Madden-Julian Oscillation

Wikipedia says it’s the largest contributor to intraseasonal (time length 30-90 days) variability in the atmosphere, and manifests as an eastward-propagating, alternating stormy/wet phase and dry phase oscillation.  The MJO is a coupling between the large-scale atmospheric circulation and mesoscale tropical deep convection.

You can see it in the Hovmoller diagram above (used for looking at waves; has latitude/longitude on the x-axis and time, increasing as you go down, on the y-axis).  So this shows that the rainy part (has precipitation and strong deep convection) propagates from west to east as time passes.

The MJO possibly plays a role in modulating tropical cyclogenesis; according to Maloney and Hartmann, twice as many North Pacific storms are found with 850 mb westerlies than with 850 mb easterlies.  In the Gulf of Mexico and western Caribbean, tropical cyclones are four times as likely to be found during the western phase of the MJO rather than the eastern phase.  The MJO also plays a considerable role in monsoons and in rainfall in parts of Asia, Africa, Australia, South America, and the west coast of North America.

Chidong Zhang wrote a really nice review article.  Some interesting points:

  • Possibly the MJO made favorable conditions for early Polynesians to sail to other islands?!
  • Possibly the MJO has an effect on a) the earth’s angular momentum and b) its magnetic field
  • As you can see from the Hovmoller diagram, the rainy region of strong deep convection (the active phase) has regions of weak convection on either side (the inactive or suppressed phases).  There’s a zonal overturning circulation between these, and apparently it goes through the whole height of the troposphere.  There are strong westerly winds at the active center and to the west of it, while there are strong easterly winds on the other side (and these are reversed in the upper troposphere).
  • The eastward propagation speed is relatively slow (5 m/s)
  • apparently both Kelvin and Rossby wave structures are “dynamically essential” to the MJO (look up Kelvin waves)
  • “The apparent eastward propagation of the large-scale convective center of the MJO is due to consecutive development of new convective systems, each on average slightly to the east of the previous one.”
  • “The diurnal cycle in deep convection is modulated by the MJO. Over the Maritime Continent the diurnal cycle is the strongest during the inactive phase of the MJO and becomes diminished during the active phase [Sui and Lau, 1992], possibly because of the interruption of the local sea breeze circulation by the large-scale circulation of the MJO”
  • MJO strength has peaks in SH summer/fall and NH summer
  • The MJO looks a lot like a Kelvin wave, but it travels much more slowly than a Kelvin wave would propagate, so there’s still a question of what the energy source (and dissipation) of the MJO are.  Could be an atmospheric response to some other kind of forcing (fluctuations in precipitation, a stochastic heating source) or just an atmospheric instability (moisture-convergence including friction, surface evaporation)
  • Apparently the MJO has been simulated in theoretical/idealized models, but it’s been a lot harder to simulate in GCMs (as of 2005), unclear why

According to Wikipedia, a Kelvin wave balances the Coriolis force against some type of topography and is non-dispersive, so it retains its shape as it propagates.  Apparently the equator can act as a waveguide (?? EDIT: because Coriolis deflection gets too big at some threshold latitude and stops it from propagating poleward, apparently, though I guess when it bounces back it must interfere constructively…???) and Kelvin waves have no meridional component to their propagation.  Kelvin waves can be free (don’t draw energy from surroundings) or forced (do), and in this case, they’d draw energy from deep convection in the troposphere.

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