Spatial and Seasonal Variations in Aridification Across Southwest North America

The Southwestern region of North America (SWNA) is projected to become more arid throughout the 21st century as precipitation (P) and evaporation (E) rates change with increasing global temperatures. Trends in the regional surface moisture balance (P−E) of SWNA vary between the arid southern subregion (mostly Northern Mexico) and the more temperate northern subregion (Southwest US), depending on the relative contributions of cold and warm season P and E. Interpreting the spatial and seasonal variations of the projected aridification is important for understanding the potential impacts of climate change on SWNA regional hydrology. We compare the projected trends in P, E, and P−E in CMIP5 global climate model projections, for the northern and southern subregions of SWNA between the extended cold and warm seasons. Both subregions become drier (i.e., negative trend in P−E) in the 21st century. The drying trend is biggest in the cold season surface moisture balance, which contributes much of the base flow to rivers in the Southwest US. We show that the downward trend in the cold season P−E is caused primarily by increasing E in the northern subregion, and by decreasing P in the southern subregion. Decreasing P is the primary contributor to small warm season drying trends in the northern and southern subregions of SWNA. P accounts for most of the interannual variability in SWNA P−E and is strongly correlated with Pacific Ocean temperature anomalies associated with the El Niño Southern Oscillation and the Pacific Decadal Oscillation during the cold season. However, SWNA aridification is distinguished from the region\u27s natural climate variability because it is caused by long-term trends in P and E that are related to global warming. This is most evident in the cold season surface moisture balance of the northern subregion, where the temperature-driven trend in E (upward) is greater than the trend in P (downward). Trends in P and E account for a considerable fraction of the total variance in 21st century P−E and cause the cold season surface moisture balance of the northern and southern subregions to drop below the range of interannual variability by the end of the 21st century