Madden–Julian oscillation

The MJO is an intraseasonal fluctuation in tropical clouds, rainfall, winds and pressure. Its characteristic pattern includes an area of enhanced convection and rainfall alongside a region of suppressed convection; the paired pattern propagates eastward through the tropics. The Australian Bureau of Meteorology describes it as a pulse of cloud and rainfall near the equator that typically recurs every 30 to 60 days. Unlike the more persistent, largely stationary tropical Pacific pattern associated with El Niño and La Niña, the MJO moves, and more than one episode may occur in a season.

Operational monitoring commonly divides the MJO's path into eight broad geographic phases around the equator. These phase labels describe the approximate location of the disturbance, not eight separate physical causes. One monitoring approach combines information about cloudiness and winds in the lower and upper atmosphere to estimate the pattern's location and strength. When the signal is weak, its phase and expected influence are less distinct. The MJO's effects also vary by season and region, so phase-based forecasts indicate shifts in probabilities rather than certainty about local weather.

By shifting tropical convection and circulation, the MJO can alter the likelihood or timing of rainfall and influence monsoon activity and tropical cyclone conditions. Its atmospheric effects can also extend beyond the tropics, where circulation changes may affect temperature, storms or precipitation. These are statistical influences, not guarantees that a particular event will occur: local conditions and other climate patterns also matter. Its week-to-week evolution makes the MJO useful in extended-range weather outlooks, between short-range forecasts and seasonal climate predictions.

Atmospheric scientists Roland Madden and Paul Julian identified the oscillation in the early 1970s while examining tropical wind and pressure records. Its broad observed behavior is well established, but the processes that initiate, sustain and move it are complex. Research reviews compare multiple theories, including mechanisms involving moisture, convection, atmospheric circulation and waves; no single simple explanation captures every observed feature.