The changing character of climate, drought, and the seasons in the Southwestern USA

The seasonality and effects of warmer temperatures during the 2000s drought in the Southwest distinguish it from the previous pronounced regional drought during the 1950s. During the typically hot and dry part of the year from May through July, higher minimum (map A) and maximum (map B) temperatures throughout much of the region during the 2000s drought relative to the 1950s drought significantly increased the atmosphere’s ability to take up moisture from reservoirs, soil, and vegetation (map C). Less moisture over parts of the region during the more recent drought at this time of year further allowed this ability to increase (map D). Hotter and drier conditions from May through July produce a disproportionate amount of negative societal impacts, including increased water demand, poorer air quality, and greater wildfire risk.

¹Difference of means test results for May-June-July (MJJ) minimum temperature. Units are °C. Color gradation quantifies differences between MJJ mean values of the 2000s and 1950s droughts (i.e., MJJ minimum temperature mean_2000-2003 – MJJ minimum temperature mean_1953-1956). Cross-hatched areas are locally significant at the 95% level. Positive (negative) values indicate warmer (cooler) minimum temperatures during the 2000s drought than the 1950s drought.

²As in map A, but for maximum temperature. Units are °C. Positive (negative) values indicate warmer (cooler) maximum temperatures during the 2000s drought than the 1950s drought.

³ As in map A, but for vapor pressure deficit. Units are kPa. Positive (negative) values indicate higher (lower) vapor pressure deficits during the 2000s drought than the 1950s drought.

⁴ As in map A, but for a three-month standardized precipitation index. Values are unitless. Positive (negative) values indicate wetter (drier) SPI-3 during the 2000s drought than the 1950s drought.