Seasonality of precipitation in the American Southwest and cultural changes in the Colorado Plateau in pre-historic times

The arid regions of the Southwest United States are characterized by essentially two regions with a seasonal cycle of precipitation that differ in the proportion of winter precipitation. The boundary, which runs roughly from southeast Arizona northward toward Farmington New Mexico and then eastward across the Colorado—New Mexico stateline separates these two regions. To the west of that line, a bimodal pattern of seasonal precipitation prevails, with significant contributions to the annual mean total coming from winter troughs, as well as contributions from the North American Southwest summer monsoon. To the east of that line, summer precipitation contributes the bulk of the annual precipitation, falling primarily during the months of July through September.

Using a newly developed daily station data set spanning the last 70 years, we analyze the variability of seasonal components of precipitation in the region of the U.S. Southwest to gain some insight into variations occurring on different timescales, that are associated with differences between the summer and winter seasonal contributions. We note that knowledge gained over the past couple of decades regarding the locations of abandonment of pre-historic cultures of the U.S. Southwest (the Anasazi or Ancestral Puebloans) suggests that the permanent abandonment of the prehistoric settlements in the region of the Colorado Plateau, occurred roughly along the modern boundary between the two precipitation regions described above. To the extent that there may have been a climatic factor for the abandonment of cultural sites in the Southwest, this observation suggests that the winter component of the annual cycle of precipitation may have been responsible.