Variability in productivity of the California Current occurs at climatic time scales, each of which must be taken into account when considering recruitment variability and fish growth. The North Pacific Ocean experiences dramatic shifts in climate every 10–20 years. These shifts occurred in 1926, 1947, 1977, and 1998 and were caused by eastward/westward jumps in the location of the Aleutian Low in winter, which result in changes in wind strength and direction. Changes in large–scale wind patterns lead to alternating states of either a "warm–ocean" or "cold–ocean" regime, with the warm ocean being less productive than the cold.
We hypothesize that during "cold PDO" (such 1999–2002), a larger amount of water enters the California Current from the Gulf of Alaska, whereas during "warm PDO" such as 2003–2005, smaller amounts of water enter from the Gulf of Alaska and more enters from the North Pacific Current offshore or from the south (Figure CPV-01). The changes in the type of source water yield the results shown in Table CPV-01.
Changes in biological productivity are best documented for the period since the 1950s, and this understanding is largely due to measurements made by the California Cooperative Oceanic Fisheries Investigations (CalCOFI) program. Zooplankton biomass, for example, was high from the 1950s through 1977, but during the warm regime of 1977–1998, zooplankton biomass in the southern sector of the California Current declined by nearly one order of magnitude. In the northern California Current, the change in zooplankton biomass between regimes was not as dramatic, ranging just over one half an order of magnitude in coastal waters off Newport Oregon. Zooplankton biomass was higher than average during the cool regime prior to 1977 and lower than average during the warm regime from 1977 to 1998. During 2000–2004, zooplankton biomass rebounded to levels comparable to those seen prior to 1977.
Since the early 1980s, the California Current has been experiencing an increased frequency of El Niño events, with large El Niño events occurring every 5-6 years: 1976-77, 1982-83, 1986-87, 1991-92, 1997-98, 2002-03 and again in 2009-10. A higher frequency of El Niño events appears to be a characteristic of the extended periods of warm ocean conditions. From 1992 to 1998, the Oregon and Washington coasts experienced almost continuous El Niño-like conditions during summer (i.e., reduced upwelling and warmer ocean conditions). Since 1998, ocean conditions have improved markedly, and another regime shift may have been initiated in late 1998. This shift to productive conditions was interrupted for 3 years (late 2002-late 2005), and again in 2013-14. Whether or not short-term (3-5 year) variability will become the norm remains to be seen.
It is unclear why ENSO activity has a variable impact on the Pacific Northwest, but one problem is that we do not know precisely how ENSO signals are transmitted from the equator to the PNW. Signals can arrive through the ocean via Kelvin waves, which propagate up the coast of North America. ENSO signals can also be transmitted through atmospheric teleconnections. El Niño conditions can strengthen the Aleutian Low pressure system 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. This results in more frequent large storms in winter and disruption of upwelling winds in spring and summer. A summary of these interactions is available from NOAA's Earth Systems Research Laboratory.
|Table CPV-01.||Summary of the manner in which the sign of the PDO influences broad–scale and local physical ocean condition indicators as well as biological indicators.|
|Cool PDO||Warm PDO|
|Broad–scale ocean indicators|
|Pacific Decadal Oscillation values||negative||positive|
|Multivariate ENSO Index values||negative||positive|
|Local physical indicators|
|Upwelling||may not be related to PDO|
|Physical spring transitiona||may not be related to PDO|
|Sea surface temperatures||cold||warm|
|Continental shelf water type||cold and salty||warm and fresh|
|Local and regional biological indicators|
|Copepod species richness||low||high|
|Northern copepod biomass||positive anomaly||negative anomaly|
|Southern copepod biomass||negative anomaly||positive anomaly|
|Euphausiid egg abundance in shelf water||usually high||usually low|
|Biological spring transition||early||late|
|Snake River Chinook SARsc||high||low|
|Forage fish abundances||many||few|
|Pacific hake abundances||few||many|
a (Logerwell et al. 2003)
b (OPIH) Oregon Production Index, Hatchery
c Smolt to adult returns (see Scheurell and Williams 2005)
These simple relationships only hold during years of persistent recurrence of one phase of the PDO. During transitional years, such as 1998-1999, 2002-2003, and 2006-07, there are time lags in the ecosystem responses. For example, after the 1998 and 2002 climate shifts, water temperatures lagged the PDO by 1-2 months, changes in copepod biodiversity lagged the PDO index by 4-6 months, and changes in copepod biomass lagged the PDO by 2 years. Similarly, increases in abundances of forage fish and juvenile salmon lagged the PDO index changes by 1-2 years. The strongly negative "cool phase" PDO of 2008 yielded good returns of salmon (particularly coho salmon) in 2009.