INTRASEASONAL INTRASEASONAL Oscillations—the Heartbeat of Oscillations—the Heartbeat of

14-100 Day Weather 14-100 Day Weather VariabilityVariability

Paul E. Roundy

Cold Air Outbreak TimelineCold Air Outbreak TimelineActive convective anomaly

amplified in Indian ocean (mid February 2003)

Anomaly approached maritime continent (around February 20)—heating deflected pacific jet stream pattern

Cold Air OutbreakCold Air OutbreakLarge edge wave formed (late

February), propagated polewardCold air stream enhanced by

strengthened west-coast ridge

Convective Anomaly PatternConvective Anomaly Pattern26 February, 200326 February, 2003

From BMRC, Matthew Wheeler, Australia

From NCEP Reanalysis

Cold Air Outbreak, February 27, 2003

The Large-Scale ConvectiveThe Large-Scale ConvectiveDisturbanceDisturbance

Tropical Intraseasonal or

Madden-Julian Oscillation

What IS the MJO?What IS the MJO?

Large-scale disturbance of deep convection and winds that controls up to half of the variance of tropical convection in some regions

Brief history

Simplified Madden-Julian Oscillation Composite OLR from A.J. Matthews, 2000.

MJO StatisticsMJO Statistics

Eastward propagation, 4 +/- 2 ms-1. Also has standing wave behavior

30-60 day periodWavenumber 1-4 (planetary scale)Interacts with midlatitudes, but some

of this is nonlinear and hard to quantify

Schematic of Mature MJOSchematic of Mature MJO

How Does It Propagate?How Does It Propagate?

Is a matter of debate, but, probably involves – interactions with equatorial waves

Kelvin wave Equatorial Rossby wave

– Feedbacks from convection– Sea surface temperatures—air-sea interaction– Land interactions

Kelvin WaveKelvin Wave

Equatorial zonal wind disturbance

H L HL

Kelvin WaveKelvin Wave

Propagation mechanism:

H LPressureRise

PressureFall

Dry wave propagates eastward at greater than 40 ms-1

PressureFall

Convective Kelvin WaveConvective Kelvin Wave

H L

Convection removesSome of the accumulating mass, slows propagation

Propagation speed: less than 20 ms-1

z

x

H

H

L

LPre

ssur

e F

alls

Pre

ssur

e R

ises

Equatorial Rossby WaveEquatorial Rossby Wave

Pre

ssur

e R

ises

Wave CooperationWave Cooperation

Kelvin and Rossby waves linked by convection, land, and air-sea interaction combine to produce the observed disturbance.

FormationRegion

Decay Region

Active Convection

Active Convection

EnhancedEasterlies

Active Convection

Energy Build-up

Deflected Jet Stream

Active Convection

Cold air outbreak enhancement

Effect of BackgroundEffect of Background

Intraseasonal oscillations are modified by convective coupling, so they must be modified by – The annual cycle– Interannual processes like ENSO

Prediction of MJOPrediction of MJO

Global weather models predict it with some skill to about 7 or 8 days

Filtering methods allow prediction up to 20 days (Wheeler and Weikmann, 2001)

Statistical schemes may allow prediction for more than 40 or 50 day lead times