Recent southwestern US drought exacerbated by anthropogenic aerosols and tropical ocean warming

The southwestern United States is currently in a multi-decade drought that has developed since a precipitation maximum in the 1980s. While anthropogenic warming has made the drought more severe, it is the decline in winter–spring precipitation that has had a more profound effect on water resources and ecosystems. This precipitation decline is not well understood beyond its attribution to the post-1980 La Niña-like cooling trend in tropical sea surface temperatures, which caused a North Pacific anti-cyclonic atmospheric circulation trend conducive to declining precipitation in the southwestern United States. Using a hierarchy of model simulations, we show that, even under El Niño-like sea surface temperature trends, there is a tendency towards a North Pacific anti-cyclonic circulation trend and declining precipitation in the southwestern United States, counter to the canonical El Niño teleconnection. This unintuitive yet robust circulation change arises from non-additive responses to tropical mean sea surface temperature warming and radiative effects from anthropogenic aerosols. The post-1980 period exhibits the fastest southwestern US soil moisture drying among past and future periods of similar length due to the combination of this forced precipitation decline and anthropogenic warming. While the precipitation trend might reverse due to future projected El Niño-like warming and aerosol emissions reduction, it is unlikely to substantially alleviate the currently projected future drought risk. Climate model simulations suggest that both anthropogenic aerosols and tropical ocean warming have contributed to reduced precipitation over the southwestern United States in recent decades, thus making the current drought more likely than previously thought.