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

Decadal-scale droughts, known as megadroughts, occurred repeatedly in the North and South American Southwest (NASW and 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, with 122 years of simultaneous megapluvial conditions and 113 years of simultaneous megadroughts. We find that megapluvials in both regions are driven by a reduction in drying La Niña-like states and 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 greater than the increase in El Niño-like conditions. Megadroughts exhibit an analogous asymmetric mechanism: they are characterized by increased La Niña-like states accompanied by a stronger reduction in wetting 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 that is 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, including anthropogenic climate change, to improve projections of decadal hydroclimate variability in the NASW and SASW.