North Pacific ocean–atmosphere responses to Holocene and future warming drive Southwest US drought

The Southwest United States is experiencing severe and persistent drought, although uncertainties regarding the causes limit our ability to predict changes in water availability. The severity of the current drought has been attributed to a combination of warming and natural changes in atmospheric circulation, suggesting that current rainfall deficits may improve as natural oscillations reverse sign. Here we use new leaf-wax stable isotope reconstructions and simulations for the mid-Holocene (6 thousand years ago) and demonstrate that moderate warming of the Northern Hemisphere can produce drought over the Southwest United States through an ocean–atmosphere response originating in the North Pacific. The patterns of ocean warming and rainfall change resemble the negative phase of the Pacific Decadal Oscillation, indicating that this mode can be excited by external forcings. A similar response to warming is evident in future projections, leading to sustained winter precipitation deficits through the mid-twenty-first century. However, the magnitudes of past and current precipitation deficits associated with this North Pacific response are systematically underestimated in models, possibly due to a weak coupling of ocean–atmosphere interactions. Projections may also underestimate the magnitude of this precipitation response to changes in the North Pacific, leading to greater drought risk in this already water-poor region.