MJO phases

• Phases follow the Wheeler–Hendon RMM convention: 1 (western Indian Ocean) → 2–3 (Indian Ocean) → 4–5 (Maritime Continent) → 6–7 (western Pacific) → 8 (Western Hemisphere / Africa). Enhanced tropical convection sits in the named region; suppressed convection sits opposite.
• The index is NOAA PSL's OMI (OLR MJO Index, Kiladis et al. 2014) mapped to RMM space (RMM1 = PC2, RMM2 = −PC1), 1979 → two months ago, with the real-time ROMI (CPC OLR) tail for the latest weeks.
• Days with amplitude < 1 are “inactive” and form their own bin.
• OMI vs VPM. The VPM switch uses NOAA PSL's velocity-potential MJO index (Ventrice et al. 2013; RMM1 = −PC1, RMM2 = PC2), the index behind the GSDM composite engine. The two indices track the same oscillation but disagree on the phase label on ~60 % of active days (VPM tends to see the upper-level circulation, OMI the convection), so composites differ in detail. VPM runs 1979 → March 2026 on NCEP/NCAR Reanalysis 1, which was then discontinued; the latest months come from NOAA PSL's CORe-reanalysis VPM, a daily-updated recalculation of the same index (r ≈ 0.97 with the original).

Statistics

• Daily anomalies vs a 31-day-smoothed 1991–2020 daily climatology. Composite maps (temperature, precipitation, heights) and the pin series (toggle) are linearly detrended per grid cell over the full record so the warming trend is not read as an MJO signal; OLR is not detrended.
• Lag: the response is read lag days after the phase day. Mid-latitude responses often peak 5–15 days after the tropical forcing.
• Terciles are computed from every day in the season. Odds are shares of phase days in the top / bottom third. With ~40 years × ~15 days per phase per season, bins hold 200–700 days but only ~40–140 independent samples.

Reading advice

The MJO is one modulator among many. Signals are strongest in the tropics and the winter Pacific–North American sector, and usually modest (a 40–45 % above-normal share against a 33 % baseline). Amplitude and propagation matter: a strong, propagating MJO is more likely to deliver the composite response than a weak or stalled one. Combine with the ENSO Explorer, since ENSO alters the MJO's teleconnections.

Data: ERA5 © ECMWF / C3S; OLR and OMI © NOAA PSL.