The Madden-Julian Oscillation

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The Madden-Julian Oscillation

• ATS 553

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Intraseasonal Oscillations

• Any quasiperiodic atmospheric fluctuation that
  is
• Longer than synoptic features, but
• Shorter than the components of the annual cycle.

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Madden-Julian Oscillation

• The most famous and most important of the ISOs.
• A pattern of tropical convection in which a
  given location experiences enhanced precipitation
  about every 40-50 days.

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How MJO was discovered
In the 1960s and 1970s, new computer power
created the ability to look for patterns in
meteorological observations. Could only look for
patterns in TIME (at a location), not SPACE (at a
given time). Tended to detect OSCILLATIONS.
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What They Found

• Surface pressure oscillates with a period of
  40-50 days.
• ZONAL winds in the lower and upper troposphere
  also oscillate at this frequency, but 180
  out-of-phase.
• The signal was limited to the deep tropics.
• They found little signal in the meridional wind,
  or in zonal winds in the midtroposphere.

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Their Interpretation
MJO is a region of low-level convergence and
convection. Propagating eastward
only. Circumnavigates the globe in 40-50 days.
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Their Interpretation
Pressure is LOW in the region of strongest
convection. Upper-level outflow is only in the
zonal direction.
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Kelvin Waves in the Atmosphere
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Kelvin Waves in the Atmosphere
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Features
Enhanced tradewinds AHEAD of the convection.
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Features
Weak tradewindsmaybe even westerliesbehind the
convection.
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Features
Alternating areas of high and low pressure.
H
L
H
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Features
Alternating areas of high and low OLR values!
H
L
H
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Features
Notice that there shouldnt be much signal in the
midtropospheric winds!
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Features
An area of upper-tropospheric DIVERGENCE, best
seen in the VELOCITY POTENTIAL.
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Equatorially trapped
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Equatorially trapped
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Circumnavigating the Globe?

• Madden and Julian originally believed that this
  area of convection propagated all the way around
  the world every 40-50 days.
• But this isnt exactly right.

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Circumnavigating the Globe?

• Rather, the convection is TRIGGERED in the
  eastern Indian Ocean (typically by intruding
  midlatitude systems).
• Convection dies out in the eastern Pacific due to
  cold SSTs

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Circumnavigating the Globe?

• However, the region of upper-level divergence
  WILL generally travel all the way around the
  world as a Kelvin Wave.

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Kelvin Waves and the Walker Circulation
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Kelvin Waves and the Walker Circulation
Initially OPPOSES the Walker Circulation
Later ENHANCES the Walker Circulation!
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Kelvin Waves and the Walker Circulation
Later, it OPPOSES the Walker Circulation and the
trade winds in the Pacific!
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Kelvin Waves and the Walker Circulation
However, these tradewinds are what maintained the
high sea surface temperatures and heights of the
western Pacific Warm Pool
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El Nino and the MJO

• MJO events can TRIGGER El Nino events by
  weakening the trade winds (or even having a
  WESTERLY WIND BURST).

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El Nino and the MJO

• Why doesnt EVERY MJO trigger an El Nino event?
• CHARGE/DISCHARGE THEORY
• MJO is the TRIGGERit happens much more often
  than the El Nino event itself.
• Not every trigger is exactly right.
• Even when the trigger is right, maybe ocean
  conditions are not yet right.
• A partial explanation for the timing of ENSO.

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An MJO thats a little off the equator
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A very short MJO
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An MJO from the southwest
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Triggering MJO Events
An MJO event in the Indian Ocean upsets the
SSTs. It takes time for the SSTs to recover. Any
intruding midlatitude systems during this period
will FAIL to trigger an MJO event. Only when the
environment is ready (another 40-50 days) will
the next midlatitude system be able to trigger a
new MJO!
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What are MJO events?

• Described by Nakazawas Hierarchy of Convection

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What are MJO events?

• Each MJO event is actually composed of a small
  number of super cloud clusters--SCCs

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What are MJO events?

• SCCs
• Move EASTWARD
• Last a day or two

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What are MJO events?

• What are SCCs?
• Made of Cloud Clusters (CCs)

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What are MJO events?

• What are CCs?
• Small groups of thunderstorms
• Last less than a day
• Move WESTWARD

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1 MJO
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4 SCCs
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Many CCs