Pathways of Ongoing Aridification in Southwest North America

Southwest North America is projected by climate models to undergo aridification, defined here as reduced soil moisture, during the coming decades. Here we use an atmosphere model forced by plausible SST scenarios to examine the mechanisms of aridification that connect, across seasons, the ocean, atmosphere and land surface. Results emphasize the role of precipitation throughout the year for soil moisture decline. The worst-case scenario is a cool tropical Pacific and warm tropical North Atlantic which reduce cool season precipitation that reduces soil moisture. Drier soils propagate into summer reducing evapotranspiration (ET) and allowing partial soil moisture recovery. In the best-case, the opposite ocean conditions increase cool season precipitation but higher ET prevents this from increasing summer soil moisture. In these scenarios atmospheric humidity is controlled by soil moisture: drier soils reduce ET which reduces humidity and increased Vapor Pressure Deficit (VPD). This is the opposite causality of the “atmosphere warming increases VPD and dries soils” that is often invoked for summer. In addition, radiative forcing reduces summer and fall precipitation such that soils enter winter drier and therefore remain drier than before even if cool season precipitation increases. It is hard to imagine a scenario for the next few decades that is other than further aridification of the southwest.

Richard Seager is the Ewing Lamont Research Professor at the Lamont Doherty Earth Observatory of Columbia University in Palisades, New York. After undergraduate studies in England he gained his Ph.D at Columbia in 1990. Then, after a postdoc at the University of Washington, he returned to Lamont for good. His work studies ocean-atmosphere coupling and its connections to the global hydrological cycle on timescales of weeks to millennia. In a paper in Science in 2007, Richard and a Lamont-GFDL team predicted the progressive aridification of the America West, work he continues today. He has been a long-term member of the NOAA Drought Task Force and led its 2015 study of the multiyear California drought. Richard is a Fellow of the AMS and AGU and the 2024 recipient of the AMS Charney Medal.