Dynamical Response to the QBO in the Northern Winter Stratosphere: Signatures in Wave Forcing and Eddy Fluxes of Potential Vorticity

Ian Whitea,b, Hua Lua, Nicholas Mitchellb and Tony Phillipsa

The Northern-hemisphere winter stratosphere is home to two large jets; the equatorial quasi-biennial oscillation (QBO) and the high-latitude stratospheric polar vortex. These two jets interact, and in particular, the former modulates the latter by the much observed Holton-Tan Effect first discovered by Holton and Tan (1980). A westerly wind anomaly in the lower stratosphere results in a stronger vortex, whereas an easterly wind anomaly in the lower stratosphere leads to a weaker vortex. However, the mechanism governing this effect is much debated and there is still no single accepted mechanism, although it is thought that large-scale Rossby waves hold the key and provide the link between the tropical and high-latitude variability. Using the ERA-Interim reanalysis dataset in isentropic coordinates, for which potential vorticity (PV) is conserved, this study has shed further light on the mechanisms behind the Holton-Tan Effect.

- Previously-suggested mechanisms cannot fully explain HTE

- New, additional mechanism suggested in lower stratosphere related to enhanced midlatitude waveguide

- Enhanced upward wave forcing drives stronger BDC- Poleward propagation of anomalies in middle

stratosphere - Different waves play different roles during the winter

1. Introductiona British Antarctic Survey, High Cross, Madingley Road, Cambridge, England, CB3 OET, UK

b University of Bath, Claverton Down, Bath, North East Somerset BA2 7AY

b) Potential Vorticity (PV)-Based Measures

- The Meridional PV Gradient determines whether Rossby waves can propagate

3. Results

d) Wavenumber Decomposition at 350 Ka) Zonal Momentum Budget2. Motivation

Seasonal evolution of the climatology and composite difference of the vertical component of the EP flux (units: kg s-2), filtered for stationary planetary waves (top), transient planetary waves (middle) and synoptic waves (bottom).

The Holton-Tan Effect (HTE)- Relationship between QBO at 50 hPa (~530 K) and Polar

Vortex at high latitudes

QBOe: Weaker, Warmer Polar Vortex

QBOw: Stronger, Colder Polar Vortex

What are the mechanism(s) that govern

the Holton-Tan Effect?Previously suggested mechanisms:

- Effect of Zero-Wind Line on stationary planetary

waves in lower stratosphere (Holton and Tan 1980);

- Effect of QBO-Induced Meridional Circulation on

waveguide in middle stratosphere (e.g. Garfinkel et al.

2012; Lu et al. 2014)

- Upper-stratospheric changes in the waveguide (e.g.

Gray et al. 2001; 2004)

(Top): DJF Climatology and composite difference between QBOe and QBOwof the zonal mean zonal wind over 1979-2014 with units of m s-1. (Bottom): Same as top row except for the zonal-mean temperature with units of K.

( )1 ˆˆ coscos cos

u v u f Qu Dt a a θϕ

ϕσ ϕ ϕ

∂ ⋅= − − − + ∂

F∇

Wave Forcing Residual Mean Meridional Circulation

(Top): DJF Climatology and composite difference of Π (shading) with units m s-2

and the Eliassen-Palm (EP) flux F (arrows). (Bottom): Same as top but for Θ(shading) and (arrows) calculated using the Downward-Control Principle.

- Clim: Strong PV gradients in vicinity of strong jets act as waveguides

- Under QBOe: Enhanced at midlatitudes and reduced in subtropics due to downward-arching zonal winds of the QBO

- Reduced migrates poleward over winter in middle stratosphere

- High-latitude anomalies associated with HTE

- Clim: Upward (downward) wave activity on poleward (equatorward) flank of subtropical jet

- Under QBOe: Upward stationary planetary waves dominate in early winter

- Transient waves dominate in late winter

White, I.P., Lu, H., Mitchell, N. J. and Phillips, T., J. Atmos. Sci., (2015). 72, 12, 4487-4507. White, I. P., Lu, H. and Mitchell, N. J., J. Geophys. Res. (Submitted)

c) Wave-Activity Conservation Law

- Up/downgradient fluxes represent wave decay/growth tendencies

Up/Downgradient Eddy PV Fluxes

DJF Climatology and composite difference of the up/downgradient eddy PVflux term Γ with units of K2 m4 kg-2 s-3 (1o)-1.

- Clim: Wave forcing (~EP flux convergence) drives the Brewer-Dobson Circulation (BDC)

- Under QBOe: Overall enhanced wave convergence drives a stronger BDC

2 2 ' '' (3)v P PP D O

t aϕ∂

= − + +∂

4. Conclusions

ˆˆ( , )v Q

u

( )F θ

Pϕ

( cos )1cos cos

u vP fa a

ϕ λϕσ ϕ ϕ

= − +

Π Θ

T

Pϕ

For more information see:

Γ

Pϕ

Clim

QBOe - QBOwClimu

T

QBOe - QBOwClimΓ

(Top): Climatologies of for the DJF mean (left), the seasonal evolution at370 K (middle) and the seasonal evolution at 850 K (right). (Bottom): Same astop but for the respective composite differences. Units in K m2 kg-1 s-1 (1o)-1.

Pϕ

QBOe - QBOwClimΠ

Θ QBOe -

QBOw

Clim

QBOe - QBOw

370 K 850 KDJF