Near the equator, weather is more chaotic and less structured than at midlatitudes. At midlatitudes, large cyclones and, sometimes, attached frontal boundaries, govern weather patterns. However, the formation of the vortices of these cyclones requires an effect called the Coriolis effect. The Coriolis effect, in essence, induces rotation in midlatitude regions because the rotation of the Earth carries points at different latitudes at different speeds. But at the equator and the area immediately around it, this effect is not significant enough to cause rotation under usual circumstances. Thus weather patterns in the tropics are rather caused by undulations of the Intertropical Convergence Zone (ITCZ), which seemed to produce shower and thunderstorm activity randomly.
However, some evidence contrary to the idea that the fluctuations were random has been known since antiquity. India's weather, for example, includes clearly demarcated dry and wet seasons in association with the monsoon, despite the fact that southern India's weather is predominantly affected by the activity of the ITCZ. In the 1970's, when more meteorological data became available, scientists Roland Madden and Paul Julian, after whom the oscillation is named, noted that there was a recurring cycle between increased precipitation and suppressed precipitation in the tropics, particularly notable in the Indian Ocean and the Pacific Ocean. More data emerged over the coming years, revealing that the anomalies in tropical precipitation tended to follow the equator, moving east around the globe over time.
The areas of increased and decreased thunderstorm activity associated with the MJO span several thousand miles; in areas of increased activity, the MJO is said to be in positive phase, and in areas of suppressed activity, in negative phase. Each "phase" propagates eastward around the globe at 4-8 meters per second, and therefore takes 30-60 days to travel around the Earth. This time interval, though not an exact value, is the period of the MJO, or in other words, the time necessary for the precipitation anomalies to return to roughly their initial conditions. At any given time, there are about 1-2 areas of increased convection and 1-2 areas of decreased convection that span the tropics.
The above water vapor imagery from the NOAA shows a dry air mass over much of the central Atlantic and Caribbean. The increased tendency for dry air masses to appear in an area of the tropics is thought to correlate with the negative phase of the MJO.
The MJO, as well as governing the onset of the monsoon in India and other locations, can also have an effect on tropical development, even in the Atlantic and Pacific basins. Dry air is often devastating to tropical cyclones, invading their circulations and weakening or even dissipating them. Though some parts of the typical Atlantic and Pacific hurricane seasons are more active then others, in any particular season, the tropical cyclone activity may modulate with the MJO, increasing during positive phase and decreasing in negative phase. In support of this theory, it is a well documented fact that the Atlantic basin tends to be quiet when the Eastern Pacific basin is active and vice versa. This is because these neighboring basins tend to be experiencing opposite phases of the MJO at any given time.
The study and modeling of the MJO is very important to meteorology becuase this oscillation brings order to the chaos of tropical weather. Greater understanding of the MJO will aid long-term and large-scale forecasts, and improve our ability to antipicate tropical cyclonogenesis.
Sources: http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/mjo.html, http://maddenjulianconversation.blogspot.com/2011/08/what-is-mjo-part-1.html, http://www.met.reading.ac.uk/~pete/mjo.html, http://en.wikipedia.org/wiki/Madden%E2%80%93Julian_oscillation, https://www2.ucar.edu/sites/default/files/news/2011/MJO_illus.jpg
