The tree rings came from a combination of dead and living pinyon pines that grow on extremely dry slopes in the Uintas. Described by Tingstad as "listening posts for climate variability and drought," the trees produce annual rings that are so sensitive to water stress that the researchers can track changes in annual precipitation on the order of an inch by studying them under microscopes.
Tingstad and MacDonald then compared their records with existing records for La Nia, the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). During a La Nia episode, the sea-surface temperature across the Pacific Ocean at the equator plummets by as much 18 degrees Fahrenheit, resulting in drops in precipitation rates of as much as 50 percent across the southwestern and southeastern United States for between five and 18 months.
The PDO is a pattern of climate variability with longer-term shifts that last between 20 and 30 years and also affect weather. A negative PDO is characterized by cooler sea-surface temperatures off the Pacific Coast of North America that can result in below-average precipitation in the southwestern U.S. The effect can be thought of as an extended La Nia event.
Each phase of the AMO can last for more than 60 years and is characterized by temperature changes in the North Atlantic Ocean. In its positive phase, the AMO has little impact on California weather if it occurs in absence of a negative PDO. But the positive AMO has bee
|Contact: Meg Sullivan|
University of California -- Los Angeles