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