Flip-Flop May Be Underway
Recent ocean data collected by the Topex/Poseidon satellite show that the Pacific Ocean may be undergoing a dramatic change in the pattern of ocean temperatures that could be signaling a major shift in the climate regime for the western U.S. A new analysis of historical data has revealed a pattern of changing climate regimes every 20-30 years in response to changing oceanic conditions.
In the Pacific Ocean, this change is known as the as the Pacific Decadal Oscillation (PDO), a long-term ocean temperature fluctuation that was first noted for the dramatic changes in biological systems (such as plankton population, salmon runs and jellyfish populations in the Pacific Northwest) as well as inland river flows.
During positive phases of the oscillation, the desert Southwest experiences a higher frequency of El Nino conditions as was the case for the period 1978 through 1997 which helped keep many western reservoir levels relatively full. However, La Nina conditions the past three years have produced consecutive dry winters and subsequent drought conditions for large areas of the western United States. The persistence of the current La Nina conditions coupled with the recent satellite data on ocean temperatures could be signaling the start of a new negative phase of the Pacific Dedacal Oscillation which was last observed in the mid-1940s and lasted for 30 years.
"This is a very strong signal," said Dr. William Patzert an oceanographer at the Jet Propulsion Laboratory who tracks ocean temperature patterns in the Pacific Ocean. "For the last 20 years, California has had above-normal rainfall. Now, we're going back to the good old days when it was really dry in California. The continuing PDO pattern (of the past three years) signals more of the unusually dry conditions that have afflicted the North American West Coast," said JPL oceanographer Dr. William Patzert
A shift to a drier climate would likely lower water levels in at least some of the reservoir systems in the western U.S. In the past, increased pumping of groundwater typically alleviated prolonged droughts. However, falling water tables and explosive population growth over the past few decades now make it more expensive and difficult for groundwater pumping to come to the rescue.
The winter snowpack of 2000/2001 was so meager that runoff from many western rivers was less than 50% of normal. Doug Evans, manager of the Mountain Regional Water Special Service District in Utah reports:
"Reservoirs low, rivers at a trickle; farms, fire suppression, drinking water affected... As the Wasatch Back faces its third consecutive dry summer, water managers are pounding the conservation drum at a volume seldom heard before.
Randall Julander, a water-supply specialist with the U.S. Department of Agriculture's Natural Resources Conservation Service in Salt Lake City reports that "nearby reservoirs have begun to recede a full month earlier than normal."
The good news, however, is that thanks to two huge reservoirs (Lake Mead and Lake Powell), the Colorado River system is capable of holding about a four year supply of Colorado River water (60 million acre-feet). The combined storage of these reservoirs is sufficient to handle moderate drought conditions for several years, although the reservoir levels can get drawn down significantly.
The last significant drawdown occurred in 1993, when a mini-drought created a water storage loss equal to half the size of Lake Mead. The historical record shows many prolonged droughts severe enough that they could have drained both reservoirs to a point so low that power generation and water deliveries to Las Vegas would have been compromised.
"You take Lake Powell out of the Colorado River water storage equation and things get interesting real fast," says Ed Weeks, an SRP Engineer who has run water management models using historical tree ring data.
"Looking back to the last century, Lake Mead [without Lake Powell] would have been completely drained several times. It also shows that for significant periods of time [without Lake Powell] there would have been massive flows over the Hoover Dam spillways, which would cause damaging flooding downstream".