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Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest
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Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest
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Journal Article
Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest
2023
Overview
The winter‐spring precipitation over the Southwestern United States (SWUS) decreased since 1980. It is frequently attributed to Pacific internal decadal variability, but recent studies found anthropogenic aerosols (AA) can also induce a transition to a negative Pacific Decadal Variability (PDV) phase. We revisit the attribution of SWUS drying by quantifying the contributions of anthropogenically forced decadal Pacific Sea Surface Temperatures (SSTs). Applying a low‐frequency component analysis to observations, Community Earth System Model version 2 (CESM2) all‐forcings and single‐forcing large ensembles, we find up to 42% of the observed precipitation trend to be related to the AA‐induced negative PDV‐like pattern, which is driven by the emission shift from the Western to the Eastern Hemisphere. In CESM2, other radiative forcings counteract the influence of AA, but it remains unclear whether the model correctly simulates this balance. This implies that the near‐future trajectories of these forcings, in particular Asian aerosols, are important for projections of SWUS precipitation. Plain Language Summary Water resources of the Southwestern United States (SWUS) rely on winter‐spring precipitation, which has been declining since 1980. To understand the reasons for the decline, we evaluate the impacts of human‐caused Pacific Sea Surface Temperature (SST) changes on SWUS precipitation. We use observations and climate model experiments together with statistical approaches. We find evidence that the shift of aerosol emissions from the Western to the Eastern Hemisphere induced a change in Pacific SSTs that in turn favors a winter‐spring SWUS precipitation decline. Additionally, we showed that other human‐caused factors, such as greenhouse gases, can offset the impact of aerosols. This means that the near‐future SWUS precipitation change depends on the trajectories and the interactions of these various human‐caused factors. Key Points We quantify the role of anthropogenic aerosols in post‐1980 Pacific sea surface temperatures and the U.S. Southwest precipitation decline Internal variability dominates the winter‐spring precipitation trend but aerosols contribute to it through their influence on Pacific SSTs Community Earth System Model version 2 simulates a tug‐of‐war between aerosols and other forcings (e.g., greenhouse gases) and its response to aerosols might be too weak
