Annual precipitation over the Southwestern United States (SWUS) has been decreasing since 1980. During this period, the tropical Pacific sea surface temperature (SST) has a multi-decadal cooling tendency, inducing teleconnections that led to decreasing SWUS precipitation. However, it is still uncertain whether this SST trend is primarily due to internal variability (i.e., natural fluctuations) or anthropogenic radiative forcings. In this study, we use a low frequency component analysis (LFCA) on SSTs from observations, the all-forcing Community Earth System Model version 2 large ensemble (CESM2-LE), and two simulations from CESM2 single forcing large ensemble (CESM2-SFLE). In one CESM2-SFLE ensemble anthropogenic aerosols forcing held fixed and in another only anthropogenic aerosols evolving, we isolate effects from anthropogenic aerosols with these simulations. We find that up to 22% of the SWUS precipitation trend is forced by anthropogenic aerosols via Pacific multi-decadal variability. To more fully sample the role of internal variability in Pacific SSTs, we use a linear inverse model (LIM) to generate SST realizations based on observed internal variability. We perform idealized model experiments with different LIM-generated tropical SSTs and historical emissions as boundary conditions. Our modeling results show that tropical SSTs only moderately influence SWUS precipitation. Instead, there is a tendency for decreasing SWUS precipitation, regardless of the tropical multi-decadal SST trend (El Niño-like vs. La Niña-like). These findings emphasize the impact of anthropogenic aerosols on SWUS precipitation. Our findings imply that near-future SWUS water resources may be subject to human emissions not just of greenhouse gases but also aerosols.