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 climate index based upon patterns of variation in the North Pacific's sea surface temperature from 1900 to the present (Mantua et al. 1997). We derive the PDO from sea surface temperature data. It is also well correlated 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 two phases, a warm and a cool phase, depending on the 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 due to onshore transport of subtropical waters. Conversely, when winds are mostly from the north, upwelling occurs in the open ocean (Ekman pumping), leading to cooler conditions in the northern California Current.
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). Another 21-year warm phase occurred From 1977 to 1998.
These decadal cycles broke down in late 1998. 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 negative phase from 2008 to 2013 (with only a short interruption during the moderate El Niño in fall/winter 2009-2010). In 2014, the PDO switched phase again and remained strongly positive until more neutral conditions in 2018.
Oceanic Nino Index
Coastal waters off the Pacific Northwest are influenced by atmospheric conditions in the North Pacific Ocean (as indexed by the PDO) and equatorial waters, especially during El Niño events. Strong El Niño events transport warm equatorial waters northward along the coasts of Central America, Mexico, and California and into Oregon and Washington's coastal waters. Since 1955, we have gauged the presence/absence of conditions resulting from the El Niño Southern Oscillation (ENSO) using the Oceanic Niño Index, or ONI.
These events affect weather in the Pacific Northwest. They often result in stronger winter storms with southwesterly winds that drive warm, offshore waters into the coastal zone. Warm waters moving toward the coast, either from the south or offshore, also result in unusual mixes of zooplankton and fish species.
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.