Oceanic Nino Index (ONI)

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. 

Figure 4.

Values of the ONI, 1955 - present. Red bars indicate warm conditions in the equatorial Pacific, blue bars indicate cool conditions in equatorial waters. Large and prolonged El Niño events are indicated by large, positive values of the index: note the > +2 value associated with the 1972, 1983 and 1998 events. Note cool anomalies (La Niña) during 1999-2002 and 2007-spring 2009. A La Niña event developed in equatorial waters in mid-2010 and remained in effect from June 2010 through June 2011, weakening to near-neutral values towards the end of the year.

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 Niño Southern Oscillation (ENSO) has been gauged using the Oceanic Niño Index, or ONI. A time series of the ONI is shown in Figure 4. Prior to 1977 (during the cool phase of the PDO), El Niño conditions were observed infrequently (note the predominance of blue bars prior to 1977).

During these 22 years, cool conditions were observed in only 98 of 266 months. During this same warm phase of the PDO, both the equatorial and northern North Pacific oceans experienced two very large El Niño events (1983-1984 and 1997-1998). There were also two smaller events in 1986 and 1987 and a prolonged event from 1990 to 1995.

Beginning in September 1998, ONI values turned negative and remained so for nearly 4 years, similar to the trend observed in the PDO. The ONI returned to positive in April 2002 and remained so through September 2005, after which negative values returned. Positive values were seen once again, beginning in spring 2009 and remaining through May 2010. In June 2010, negative values became established and persisted into mid 2011 before becoming near-neutral.

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 5) and a later decrease in copepod biodiversity (Figure 21).