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Madden-Julian Oscillation (MJO)

The Madden-Julian Oscillation is interesting because it varies rapidly over time. In fact, it is referred to as an intraseasonal oscillation because it changes over a period of 30-60 days, a rate that is much faster than other climate indices. The Madden-Julian Oscillation is defined as a tropical disturbance that moves eastward at about 4-8 meters per second (8 – 18 mph) across tropical latitudes. Despite its location, the MJO affects weather in many locations around the world. The MJO itself is identified by the anomalous rainfall it produces.

Impact of the Madden-Julian Oscillation

As was said, the MJO is identified easily by the anomalous rainfall it generates. If one starts tracking it over the Indian Ocean, it can be followed eastward into the Pacific, where it becomes difficult to recognize before it reappears again over the tropical Atlantic. Impacts of a positive MJO are complicated, but can be roughly defined as follows (Note that winter/summer are in regard to the Northern Hemisphere and that negative MJO states will have the opposite impact).

  • Winter – Increased rainfall over Brazil, Southeast Africa, and Indonesia.
  • Summer – Increased rainfall over Central America, Mexico, and Southeast Asia

If the MJO is present and in a positive phase during Monsoon seasons in any of the above locations, it clearly augments the rainfall and makes for flood conditions. Other impacts are more generalized as follows:

  • ENSO (El Nino – Southern Oscillation) – MJO is known to enhance formation as well as intensity of El Nino (warm ENSO) events. The MJO is most active during neutral ENSO years and is absent in both strong El Nino and La Nina events. The two indices appear to be linked, though there have been periods when the MJO has been quite active despite strong ENSO events.
  • Tropical Cyclones – The MJO creates favorable conditions for cyclone formation and tends to increase precipitation associated with these storms. The U. S National Hurricane Center monitors the MJO carefully during the Atlantic hurricane season (August – November) for this reason
  • Precipitation in North America – Increased rainfall in the western and central Pacific due to MJO creates extreme rainfall in western North America, particularly in the higher latitudes. In winter months, the MJO is associated with dramatic winter storms and what is known as the Pineapple Express in North America.  The most recent of these events occurred in 1996-1997 and caused some 3 billion USD worth of damage in the state of California.

Patterns

The speed of the Madden-Julian Oscillation allows it to circle the equator in just 30-60 days. Active (positive) phases can be tracked by weather satellites and indicate the presence of thunderstorms. Influence on weather during these phases extends up to the 30 degree latitude mark in both the north and south directions. The active phase is followed by a dry phase in which thunderstorm activity is suppressed. This dry phase is responsible for the midsummer break in monsoon rainfall in Southeast Asia.

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