Pacific Decadal Oscillation (PDO)
The Pacific Decadal Oscillation is a climate index based upon patterns of variation in sea surface temperature of the North Pacific from 1900 to the present (Mantua et al. 1997). While derived from sea surface temperature data, the PDO index is well correlated with many records of North Pacific and Pacific Northwest climate and ecology, including sea level pressure, winter land–surface temperature and precipitation, and stream flow. The index is also correlated with salmon landings from Alaska, Washington, Oregon, and California.
The PDO is highly correlated with sea surface temperature in the northern California Current (CC) area; thus we often speak of the PDO as being in one of two phases, a "warm phase" and a "cool phase," according to the sign of sea–surface temperature anomalies along the Pacific Coast of North America. These phases result from winter winds in the North Pacific: winter winds blowing chiefly from the southwest result in warmer conditions in the northern CC. Conversely, when winds blow chiefly from the north, upwelling occurs, leading to cooler conditions in the northern CC.
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| Figure 2. |
Time series of shifts in sign of the Pacific Decadal Oscillation (PDO), 1925 to 2008. Values are averaged over the months of May through September. Red bars indicate positive (warm) years; blue bars negative (cool) years. Note that 2008 was the most negative since 1956. |
Warm and cold phases can persist for decades. For example, a warm phase continued from 1925 to 1946 (red bars in Figure 2), and a cool phase from 1947 to 1976 (blue bars). From 1977 to 1998, another 21‑year warm phase occurred. Recently, however, these decadal cycles have broken down: in late 1998, the PDO entered a cold phase that lasted only four years followed by a warm phase of three years from 2002 to 2005. The PDO has remained in positive (warm) phase through at least July 2006.
Dr. Nathan Mantua and his colleagues were the first to show that adult salmon catches in the Northeast Pacific were correlated with the Pacific Decadal Oscillation (Mantua et al. 1997). They noted that in the Pacific Northwest, the cool PDO years of 1947–1976 coincided with high returns of Chinook and coho salmon to Oregon rivers. Conversely, during the warm PDO cycle that followed (1977‑1998), salmon numbers declined steadily.
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| Figure 3. |
Summer average PDO (top) vs. adult spring Chinook passing Bonneville Dam (middle) and survival of hatchery coho salmon (bottom), 1955–2006. Vertical lines indicate climate–shift points in 1977 and 1998. |
The listing of several salmon stocks as threatened or endangered under the U.S. Endangered Species Act coincides with a prolonged period of poor ocean conditions that began in the early 1990s. This is illustrated in Figure 2, which shows average PDO values in summer vs. anomalies in counts of adult spring Chinook at Bonneville Dam. Also shown are percentages of hatchery juvenile coho salmon that returned as adults to hatcheries in SW Washington and NE Oregon during this period. These percentages have been recorded since 1961 as the Oregon Production Index, Hatchery (OPIH).
The OPIH includes fish taken in the fishery as well as those that returned to hatcheries. Figure 3 shows a clear visual correlation between the PDO, adult spring Chinook counts and hatchery coho adult returns. Note that during the 22–year cool phase of the PDO (1955 to 1977), below–average counts of spring Chinook at Bonneville Dam were seen in only 5 years (1956, 1958‑60, and 1965).
In contrast, below–average counts were common from 1977 to 1998, when the PDO was in warm phase: below–average counts were observed in 16 of these 21 years. The dramatic increase in counts from 2000 to 2004 coincided with the return to a cool phase PDO in late 1998. Note also from Figure 3 that a time lag of up to 2 years exists between PDO phase changes and spring Chinook returns: Chinook runs remained above average in 1977 and 1978, 2 years after the 1976 PDO shift. Similarly, increased returns of spring Chinook adults in 2000 lagged 2 years behind the PDO shift of 1998.
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