"It's three different things working on different times scales," MacDonald said. "You may not get them to line up that frequently."
Tingstad and MacDonald found a "striking and significant propensity" for droughts in northeastern Utah when cool sea-surface temperatures in the Pacific associated with La Nia and the negative phase of the PDO were coupled with warm temperatures in the North Atlantic linked to the positive phase of the AMO. During such episodes, snowpack declined on average between 9 percent and 10 percent, and river discharge decreased on average by 18 percent.
The three conditions last converged at least five times between 1945 and 1965, a period that was characterized by generally depressed but variable flows in the river, they said.
The findings are troublesome because not only are all three conditions predicted for 2010, but they are expected to be particularly strong, the researchers say. The coming year's La Nia and AMO are at this point supposed to be the strongest in 10 years, and a strong negative PDO is also building.
The convergence increases the likelihood that Lake Mead, already diminished by 11 years of drought, will fall below 1,075 feet above sea level a threshold that can result in the reduction of water allocations in Nevada and Arizona, the researchers say. Under a series of agreements among seven U.S. states along the Colorado River and Mexico, California has first rights to the water, so it would not face the same restrictions. Water levels at Lake Mead currently stand just nine feet from the critical threshold, at 1,084 feet above sea level.
"We're looking at a situation that could pit us against our neighbors," said MacDonald, who is also a UCLA professor of geography. "We've never had to face such a severe decline in Lake Mead and
|Contact: Meg Sullivan|
University of California -- Los Angeles