Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation

Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation

Hai LinMeteorological Research Division, Environment Canada

Acknowledgements:Gilbert Brunet, Jacques Derome

BIRS Workshop, Banff, Alberta, April 29, 2009

Outlines

• Observed MJO – NAO connection• Intraseasonal variability in a dry GCM

NAO and MJO connection

• NAO: dominant large scale pattern in the extratropics with significant influence on weather from eastern North America to Europe

• MJO: dominant tropical intraseasonal mode, coupled with convections and variability in diabatic heating

• One-way impact, or two-way interaction?• A possible mechanism for both the NAO and MJO

Data and methodology

Definition of the NAO: 2nd REOF of monthly Z500

NAO index: projection of pentad Z500 anomaly onto this patternPeriod: 1979-2003Extended winter, November to April (36 pentads each winter)

Data and methodology

Definition of the MJO: combined EOF of OLR, u200 and u850 in the band of 15°S – 15°N (Wheeler and Hendon, 2004)

NAO index: RMM1 and RMM2Period: 1979-2003Extended winter, November to April (36 pentads)

Composites of tropicalPrecipitation rate.

Winter half yearNovember-April

Xie and Arkin pentad data, 1979-2003

Pentads in MJO phases

Extended winter from 1979 to 2004

Phase 1 2 3 4 5 6 7 8

Number of pentads

55 79 78 78 63 71 87 66

Mean amplitude

1.67 1.66 1.81 1.78 1.66 1.70 1.62 1.75

Lagged composites of the NAO index

Phase 1 2 3 4 5 6 7 8

Lag −5 −0.39 0.28

Lag −4 −0.26 0.28

Lag −3 −0.29

Lag −2 0.26

Lag −1

Lag 0 −0.41

Lag +1 0.26 0.27 0.26 −0.25 −0.35

Lag +2 0.34 0.36 −0.31 −0.33 −0.29

Lag +3 0.35 −0.35 −0.41

Lag +4 −0.35 −0.31

Lag +5 −0.27

Lagged composites of the NAO index

Phase 1 2 3 4 5 6 7 8

Lag −5 −0.39 0.28

Lag −4 −0.26 0.28

Lag −3 −0.29

Lag −2 0.26

Lag −1

Lag 0 −0.41

Lag +1 0.26 0.27 0.26 −0.25 −0.35

Lag +2 0.34 0.36 −0.31 −0.33 −0.29

Lag +3 0.35 −0.35 −0.41

Lag +4 −0.35 −0.31

Lag +5 −0.27

Lagged probability of the NAO indexPositive: upper tercile; Negative: low tercile

Phase 1 2 3 4 5 6 7 8

Lag −5 −35% −40% +49% +49%

Lag −4 +52% +46%

Lag −3 −40% +46%

Lag −2 +50%

Lag −1

Lag 0 +45% −42%

Lag +1 +47% +45% −46%

Lag +2 +47% +50% +42% −41% −41% −42%

Lag +3 +48% −41% −48%

Lag +4 −39% −48%

Lag +5 −41%

Tropical influence

Wave activity flux and 200mb streamfunction anomaly

Benefit to Canadian surface air temperature forecasting

Lagged winter SAT anomaly in Canada

Lagged regression of 200mb U to NAO index

Extratropical influence

U200 composites

Lagged regression of 200mb U to NAO index

Extratropical influence U200 composites

Tropical intraseasonal variability (TIV) in a dry GCM

Model and experiment

• Primitive equation AGCM (Hall 2000).

• T31, 10 levels• Time-independent forcing to maintain the winter climate

(1969/70-98/99) all variabilities come from internal dynamics

• No moisture equation, no interactive convection• 3660 days of integration

Unfiltered data

20-100 day band-pass

Zonal propagation

10S-10N

Model Result

Stronger in eastern Hemisphere

Wavenumber-frequency spectra

Equatorial velocity potential

10S-10N average

wav

enum

ber

Wavenumber-frequency spectraEquatorial U

10S-10N average

wav

enum

ber

EOF analysis of 20-100 day band-passed 250 hPa velocity potential

TIV index:

2)8()()( 12

tPCtPCtI

Horizontal structure250 hPa u’,v’,z’

Vertical structure

Regression to Day +8

TIV in the dry model

• Kelvin wave structure• Phase speed: ~15 m/s (slower than free Kelvin wave, similar to

convective coupled Kelvin wave, but there is no convection)

What causes the TIV in the dry model?

• 3-D mean flow instability (Frederiksen and Frederiksen 1997)• Tropical-extratropical interactions (all wave energy generated in the

extratropics)

Moisture and convection related mechanisms are excluded

Possible mechanisms

Tropica-extratropical interactions in the dry GCM

Regression to TIV index

Color: 250mb velocity potentialContour: 250mb streamfunction anomaly

TIV index: In phase with PC2

















































ISO in a dry model

Linked to tropical eastward propagation in the eastern Hemisphere Global propagation of low-frequency wave activity

250 hPa PV’ and wave activity flux

Response to subtropical forcing

30N,30W, 10days 30N,30W, 30days 30N,180, 30days

Response to subtropical forcing

Summary

• Two-way interaction between the MJO and the NAO• Increase of NAO amplitude 5~15 days after the MJO-related

convection anomaly reaches western Pacific• Certain MJO phases are preceded by strong NAOs• TIV generated in a dry GCM• Tropical-extratropical interactions are likely responsible for the

model TIV

Implication to the MJO

• A possible mechanism for the MJO: triggering, initialization

• Contribution of moisture and tropical convection: spatial structure, phase speed

Source of LFV in extratropics

• From mean flow• From high-frequency transients

250 hPa kinetic energy conversion from mean flow

E vector

C

250 hPa Z tendency caused by vorticity flux convergence of high-frequency transients

Vertical structure

Regression to Day +8

Documents

Interactions between the Madden-Julian Oscillation and the North Atlantic Oscillation