Climate and Atmospheric Indicators

Pacific Decadal Oscillation

The Pacific Decadal Oscillation, or PDO, is a climate index based upon patterns of variation in the North Pacific's sea surface temperature from 1900 to the present (Mantua et al. 1997). 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 salmon landings from Alaska, Washington, Oregon, and California.

Warm and Cold Phases

The spatial pattern of the PDO has a warm phase and a cool 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 cool 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 in the open ocean (via Ekman pumping), leading to cooler conditions in the NCC.

Figure PDO-01. Time series of the Pacific Decadal Oscillation (PDO) from 1925 to present. Values are summed over May through September. Red bars indicate positive (warm) years; blue bars indicate negative (cool) years. Credit: NOAA Fisheries

Warm or cool 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). Another 21-year warm phase occurred from 1977 to 1998.

These decadal cycles broke down in late 1998 when the PDO entered a cold phase that lasted only four years. This cold phase was followed by a warm phase from 2002 to 2005, a neutral phase from 2006 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 2020. This period coincided with a large marine heatwave and impacts from El Niño in the NE Pacific Ocean.

Oceanic Nino 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.

Figure ONI-01. Values of the Oceanic Niño Index from 1955 - present. Red bars indicate warm conditions and blue bars indicate cool conditions in the equatorial Pacific. Strong and prolonged El Niño events are indicated by large, positive values of the index >1.5°C- note the +2°C value associated with the 1972, 1983, 1998, and 2015-2016. Examples of cool anomalies (La Niña) were present during 1974-1976, 1989,1999-2002, 2007-2009, and 2010-2012. Credit: NOAA Fisheries

Atmospheric teleconnections can also transmit El Nino signals. El Niño conditions can strengthen the Aleutian Low, a persistent low–pressure air mass over the Gulf of Alaska. Adjustments in the strength and location of low–pressure atmospheric cells at the equator can affect our local weather. We may experience more frequent large storms in winter and possible disruption of upwelling winds in spring and summer.

The equatorial and northern North Pacific oceans experienced several very intense El Niño events (1972–72, 1983–1984, 1997–1998, and 2015) and prolonged events from 1990 to 1995 and 2002–2005 and a short but relatively strong event in early 2010 and 2015.