Madden Julian Oscillation Update: Influence on La Nina and Potential SSW Events

Climate Impact Company Madden Julian Oscillation Outlook

Issued: Wednesday, October 29, 2025

Highlight: Madden Julian oscillation is active and important to follow…influence on La Nina (strength) and possible SSW trigger.

Fig. 1: The ECM 30-day MJO phase forecast.

Discussion: The strongest convection phase of the Madden Julian oscillation (MJO) in several months has emerged during the past 2 weeks shifting across the Indian Ocean and helping to inspire tropical cyclone activity while recently reaching eastward to Maritime Continent (Fig. 1). The ECM 30-day forecast reveals some weakening shifting toward the West Pacific in the short-term with re-emergence in the West Pacific for mid-to-late November. MJO in this position causes the southern oscillation index (SOI) to shift to positive phase (Fig. 2) as sea level pressure lowers at Darwin and increases at Tahiti. +SOI inspires trade winds which up-wells cool water to make the Nino34 sea surface temperature anomaly (SSTA) cooler. Currently, the Nino34 SSTA is marginally cool (weak La Nina). Interestingly, the subsurface equatorial East Pacific has about 1/3 of the cool anomaly strength of the La Nina episode from late last year (Fig. 3). Consequently, the strength potential of La Nina 2025-26 is minimal including influence on climate. The SOI 15-day forecast maintains positive phase (Fig. 4) suggesting some marginal strengthening of the weak cold ENSO episode. The strengthening of the MJO in the tropical West Pacific during mid-to-late November is important. In that position, a strong MJO can create warming generated by the attendant convection traveling poleward from the tropical atmosphere via a Rossby Wave into the polar stratosphere causing sudden stratospheric warming (SSW) events. This process has a better chance of occurrence during the easterly phase of the quasi-biennial oscillation (QBO) which has recently emerged. SSW events lead to a split in the polar vortex, and the southward travel increases risk of mid-latitude cold and snow.

Fig. 2-4: The daily southern oscillation index observations, upper ocean heat anomalies across the equatorial Pacific to the east of the Dateline, and 15-day SOI forecast.