MJO as a fluctuation of the monsoon
Over the wettest parts of the tropics, the climate tends to be monsoonal. This means two things:
1. There is a wet season and a dry season,
2. The wind blows in different directions in the two seasons. Westerly (from the west to the east) in the rainy season, and easterly (from the east to the west) in the dry season.
In monsoonal climates, most of the rain that falls in the entire year falls in the wet season; the dry season tends to be almost completely dry. Temperature, on the other hand, doesn’t differ all that much between the seasons. This is the opposite of what happens in some midlatitude climates. For example, in New York City, the temperature is tremendously different in summer and winter, but the amount of precipitation is pretty similar in the two seasons (though some of it is snow in the winter). Another difference is when it is hottest or coldest. In monsoon climates, the rainy season comes at what would be the peak of summer, when the sun is high and you would think it would be hottest; but all the clouds and rain cool things down. The hottest time is actually the late dry season, just before the monsoon, which would be “spring” at higher latitudes.
We think of the monsoons as occurring over land. India in particular is the most famous monsoon region. The Indian monsoon is not just India’s, but extends into southeast Asia as well. Northern Australia and northern South America also have monsoons, as does West Africa, and (arguably) southwestern north America. But the land monsoons are not the whole story. Actually the climate is monsoonal over large regions of tropical ocean also. The tropical Indian ocean experiences the monsoon just as much as India itself does, and so does the tropical western Pacific (say, from Asia to the date line, within 10 or 20 degrees of the equator). By this we mean that over these parts of the oceans, there is a season when it rains a lot and a season when it doesn’t, and the wind shifts from easterly in the dry season to westerly in the rainy season.
Just like summers and winters at higher latitudes, monsoon seasons aren’t exactly the same from one year to the next. Some wet seasons are wetter and some drier than others. Some start earlier, or later; or end earlier or later. Once the rains do start, they may keep going for a long time, but on the other hand they may stop and start. Sometimes there are “breaks” in the middle of monsoon season, during which it may rain little or not at all.
While rainy periods and breaks don’t come and go with complete regularity, there is some rhythm to them. The weather tends to switch from rainy to dry a few times over the course of a rainy season. This is what we mean by “intraseasonal” variability. The period of “intraseasonal oscillations” – the time for one complete cycle to complete itself - is sometimes quoted as 40-50 or 30-60 days, but it’s just as well to think of it as “a few times in a season”. These intraseasonal oscillations affect the land monsoon regions, but are actually strongest over the oceans.
What Roland Madden and Paul Julian discovered in the early 1970s is not just that these active and break periods occur over the oceans (to people living on the islands or adjacent continents this surely was known already) but that there are connections between all the active and break cycles occurring in different places. The activeness or breakness – the “phase”, in jargon – occupies a large chunk of real estate at any given time, but moves at a leisurely pace from west to east. (In northern hemisphere summer, the Indian and southeast Asian monsoon season, it can also move from south to north.) This is the MJO – the eastward-propagating, intraseasonal variability of the monsoons, especially (but not only) over the oceans.