The MJO for Dummies

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Transcript The MJO for Dummies

INTRASEASONAL
Oscillations—the Heartbeat of
14-100 Day Weather
Variability
Paul E. Roundy
Cold Air Outbreak Timeline
 Active
convective anomaly
amplified in Indian ocean (mid
February 2003)
 Anomaly approached maritime
continent (around February 20)—
heating deflected pacific jet stream
pattern
Cold Air Outbreak
 Large
edge wave formed (late
February), propagated poleward
 Cold air stream enhanced by
strengthened west-coast ridge
Convective Anomaly Pattern
26 February, 2003
From BMRC, Matthew Wheeler, Australia
Cold Air Outbreak, February 27, 2003
From NCEP Reanalysis
The Large-Scale Convective
Disturbance
 Tropical
Intraseasonal or
Madden-Julian Oscillation
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 Statistics
propagation, 4 +/- 2 ms-1.
Also has standing wave behavior
 30-60 day period
 Wavenumber 1-4 (planetary scale)
 Interacts with midlatitudes, but some
of this is nonlinear and hard to
quantify
 Eastward
Schematic of Mature MJO
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 Wave

L
Equatorial zonal wind disturbance
H
L
H
Kelvin Wave

Propagation mechanism:
Pressure
Fall
H
Pressure
Rise
L
Pressure
Fall
Dry wave propagates eastward at greater
than 40 ms-1
Convective Kelvin Wave
z
Convection removes
Some of the accumulating
mass, slows propagation
H
L
Propagation speed: less than 20 ms-1
x
Pressure Rises
H
H
Pressure Falls
L
L
Pressure Rises
Equatorial Rossby Wave
Wave Cooperation
 Kelvin
and Rossby waves
linked by convection, land, and
air-sea interaction combine to
produce the observed
disturbance.
Formation
Region
Decay
Region
Active
Convection
Enhanced
Active
Convection Easterlies
Deflected
Jet Stream
Active
Convection
Energy
Build-up
Cold air outbreak enhancement
Active
Convection
Effect of Background

Intraseasonal oscillations are modified by
convective coupling, so they must be
modified by
– The annual cycle
– Interannual processes like ENSO
Prediction 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