Hydrologic Impacts of the Pacific Decadal Oscillation on ENSO Influenced Regions
Presented at the American Geophysical Union (AGU) Fall Meeting, December 13-17, 2004, San Francisco, California.
Several regions in the United States [Pacific Northwest (PNW), Southwest (SW), North Central (NC), Northeast (NE) and Gulf of Mexico (GM)] display a strong El Niño-Southern Oscillation (ENSO) signal in which monthly precipitation (and resulting streamflow) vary in the winter-spring season following the onset of a summer season warm (El Niño) or cold (La Niña) phase of ENSO. Additionally, numerous studies have shown that in several of these regions (PNW, SW and GM), a distinct lag exists between the occurrence of a summer El Niño (or La Niña) and the resulting winter-spring streamflow variation. The concern for water planners is "how likely (probability) will winter-spring streamflow be above (or below) median if a summer season ENSO event occurs?" and, "can this probability be provided prior to the beginning of the water year (01 October)?" Research has shown that the Pacific Decadal Oscillation (PDO) can increase (or reduce) the effects of ENSO. PDO tends to oscillate with a characteristic period on the order of 50 years (a particular phase of the PDO will typically persist for about 25 years). This period differs greatly from ENSO events, which typically persist for 6 to 18 months. Water-year (October to September) average values of the PDO were examined for the period of 1941 to 2001. During this period, three distinct phases were observed: 1941 to 1961 (21 years - Declining), 1962 to 1987 (26 years - Rising) and 1988 to 2001 (14 years - Declining). Currently, we are in a Declining PDO phase, which should persist for approximately 10 more years. An additional concern for water planners is "will the current Declining PDO phase enhance or dampen a summer ENSO event?" Data was obtained for unimpaired streamflow stations in the PNW, SW and GM basins from 1942 to 2002 (61 years of data). Preliminary results in the SW basin reveal that the current Declining PDO phase does result in a higher likelihood of below median winter-spring streamflow, as a result of a summer La Niña event (based on the Southern Oscillation Index), when compared to the Rising PDO phase.