The Madden-Julian Oscillation

transcript

ATS 553

Intraseasonal Oscillations

• “Any quasiperiodic atmospheric fluctuation that is:– Longer than synoptic features, but– Shorter than the components of the annual

cycle.”

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.”

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.

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.

Their Interpretation

MJO is a region of low-level convergence and convection.

Propagating eastward only.

Circumnavigates the globe in 40-50 days.

Their Interpretation

Pressure is LOW in the region of strongest convection.

Upper-level outflow is only in the zonal direction.

Kelvin Waves in the Atmosphere

Features:Enhanced tradewinds AHEAD of the convection.

Features:Weak tradewinds—maybe even westerlies—behind the convection.

Features:Alternating areas of high and low pressure.

Features:Alternating areas of high and low OLR values!

Features:Notice that there shouldn’t be much signal in the midtropospheric winds!

Features:An area of upper-tropospheric DIVERGENCE, best seen in the VELOCITY POTENTIAL.

Equatorially trapped

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 isn’t exactly right.

• 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

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

Kelvin Waves and the Walker Circulation

Initially OPPOSES the Walker Circulation

Later ENHANCES the Walker Circulation!

Later, it OPPOSES the Walker Circulation and the trade winds in the Pacific!

However, these tradewinds are what maintained the high sea surface temperatures and heights of the western Pacific Warm Pool…

El Nino and the MJO

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

El Nino and the MJO

• Why doesn’t EVERY MJO trigger an El Nino event?– CHARGE/DISCHARGE THEORY:– MJO is the TRIGGER—it 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.

An MJO that’s a little off the equator

A very short MJO

An MJO from the southwest

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!

What are MJO events?

• Described by “Nakazawa’s Hierarchy of Convection”

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

• SCCs:– Move EASTWARD– Last a day or two

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

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

4 SCCs

Many CCs

The Madden-Julian Oscillation

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