Spatial and Seasonal Variations in Aridification across Southwest North America

Southwestern North America (SWNA) is projected to become drier in the twenty-first century as both precipitation (P) and evaporation (E) rates change with increasing greenhouse gas concentration. The authors diagnose the relative contributions of changes in P and E to the local surface moisture balance (P - E) in cold and warm halves of the year across SWNA. Trends in P - E vary spatially between the arid southern subregion (mostly northern Mexico) and the more temperate northern subregion (southwest United States), although both subregions exhibit a negative trend in P - E (trending toward more arid conditions) in CMIP5 projections for the twenty-first century. The P - E trend is biggest in the cold season, when much of the base flow to rivers in the southwest United States is generated. The downward trend in cold season P - E across SWNA is caused primarily by increasing E in the north and decreasing P in the south. Decreasing P is the primary contributor to modest warm season drying trends in both northern and southern subregions. Also, P accounts for most of the interannual variability in SWNA P - E and is strongly correlated with modes of oceanic natural variability during the cold season. SWNA aridification is therefore most readily distinguished from the region's large natural climate variability in the cold season in the northern subregion, where the projected temperature-driven increase in E is greater than the projected decrease in P.