Evidence of Anthropogenic Aerosols Impacts on the Southwestern U.S. Droughts since 1980

The freshwater resources of the Southwestern United States (SWUS) largely depend on wintertime precipitation, which has declined since 1980. During this period, the tropical Pacific sea surface temperature (SST) has exhibited a multi-decadal La Niña-like trend, inducing a teleconnection that reduces SWUS precipitation. However, the extent to which this SST trend is driven by internal variability (i.e., natural fluctuations) versus anthropogenic radiative forcings remains uncertain. This study explores two aspects of the impact of anthropogenic aerosols on the Pacific sector and their influence on the wintertime SWUS precipitation trend. First, we demonstrate that the La Niña-like SST trend is partially forced by anthropogenic aerosols, as evidenced by simulations from the Community Earth System Model version 2 (CESM2) large ensemble and its single-forcing large ensemble. This forced La Niña-like SST trend, in turn, drives a decline in SWUS precipitation through its teleconnection. Second, we show that the teleconnection pattern associated with internal Pacific decadal variability during the post-1980 period differs from the pre-industrial condition. Specifically, using a hierarchy of model simulations, we find that even under El Niño-like SST trends, there is a tendency toward a North Pacific anticyclonic circulation trend and reduced SWUS precipitation during post-1980 — contrary to the canonical El Niño teleconnection. This unintuitive yet robust circulation change arises from nonadditive responses to tropical mean warming and radiative effects from anthropogenic aerosols. As the forced SWUS precipitation decline combines with anthropogenic warming, the post-1980 period exhibits the most rapid SWUS soil moisture drying among past and future periods of similar length. Although future projected El Niño-like warming and aerosol emission reductions could potentially reverse the precipitation trend, these changes are unlikely to mitigate the currently projected drought risk in the region.