External and internal controls on decadal precipitation variability over North America

Precipitation variability across North America (NA) substantially impacts regional water security, agricultural productivity, ecosystem stability, and the frequency of extreme climatic events. Variations in annual precipitation play a critical role in drought occurrence and dryland expansion. The southwestern NA, a typical semi-arid region, has experienced rising agricultural and industrial water demands in recent decades, increasing its vulnerability to droughts. Since 1980, this region has grown drier, intensifying risks of moisture deficits and wildfires. Previous studies have identified both anthropogenic forcing and internal variability as key factors driving NA precipitation changes. External forcing from greenhouse gas and aerosol emissions has influenced regional precipitation patterns, while internal variability associated with large-scale teleconnection patterns plays a crucial role in modulating these changes, particularly on interannual to decadal timescales. However, most studies have focused on either external forcing or internal variability in specific NA regions, neglecting their combined effects across the entire continent.

Here, we combine long-term observational data and CMIP6 simulations to find the distinct roles of anthropogenic forcing and low-frequency internal variability. Our results identify a long-term wetting trend primarily driven by greenhouse gas forcing, though state-of-the-art climate models tend to underestimate the influence of external forcing on NA precipitation. Decadal precipitation oscillations are modulated by internal variability, especially the Interdecadal Pacific Oscillation (IPO), whose sea surface temperature anomalies trigger large-scale Rossby waves. The wave train originating from the Pacific propagates downstream, influencing atmospheric circulation and moisture transport, ultimately shaping the tripolar precipitation pattern observed in NA. Climate model simulations confirm that the impact of the Atlantic Multidecadal Oscillation (AMO) on NA precipitation is significantly weaker than that of the IPO. This tripolar precipitation pattern dominates NA precipitation variability at decadal scales, surpassing anthropogenic influences. From 2021 to 2050, the tripolar pattern is projected to persist, contingent on IPO phase. By 2100, constrained projections under the SSP2-4.5 and SSP5-8.5 scenarios suggest a further intensification of precipitation increases. This study shows how NA rainfall responds differently to human influence and natural oscillations over decades, with implications for improving our ability to predict and attribute regional climate changes.