Geoscience Reference
In-Depth Information
Some of the deepest droughts and most catastrophic l oods are caused
by the interactions between the PDO and the ENSO. When the PDO is in
a positive (warm) phase, El Niño events bring more rain to the American
Southwest and less to the Pacii c Northwest, as was the case during the
period between 1977 and 1999. In contrast, during a negative (cool) PDO
phase, La Niña events bring drier conditions to the American Southwest
and wetter conditions to the Pacii c Northwest, such as during the period
from 1945 to 1976.
A negative (cool) PDO regime began in 1999, marked by a deep drought
in the Southwest. But this time, the cold-phase PDO lasted only four years,
l ipping back to a warm phase from 2002 to 2005. It then returned to a cool
phase again, where it has remained until the present time (2013). h ese recent
l uctuations in the PDO are not typical for the twentieth century and are
currently under investigation by atmosphere and ocean scientists to assess
whether they are possibly related to global warming.
atmospheric river storms
Water Resources
Scientists at the U.S. Geological Survey and elsewhere have found that
water vapor from the tropical Pacii c Ocean is carried to higher latitudes
through narrow corridors of concentrated moisture, dubbed “atmospheric
rivers.” Satellite technology over the past decade has allowed research-
ers to observe and image these storms. h ey discovered currents in the
lower atmosphere (about one mile high, 250 miles wide, and extending
thousands of miles) carrying water vapor and warm air from the eastern
North Pacii c to the West Coast. Once these atmospheric rivers hit the
mountain slopes, they are forced upward and cooled, releasing copious
amounts of precipitation along the Coast Ranges, Sierra Nevada, and
Cascades.
Atmospheric rivers are actually a type of cyclone that forms at higher
latitudes than hurricanes. Although they lack a cyclone's characteristic cir-
cular pattern, they carry hurricane-strength winds and a similar amount of
rainfall. Unlike hurricanes, which obtain their energy from surface ocean
heat content, these atmospheric rivers derive their energy from the contrasts
in temperature between the tropics and the poles. Atmospheric river storms
contribute between 30 and 50 percent of the total amount of precipitation
