Multivariate El Niño Southern
Oscillation Index (MEI)
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, often resulting in stronger winter storms and 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.
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Figure 4. |
Anomalies of the MEI from 1955 to 2008. Red bars indicate warm ocean conditions in the equatorial Pacific, blue bars indicate cool conditions in equatorial waters. Large and prolonged El Niño events are indicated by large values of the index: note the +3 anomaly associated with the 1983 and 1998 events and the prolonged period of warm conditions at the equator from 1977 to 1996. Note cool anomalies from 1999-2002 and from 2007 present. |
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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 Multivariate ENSO Index, or MEI. A time series of the MEI 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, MEI values turned negative and remained so for nearly four years, similar to the trend observed in the PDO. The MEI returned to positive in April 2002 and remained so through September 2005, after which negative values returned.
Both the PDO and MEI can be viewed as "leading indicators" since after a persistent change in sign of either index, ocean conditions in the California Current soon begin to change. Most recently, in September 2005, the MEI appears to have signaled a return to warm ocean conditions. In fact, NOAA recently issued a report that indicates El Niño conditions are now evident in the tropical Pacific and should intensify during the next one to three months. However, this episode is expected to be much weaker than the very strong 1997–1998 El Niño event.
However, if the prediction of an El Niño event holds true, warm ocean conditions can be expected for the next year. The impact of El Niño events. The impact of El Niño events on survival of coho salmon is well documented (Pearcy 1992). For example, the large events of both 1983 and 1998 were followed by low adult return rates for coho salmon during 1983–1984 and 1999, respectively.
Likewise, the extended period of El Niño conditions in 1977–1983 was accompanied by declines in adult coho returns during the same years. A second extended El Niño period during 1990–1996 was followed by extremely low returns of adult fish that migrated to sea as juveniles from 1991 to 1998. For spring Chinook, the two large (but brief) El Niño events resulted in lower than average smolt–to–adult return rates, but the lowest adult return rates were observed during the weaker but prolonged El Niño events of 1990–1998.
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