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INTRASEASONAL INTRASEASONAL Oscillations—the Heartbeat of Oscillations—the Heartbeat of
14-100 Day Weather 14-100 Day Weather VariabilityVariability
Paul E. Roundy
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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
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Cold Air OutbreakCold Air OutbreakLarge edge wave formed (late
February), propagated polewardCold air stream enhanced by
strengthened west-coast ridge
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Convective Anomaly PatternConvective Anomaly Pattern26 February, 200326 February, 2003
From BMRC, Matthew Wheeler, Australia
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From NCEP Reanalysis
Cold Air Outbreak, February 27, 2003
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The Large-Scale ConvectiveThe Large-Scale ConvectiveDisturbanceDisturbance
Tropical Intraseasonal or
Madden-Julian Oscillation
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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
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Simplified Madden-Julian Oscillation Composite OLR from A.J. Matthews, 2000.
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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
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Schematic of Mature MJOSchematic of Mature MJO
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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
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Kelvin WaveKelvin Wave
Equatorial zonal wind disturbance
H L HL
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Kelvin WaveKelvin Wave
Propagation mechanism:
H LPressureRise
PressureFall
Dry wave propagates eastward at greater than 40 ms-1
PressureFall
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Convective Kelvin WaveConvective Kelvin Wave
H L
Convection removesSome of the accumulating mass, slows propagation
Propagation speed: less than 20 ms-1
z
x
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H
H
L
LPre
ssur
e F
alls
Pre
ssur
e R
ises
Equatorial Rossby WaveEquatorial Rossby Wave
Pre
ssur
e R
ises
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Wave CooperationWave Cooperation
Kelvin and Rossby waves linked by convection, land, and air-sea interaction combine to produce the observed disturbance.
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FormationRegion
Decay Region
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Active Convection
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Active Convection
EnhancedEasterlies
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Active Convection
Energy Build-up
Deflected Jet Stream
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Active Convection
Cold air outbreak enhancement
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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
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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