Evolution of Extreme Madden-Julian Oscillation Events and their Impacts on South America

TL;DR

This paper investigates the evolution of extreme Madden-Julian Oscillation events, their seasonal and ENSO phase modulation, and their significant impacts on South American climate, especially rainfall patterns, during the austral summer.

Contribution

It provides a detailed analysis of the characteristics, initiation phases, and impacts of extreme MJO events and how ENSO phases modulate their duration and influence on regional climate.

Findings

Extreme MJO events often initiate in phases 2-3 and 8-1.

Extreme events during La Niña last longer than during El Niño.

Extreme MJO events cause more intense and widespread rainfall anomalies.

Abstract

This study examines the evolution of extreme Madden-Julian Oscillation (MJO) events and their impacts on South America during the austral summer. Furthermore, the study explores how the different ENSO phases modulate extreme MJO events, and how the combined effects impact South American climate. Extreme MJO events are defined as those exceeding a specific threshold based on the events distribution, distinguishing them from weak events. Our analysis shows 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. This distribution is also characteristic of winter, while in summer, initiation is more balanced between phases 2-3 and 8-1. 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. Seasonally, weak event initiation prevails in phases 4-5 during summer, while in winter, it is evenly distributed between phases 8-1 and 4-5. Additionally, during La Ni\~na, extreme events tend to last longer than during El Ni\~no, a pattern not observed in weak events. A composite analysis of outgoing longwave radiation (OLR), eddy streamfunction, and velocity potential was conducted, with particular focus on the initiation phases 2-3 and 6-7 to determine which phases result in the most significant impacts and how the associated anomalies evolve. The findings show that enhanced (suppressed) convection centers in the equatorial region during extreme events are more intense and exhibit a southeastern displacement compared to those during weak events. These extreme MJO events influence the South American rainfall Dipole (SAD), a key feature of regional climate variability and results show that extreme MJO events induce more intense rainfall anomalies of larger spatial extent compared to weak events.