Coastal waters off the Pacific Northwest are influenced by atmospheric conditions not only in the North Pacific Ocean (as indexed by the PDO), but also in equatorial waters, especially during El Niño events. Strong El Niño events result in the transport of warm equatorial waters northward along the coasts of Central America, Mexico, and California and into the coastal waters off Oregon and Washington.
These events affect weather in the Pacific Northwest as well, often resulting in stronger winter storms with southwesterly winds that drive the transport of warm, offshore waters into the coastal zone. The transport of warm waters toward the coast, either from the south or from offshore, also results in the presence of unusual mixes of zooplankton and fish species.
El Niño events have variable and unpredictable effects on coastal waters off Oregon and Washington. While we do not fully understand how El Niño signals are transmitted northward from the equator, we do know that signals can travel through the ocean via Kelvin waves. Kelvin waves propagate northward along the coast of North America and result in transport of warm waters from south to north.
El Niño signals can also be transmitted through atmospheric teleconnections in that El Niño conditions can strengthen the Aleutian Low, a persistent low–pressure air mass over the Gulf of Alaska. Thus adjustments in the strength and location of low–pressure atmospheric cells at the equator can affect our local weather, resulting in more frequent large storms in winter and possible disruption of upwelling winds in spring and summer.
Since 1955, the presence/absence of conditions resulting from the El Nino Southern Oscillation (ENSO) has been gauged using the Oceanic Nino Index, or ONI. A time series of the ONI is shown in Figure ONI–01. The equatorial and northern North Pacific oceans experienced several very intense El Nino events (1972–72, 1983–1984, 1997–1998, and 2015) along with prolonged events from 1990 to 1995 and 2002–2005, and a short, but relatively strong event in early 2010.
Both the PDO and ONI can be viewed as "leading indicators" of ocean conditions, since after a persistent change in sign of either index, ocean conditions in the California Current soon begin to change. The ONI is a good index of El Niño conditions, and one can find information on the status of both El Niño and La Niña at the Climate Prediction Center and other websites maintained by the NOAA National Weather Service. Following the relatively strong El Niño during the winter of 2009-2010, the northern California Current experienced a rapid switch to La Niña conditions. The switch was reflected in both a drop in sea surface temperatures (Figure TA-01) and a later decrease in copepod biodiversity (Figure CB-02).