude troposphere This is responsible for the disappearing of the HT effect during 1977-1997

13. Summary  The QBO induced meridional circulation plays a more important role than the critical line effect in the lower stratosphere  At 5-20 hPa, there is a fine balance between poleward circulation anomaly at low latitudes and equatorward circulation anomaly at high latitudes  More planetary wave breaking in the upper stratosphere in the winter when the previous QBO phase transition occurs during NH winter, causing a stronger meridional circulation and a warmer, more disturbed polar vortex  This leads a cancellation/contamination of the QBO induced residual mean meridional circulation, thus a substantially weakened HT effect  As the winter transitions occurred much more frequently during 1977-1997, the excess planetary waves from the high latitude troposphere is responsible for the disappearing of the HT effect during that decadal period  The cause of the decadal variation of high latitude wave anomalies remains to be studied

14. Thank you !

15. Latitude QBO – Polar Vortex Relationship - the HT effect QBO composites of zonal wind and temperature (Lu et al. 2008, JGR) weaker polar vortex warmer Arctic lower stratosphere colder Arctic upper stratosphere Latitude stronger polar vortex colder Arctic lower stratosphere warmer Arctic upper stratosphere eQBOwQBO Early winter Late winter the polar signal descends over the winter

16. Contribution from Horizontal and Vertical Components of EP fluxes Time-height cross-section of EP-flux divergence Vertical component controls early winter signal Horizontal component controls middle to late winter signal The QBO signal in total EP flux divergence is very small in the lower stratosphere Horizontal Vertical Total EP-Div ( wQBO  eQBO)

17. Effect of the Timing of QBO phase transition on the length of QBO cycle Composite differences of Winter  Summer transitions the mean cycle length of eQBO is ~6 months longer winter transition than summer transitions. The QBO starts at 3 -5 hPa under if the phase transition at 50 hPa occurred in NH summer but at ~7 hPa if the phase transition at 50 hPa occurred in NH winter

18. Effect of the Timing of QBO phase transition on Zonal Wind Composite differences of Winter  Summer transitions Weaker polar vortex associated with winter transition The effect at low latitude is sensitive to the QBO phases Dynamically consistent with the QBO induced residual mean circulation

19. EQ Pole tropopause stratospause Schematics of the Dynamics Behind the Seasonal QBO Phase Transition