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 the direction of winter winds in the North Pacific: winter winds blowing chiefly from the southwest result in warmer conditions in the northern CC. The CC warms at such times due to onshore transport of warm waters that normally lie offshore. Conversely, when winds blow chiefly from the north, upwelling occurs both in the open ocean and at the coast, leading to cooler conditions in the northern CC.
Warm and cold phases can persist for decades. For example, a warm phase continued from 1925 to 1946 (red bars in Figure PDO-01), and a cool phase from 1947 to 1976 (blue bars). From 1977 to 1998, another 21–year warm phase occurred. However, these decadal cycles have recently broken down: in late 1998, the PDO entered a cold phase that lasted only 4 years followed by a warm phase of 3 years, from 2002 to 2005. The PDO was in a relatively neutral phase through August 2007, but abruptly changed in September 2007 to a negative phase that lasted nearly 2 years, through July 2009. The PDO then reverted to a positive phase in August 2009 (Figure TA-01) because of a moderate El Niño event that developed at the equator during fall/winter 2009–2010. This positive signal continued for 10 months (August 2009–May 2010) until June 2010, when persistently negative values of the PDO initiated and have remained strongly negative through autumn 2012.
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 salmon and coho salmon to Oregon rivers. Conversely, during the warm PDO cycle that followed (1977–1998), salmon numbers declined steadily.
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 PDO-02, which shows average PDO values in summer vs. anomalies in counts of adult spring Chinook salmon 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 PDO-02 shows a clear visual correlation between the PDO, adult spring Chinook salmon counts and hatchery coho salmon adult returns. Note that during the 22–year cool phase of the PDO (1955 to 1977), below–average counts of spring Chinook salmon 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 PDO-02 that a time lag of up to 2 years exists between PDO phase changes and spring Chinook salmon returns: Chinook salmon runs remained above average in 1977 and 1978, 2 years after the 1976 PDO shift. Similarly, increased returns of spring Chinook salmon adults in 2000 lagged 2 years behind the PDO shift of 1998.
Adult spring Chinook salmon runs declined again, beginning with fish that had entered the sea in 2003 and had experienced poor conditions associated with the positive PDO signal in that year. This decline continued for 3 years, until 2008 and 2009, when returns began to increase, as we predicted based on ocean conditions during 2006–2007. With the strongly negative PDO in effect for juvenile Chinook salmon that entered the ocean in spring 2008, we predicted high adult returns of these fish in 2010. In fact, the third highest returns on record were recorded in 2010.