Simultaneous Megapluvials in Southwestern North and South America During the Last Millennium

Earth's climate has experienced prolonged hydroclimatic extreme events that can manifest decadal or longer anomalies. Such events can shape ecological systems and human societies. Decadal‐scale droughts, megadroughts, occurred in southwestern North and South America (North American Southwest, NASW; South American Southwest, SASW) over the past millennium, including simultaneous events. Similarly, these regions experienced prolonged wet periods, megapluvials, including well‐documented episodes over the 20th century. Using a paleoclimate data assimilation product, we identify 18 megapluvials in each region (12 overlapping), 13 NASW and 15 SASW megadroughts (9 overlapping). Both phenomena show similar duration and severity. We find that megapluvials in both regions are driven by a reduction in drying La Niña‐like states, not solely by an increase in wetting El Niño‐like states; while both changes are statistically significant, the decrease in La Niña‐like conditions is stronger than the increase in El Niño‐like conditions. Megadroughts exhibit an analogous asymmetric mechanism: an increase in La Niña‐like states and a stronger reduction in El Niño‐like states. We also find volcanic forcing influences these events through the El Niño‐Southern Oscillation (ENSO): large eruptions reduce the frequency of La Niña‐like states, causing overall wetting. This mechanism is most clearly seen in the SASW where ENSO teleconnections are stronger in the paleoclimate reconstruction. These findings demonstrate that megapluvials exhibit interhemispheric synchronization, similar to megadroughts, and are similarly influenced by Pacific variability on decadal timescales. Our results highlight the need for better understanding and representation of ENSO's response to external forcing to improve projections of decadal hydroclimate variability in the NASW and SASW.