The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropics: a pulse of enhanced tropical convection (clouds and rainfall) that propagates eastward along the equator every 30-60 days, at a speed of about 5 m/s. It was discovered by Roland Madden and Paul Julian in 1971.
Structure and propagation
The MJO consists of a region of active convection (cloudy skies, heavy rainfall, convergent surface winds) followed by a region of suppressed convection (clear skies, subsidence). This "dipole" propagates from the Indian Ocean to the western Pacific in about 2-4 weeks. Upon reaching the central Pacific, convection weakens but the dynamic signal continues circumnavigating the globe.
The MJO modulates numerous meteorological and climatic phenomena globally: it strengthens or weakens the Asian monsoons, influences tropical cyclone genesis (more hurricanes during the active phase), affects rainfall in East Africa and the Americas, and can modify the teleconnection patterns that determine mid-latitude weather.
For Europe and Spain, the MJO has an indirect but significant influence: certain MJO phases are associated with anticyclonic blocking or with intensified Atlantic storm tracks. It is a key tool for subseasonal prediction (15-45 days). See also: El Nino, teleconnection.