The Madden-Julian oscillation (MJO), also known as the tropical intraseasonal oscillation, is a cyclic weather pattern (wave) that travels from west to east along the equator every 30–90 days. The MJO, discovered by in 1971 by Roland Madden and Paul Julian of the National Center for Atmospheric Research in Boulder, Colorado, is responsible for enhancing and suppressing the wind, sea surface temperature, cloudiness, and rainfall in the tropics. See also: Equator; Tropical meteorology; Precipitation (meteorology); Wind
The strength of the MJO has been linked to the El Niño Southern Oscillation (ENSO), with strong MJO activity in La Niña years or ENSO-neutral years and weak MJO activity in El Niño years. The MJO also has been found to influence the formation of hurricanes (tropical cyclones) in the Northern Hemisphere from May to November. See also: El Niño; Cyclone; Hurricane
In 2004, Matthew C. Wheeler and Harry H. Hendon of the Bureau of Meteorology Research Centre, Melbourne, Australia, defined eight phases for tracking the west–east progress of enhanced rainfall for the MJO. Phase 1 extends from Africa to the Indian Ocean, Phases 2 and 3 cross the Indian Ocean, Phases 4 and 5 represent the Maritime Continent (Southeast Asia), Phases 6 and 7 extend from the Western Pacific to the International Date Line, and Phase 8 represents the Eastern Pacific to Africa. The MJO phase diagram below shows the location of the MJO and illustrates its relative strength. Inside the circle, the MJO is very weak or inactive and outside the circle the MJO is strong. See also: Africa; Asia; Indian Ocean; International Date Line; Pacific Ocean
In relation to tropical cyclone formation, Phases 1 and 2 are associated with a favorable MJO state for increased Atlantic activity and decreased Pacific activity, while Phases 6 and 7 are associated with decreased Atlantic activity and increased Pacific activity. For example, during the week of September 4–10, 2013, which corresponded to the Phase 1-to-Phase 2 portion of the MJO diagram, tropical cyclones Gabrielle and Humberto formed in the Atlantic.