Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest

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.