Evolution and Impact of Extreme and Weak Madden–Julian Oscillation Events on Precipitation Over South America

ABSTRACT We examine the evolution of extreme Madden–Julian oscillation (MJO) events, how the different ENSO phases modulate extreme MJO events, and how their combination impacts South American climate. Extreme MJO events are defined as those exceeding a specific threshold based on the events distribution, distinguishing them from weak events. We show that extreme MJO events most frequently initiate in phases 2–3 throughout the year, with similar distributions across phases 8–1, 6–7 and 4–5. In contrast, weak events predominantly start in phases 4–5 year‐round, followed by phases 2–3, with phases 8–1 and 6–7 occurring at similar frequencies. Additionally, during La Niña, extreme events tend to last longer than during El Niño, a pattern not observed in weak events. A composite analysis of outgoing longwave radiation, rainfall, vertically integrated water vapour flux, eddy streamfunction and velocity potential was conducted to determine how the associated anomalies evolve. The findings show that enhanced (suppressed) convection centres in the equatorial region during extreme events are more intense compared to those during weak events. Moreover, hemispheric and regional circulation anomalies are stronger during extreme MJO events influencing the South American rainfall Dipole (SAD), a key feature of regional climate variability, resulting in more intense rainfall anomalies and of larger spatial extent compared to weak events. This is because extreme events are characterised by tropical–tropical and tropical–extratropical teleconnections, whereas weak events primarily feature tropical–tropical teleconnections. Moreover, El Niño increases the persistence of the SAD during extreme events starting in phases 2–3. We also found that rainfall anomalies develop over some regions of South America irrespective of the initiation phase, while in other regions this is not the case, suggesting larger sub‐seasonal predictability in the former. Finally, a large increase in extreme MJO events during the last decade was found, mainly due to their occurrence during El Niño years.