Biology Reference
In-Depth Information
WATER YEAR TYPES
Every year, based largely on the snowpack in the Sierra, California's Depart-
ment of Water Resources announces a “water year type” for the Sacramento
Valley. The type can be wet, above normal, below normal, dry, or critical. The
designation is based on a mathematical formula that considers two factors:
the amount of natural flow from the current water year (from April to July and
October to March), and the wetness of the previous year (essentially a mea-
sure of reservoir storage). The flow is measured on the Sacramento River 15
miles north of Red Bluff, as well as on the Yuba, American, and Feather rivers.
The year type is often used to make water management and allocation deci-
sions. Other state agencies calculate year types differently.
share of rain, up to 52 inches on average, compared to the 14-inch average
for San Jose.
Several large ocean cycles inject variety into seasonal weather patterns.
The best known of these is the El Niño Southern Oscillation (ENSO).
During an El Niño year, waters in the eastern tropical Pacific grow unusu-
ally warm. Normal upwelling currents get reversed. Meanwhile, the jet
stream often diverts storms that fall over the Pacific Northwest to Califor-
nia, drenching the Bay Area. Because water expands as it warms, the tides
swell beyond their normal reach. Now and then, instead of El Niño, the
region can come under the influence of La Niña conditions, which are
marked by cooler-than-average tropical waters and extraordinarily strong
upwelling. Each phase tends to last between 6 and 18 months, but one
round of the cycle may take three to four years to complete.
Two other major ocean cycles operate closer to home. The first to be
discovered was the Pacific Decadal Oscillation (PDO) in the early 1990s.
Reviewing salmon catch numbers in the North Pacific for the past century,
fisheries biologist Steven Hare noticed a distinct pattern. When nets were
full in Alaskan waters, those off Washington and Oregon would be empty.
Intrigued, Hare and oceanographer Bob Francis of the University of
Washington dug through more ocean records and found an oscillation
that in essence regulates the thermostat of the North Pacific. It keeps tem-
peratures around the Gulf of Alaska cool when the west coast of North
America is warm, and vice versa. It swings average sea surface tempera-
tures in each region up and down by about 12 degrees Fahrenheit, repeat-
ing the exchange every 20 to 30 years.
Scientists didn't discover the second ocean cycle for another 15 years.
While studying computer models of the North Pacific, climate scientist
Emanuele DiLorenzo of the Georgia Institute of Technology noticed a
