Climate and Atmospheric Indicators
Atmospheric conditions experienced by juvenile salmon entering the northern California Current.
Pacific Decadal Oscillation
The Pacific Decadal Oscillation, or PDO, is a basin-scale climate index based upon patterns of variation in the North Pacific's sea surface temperature from 1950 to the present using the Extended Reconstructed Sea Surface Temperature version 3b (ERSST 3b; Wen et al. 2014). The PDO index correlates well with many North Pacific and Pacific Northwest climate and ecology records, including sea level pressure, winter land–surface temperature and precipitation, and stream flow. Further, the index correlates with commercial salmon landings from Alaska, Washington, Oregon, and California.
Warm and Cold Phases
The spatial pattern of the PDO has a warm phase and a cold phase depending on the positive or negative sign of sea surface temperature anomalies along the Pacific coast of North America.
The North Pacific wind direction modulates the oscillation between the warm and cold phases. When the winds are primarily from the southwest, warmer conditions occur in the northern California Current (NCC) due to the onshore transport of subtropical waters. Conversely, when winds are mainly from the north, upwelling occurs—via Ekman pumping—leading to cooler conditions in the NCC.
Warm or cold phases can persist for decades. For example, a predominately cold phase occurred from 1947 to 1976 (blue bars in Figure PDO-01), and a 21-year warm phase occurred from 1977 to 1998 (red bars).
These decadal cycles broke down in late 1998 when the PDO entered a cold phase that lasted only five years. This cold phase was followed by a warm phase from 2003 to 2007 and an abrupt change to a cold phase from 2008 to 2013 (with a short interruption during the moderate El Niño in fall/winter 2009-2010). The PDO then switched phases again in 2014 and remained positive until 2018. This period coincided with a large marine heatwave and El Niño that negatively impacted the marine ecosystem in the NE Pacific during that time. Since 2020, the PDO has been consistently negative, reaching the most negative values since 1955 in both 2022 and 2023.
Oceanic Niño Index
Atmospheric conditions influence coastal waters off the Pacific Northwest in the North Pacific Ocean (as indexed by the PDO) and equatorial waters (as indexed by the Oceanic Niño Index or ONI). Strong El Niño events include above-average SSTs in equatorial waters, which are transported northward along the coasts of Central America, Mexico, and California and into Oregon and Washington's coastal waters.
Along with changes in local SST, El Niño events can affect the weather in the Pacific Northwest. Atmospheric teleconnections can transmit El Niño signals northward, strengthening the Aleutian Low, a persistent low–pressure air mass over the Gulf of Alaska. The shift in atmospheric pressure often results in more frequent, large winter storms and possible disruption of upwelling winds during the spring and summer in Washington and Oregon. The strong southwesterly winds—that occur during winter storms—drive warm, offshore waters into the coastal zone resulting in unusual mixes of zooplankton and fish species which can affect the early marine survival of juvenile salmon.
We can see in Figure ONI-01 that the equatorial, and therefore the northern Pacific Ocean, has experienced several very intense El Niño events (1971–1972, 1983–1984, 1997–1998, and 2015), prolonged events from 1990-1995 and 2002–2005, and short but relatively strong events in 2009-10 and 2015-16.
