DROUGHT PERIODICITY OVER THE PAST 2000 YEARS IN THE AMERICAN SOUTHWEST

Tree ring records from 117 precipitation sensitive sites in the Four Corners area of northeastern Arizona, northwestern New Mexico, and southern Utah and Colorado were used to reconstruct annual precipitation variability over the region for the last 2000 years. Response functions for individual chronologies suggest that total precipitation for a 12-month period from the prior September to August of the growing season can be usefully reconstructed at these individual sites. Because of general similarities in the precipitation response for all 117 chronologies they were combined to produce a master regional chronology with greatly enhanced chronology depth.

The reconstructed series shows that precipitation has varied in a regular manner with alternating wet and dry periods over the last 2000 years. The reconstruction for individual years ranges from a minimum of 19.2cm (AD 980) to a maximum of 56.8cm (AD 636). The wettest 20-year interval occurs from AD 1905-1924 (mean 43.4 cm/yr) while the driest occurs from AD 1573-1592 (mean 30.7 cm/yr). This reconstruction confirms that some earlier droughts were multidecadal with several (centered on AD 615, 710, 1090, 1280, 1450 and 1580) exceeding drought magnitudes experienced in the 20th century.

Long-term trends in the reconstruction are indicative of an ~70-year periodicity in precipitation totals. Documented wet and dry periods over the instrumental record and drought periods prior to 1895 reconstructed from other proxy data sources appear to be closely linked to this variability. Superimposed on this longer periodicity is an interval from 800 to 1200 AD during which shorter period precipitation variability dominates the record. A change to higher magnitude events and rapid transitions from wet to dry conditions may be indicative of a climatic shifts in the 13th and 20th centuries.