Interdecadal Pacific oscillation phase dominants near-term global monsoon precipitation changes

Abstract Precipitation is projected to increase in response to human-induced global warming over global monsoon region, with significant implications for water resources in these densely populated areas. While most climate models agree on the externally forced wetting signal in global monsoon precipitation (GMP), the chaotic nature of internal variability introduces substantial uncertainty in future projections, particularly in the near term. In this study, we utilize multi-model simulations and four sets of large-ensemble simulations to investigate the role of internal variability, particularly the interdecadal Pacific Oscillation (IPO), in influencing uncertainty in future monsoon precipitation changes. We find that the externally forced wetting trend in GMP surpasses the range of internal variability only after the 2040s under both the SSP2-4.5 and SSP5-8.5 scenarios. Prior to this, internal variability contributes over 40% of the uncertainty in GMP projections, with the IPO accounting for nearly half of this internally induced uncertainty. To inform adaptation and mitigation efforts, we further propose near-term storylines of monsoon precipitation changes at both global and regional scales. Specifically, a transition from a negative to a positive phase of the IPO at a rate of +1 SD per decade during 2021–2040 leads to a decrease in GMP by −0.41% (−0.23%) per decade relative to 1995–2014 under the SSP2-4.5 (SSP5-8.5) scenario, while the reverse transition leads to an increase of 1.35% (1.53%) per decade. These findings underscore the need to account for the impacts of the IPO in near-term climate change planning for monsoon-affected regions.