Decadal Variation of the Holton-Tan Effect

Hua Lu, Thomas Bracegirdle, Tony Phillips , Andrew Bushell

DynVar/SNAP Workshops, 22-26 April, 2013, Reading, UK

QBO – Polar Vortex Relationship - the HT effectComposite 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 T

Oct-Dec

Jan-Mar

where the phase of the QBO is defined

Decadal change of the QBO-Polar Vortex yet to be Explained update based on ERA-40+Interim: 1958-2011

-20

-10

0

10

20

U 60N

10h

Pa

(m s

-1)

(a) r = 0.38 (1e+02%) r = 0.82 (1e+02%) r = -0.043 (64%) r = 0.58 (98%)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010-20

-10

0

10

20

U eq, 5

0hPa

(m

s-1

)

-16

-8

0

8

16

T 80N

100

hPa (

K)

(b) r = -0.26 (96%) r = -0.74 (1e+02%) r = 0.12 (62%) r = -0.4 (95%)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010-20

-10

0

10

20

U eq, 5

0hPa

(m

s-1

)-2

-1

0

1

2

T EQ 1

00hP

a (K)

year

(c) r = 0.33 (1e+02%) r = 0.8 (1e+02%) r = 0.24 (73%) r = 0.79 (1e+02%)

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010-20

-10

0

10

20

U eq, 5

0hPa

(m

s-1

)

JFM polar mean U & T DJF QBO

• The HT effect in late winter is substantially weaker during 1977-1997

• But why?

year

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

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

QBO Signal in EP flux Div & Mean Residual CirculationTime-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

*f v

wQBO eQBO

F

*f v

25-45N 55-75N

A schematic of the Dynamics of the HT EffectModified from Yamashita et al. (2011)

EQ Pole

tropopause

stratospause

0T 0T W

E

0 F 0 FW

• QBO induced residual circulation play an important role in the lower stratosphere

• consistent with Yamashita et al. (2011) and Garfinkel et al. (2012)

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

Decadal-scale Variation of the QBO Phase TransitionA significantly stronger meridional circulation during 1977-1997

winter QBO phase transition occurred primarily during 1977-1997!

Effect of the QBO phase transition on EP fluxes & Divergence

OND JFM

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

Effect of Timing of QBO Phase Transition on Mean Residual CirculationTime-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.

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

0 F

0T 0T

0T 0T

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

Thank you !

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

eQBO wQBO

Early winter

Late winter

the polarsignal descends over the winter

Contribution from Horizontal and Vertical Components of EP fluxesTime-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)

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

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

EQ Pole

tropopause

stratospause

0T 0T

0 F

Schematics of the Dynamics Behind the Seasonal QBO Phase Transition