seasonmap field guide
What is the MJO?
The Madden–Julian Oscillation (MJO) is the dominant mode of intraseasonal tropical variability: an envelope of enhanced convection that circles the equator every 30–60 days and is one of the few reliable sources of US forecast skill one to three weeks out. This page covers how the envelope moves, how to read the 8-phase RMM diagram, and its documented CONUS impacts.
The convective envelope
Identified by Roland Madden and Paul Julian in 1971, the MJO is a region of enhanced thunderstorm activity thousands of kilometers wide, paired with a suppressed-convection region on its flanks. Unlike El Niño, which sits in place for months, the envelope moves — eastward along the equator at roughly 5 m/s, from the Indian Ocean across the Maritime Continent into the western Pacific, before it weakens over cooler eastern-Pacific water and the cycle restarts.
The heating anomaly excites Rossby waves that propagate through the jet stream and into the extratropics over one to three weeks. This is the mechanism by which a convective cluster near Indonesia is followed, at that lag, by a pattern change over North America.
Reading the phase diagram
The standard tracking tool is the Wheeler–Hendon RMM index: two numbers (RMM1, RMM2) derived from satellite outgoing longwave radiation and upper- and lower-level zonal winds, plotted as a point on a circular diagram split into 8 phases. Each phase marks where the enhanced convection currently sits:
| Phases | Convection over |
|---|---|
| 2–3 | Indian Ocean |
| 4–5 | Maritime Continent (Indonesia) |
| 6–7 | Western Pacific |
| 8–1 | Western Hemisphere / Africa |
Two values determine how to read the diagram. Distance from center is amplitude — below 1 (inside the unit circle) there is no coherent event and the phase number carries little signal. Direction of travel is phase progression: a well-organized event moves counterclockwise around the diagram, completing a circuit in 30–60 days.
CONUS impacts by phase
Teleconnections arrive one to three weeks after the convective phase is observed, which is what makes the MJO a source of week-2 and week-3 forecast skill. The associations are strongest in winter:
- Phases 8–1 (strong), winter: the canonical cold signal for the central and eastern US — ridging toward Alaska, troughing downstream. Several documented eastern cold outbreaks have followed a strong phase-8 passage by one to two weeks.
- Phases 4–6, winter: the opposite lean — Pacific air floods the CONUS and the east tilts mild.
- Warm season: the amplitude is smaller but the signal persists — MJO phase modulates Gulf moisture feeds, monsoon bursts, and windows of Atlantic hurricane activity, favored when the enhanced-convection phase sits in the Western Hemisphere.
These are probability shifts, not certainties. A strong ENSO base state or a stratospheric sudden warming can dominate the MJO signal at the surface; week-2 outlooks weigh all three together.
The Maritime Continent barrier
Events frequently weaken or stall crossing Indonesia's islands and mountainous terrain, a documented feature known as the Maritime Continent barrier; forecast models have historically dissipated the MJO there too readily compared to observations. Whether an event crosses to the West Pacific (phases 6–7) or decays at the barrier is often the deciding factor in a two-week outlook.
Madden, R. A., & Julian, P. R. (1971). Detection of a 40–50 day oscillation in the zonal wind in the tropical Pacific. J. Atmos. Sci., 28, 702–708.
Wheeler, M. C., & Hendon, H. H. (2004). An all-season real-time multivariate MJO index. Mon. Wea. Rev., 132, 1917–1932.
Zhang, C. (2005). Madden–Julian Oscillation. Rev. Geophys., 43, RG2003.