UW News

March 2, 1999

Shift in climate cycle would mean winters that are wetter than average

News and Information

Climate scientists may have detected a shift in a regional climate cycle that could partially explain this winter’s exceptional amount of precipitation and could mean we’re in for wetter than average winters for the next several decades.

During a weekend presentation at a Northwest weather workshop in Seattle, University of Washington researchers Philip Mote and Alan Hamlet presented what they consider to be mounting evidence of a shift in the cycle that influences Alaska and Pacific Northwest climate for 10, 20 or 30 years at a time. Other scientists say that while recent observations make it appear there has been a shift, it is still too soon to say if it will persist.

The climate pattern of the Pacific decadal oscillation, or PDO, has tended to be in one of two phases in the past century. When there is a large pool of cooler-than-average surface water in the central North Pacific Ocean and a narrow band of warmer-than-average sea surface temperature near the coast of the Americas, then the PDO is in a phase in which the weather is drier than normal. The PDO entered this phase in 1977. The PDO enters its wetter phase when the surface waters in the central North Pacific Ocean are warmer than average and there is a narrow band of cooler-than average sea surface water near the coast.

Wet phase PDO conditions in the North Pacific have persisted for the last six months, according to Mote, UW research scientist with the Joint Institute for the Study of Atmosphere and Ocean, and Hamlet, a UW civil and environmental engineer.

The question, however, is whether the conditions typical of a wet PDO will continue beyond this winter, according to one of their colleagues, Nate Mantua, a research scientist with the Joint Institute for the Study of Atmosphere and Ocean. Since the shift to the drier PDO in 1977, there were periods in 1989 and 1994 when it appeared the cycle was turning to the wet phase, but the changes were short lived and lasted only six months and 30 months, Mantua says.

Mote and Hamlet said Saturday that other key reasons they think the PDO has shifted is because of changes in the streamflows in the Columbia River Basin and changes in salmon abundance in Alaskan waters, two things that past research has shown to be closely correlated to the phases of the PDO.

For example, there was extremely high natural streamflow in the Columbia River in 1996-97, something Hamlet says has never occurred in the dry phase PDO periods of the past 100 years. As contrast, he says runoff was unexceptional in the two years when ocean conditions led scientists to mistakenly think a shift in the PDO was happening – 1989 and 1994.

There was an El Ni?vent under way during the winter of 1998 and it was expected that warm, dry conditions would reduce streamflow in the Columbia River Basin. Yet flows were about normal and one explanation is that the wet phase of the PDO mediated the effects of the strong El Ni?Mote says. Finally there is this winter when most of the Columbia basin has snowpack ranging from 30 to 70 percent of normal and it’s been raining like mad in the lowlands.

“Considering how closely streamflows have reflected the phase of the PDO in the past, we think the Columbia River is telling us a shift might have happened,” Mote says.

If so, then most winters could be wetter than average for the next several decades. That doesn’t mean they will be as wet as this year, which is a record-breaker, but many will be wetter than average, Mote says. The sunny side to this news is that we’re less likely to see shortages of summer water for agriculture and urban use, and higher streamflows will mean some easing of pressures on salmon and hydropower producers.


For more information:
Philip Mote, (206) 616-5346, pager 994-6945 or philip@atmos.washington.edu
Alan Hamlet, (206) 633-5660, 543-5454 or hamleaf@u.washington.edu
Nate Mantua, (206) 633-5660 or 616-5347 or mantua@atmos.washington.edu

Also see: http://tao.atmos.washington.edu/PNWimpacts/climvar.html