MJO Tracker — Sky Islands Climate

How to read this diagram

The Wheeler-Hendon diagram tracks a weather pattern called the Madden-Julian Oscillation (MJO) — a pulse of enhanced cloudiness and rainfall that slowly circles the globe through the tropics every 30 to 90 days. The diagram is divided into eight phases based on where that pulse is located geographically. The red dot shows today's position. The trail behind it shows the past 40 days of movement.

Distance from the center matters: the further the dot is from the center, the stronger the MJO signal and the more likely it is to influence weather here. Dots inside the dashed circle indicate a weak or disorganized MJO with little predictive value. The MJO typically moves counterclockwise through the phases over several weeks.

The dot's position comes from two numbers, RMM1 and RMM2, which combine satellite-measured cloudiness with upper- and lower-level tropical winds into a single daily index (the Real-time Multivariate MJO index, or RMM). RMM1 is the dot's left-right position and RMM2 its up-down position; the phase is the slice of the diagram the dot sits in, and the amplitude is its distance from the center. An amplitude of 1.0 or more is the usual cutoff for an active MJO.

For southern Arizona and the Sky Islands, phases 1 and 2 tend to favor monsoon rainfall during summer, while phases 6 and 7 tend to favor Pacific winter storms. The MJO is one piece of the puzzle — it shifts the odds but doesn't guarantee any particular outcome.