Large–scale Ocean and Atmospheric Indicators

Local and Regional Physical Indicators

Local Biological Indicators

Indicators Under Development

Introduction to Pacific Northwest Oceanography

Ocean Sampling Methods

Coastal Upwelling

Perhaps the most important process affecting plankton production off the Pacific Northwest is coastal upwelling.  Upwelling is caused by northerly winds that blow along the Oregon coast from April to September.  These winds transport offshore surface water southward (yellow arrow in Figure 6), with a component transported away from the coastline (to the right of the wind, light blue arrow).  This offshore, southward transport of surface waters is balanced by onshore, northward transport of cool, high–salinity, nutrient–rich water (dark blue arrow).

	Figure 6.	Forces affecting coastal upwelling.  Drawing courtesy of Environmental Research Division, Pacific Fisheries Environmental Research Laboratory, NOAA.

The strength of an upwelling process can be calculated based on estimates of wind speed.  Using such data, Dr. Andy Bakun (1973) developed the Coastal Upwelling Index (CUI).

The CUI is, as its name implies, a measure of the volume of water that upwells along the coast; it identifies the amount of offshore transport of surface waters due to geostrophic wind fields.  Geostrophic wind fields are calculated from surface atmospheric pressure fields measured and reported provided by the U.S. Navy Fleet Numerical Meteorological and Oceanographic Center (FNMOC) in Monterey, California. 

The CUI is calculated in 3–degree intervals from 21°N to 60°N latitude, and data is available from 1947 to present.  For the northern California Current, relevant values are from 42, 45, and 48°N.  Year–to–year variations in upwelling off Newport (45°N) are shown as CUI anomalies in Figure 7.  The years of strongest upwelling were 1965–1967.

	Figure 7.	Anomalies of the Coastal Upwelling Index during May to September each year from 1947 to 2006.

Upwelling was anomalously weak in all but 8 of the 21 years from summer 1976 to summer 1997, and this is expected during warm PDO phases.  When the PDO was in a cool phase (late 1998–2003), upwelling strengthened.  With the change in PDO sign to positive in 2004–2005, upwelling again weakened.

Many studies have shown correlations between the amount of coastal upwelling and production of various fisheries.  The first to show a predictable relationship between coho survival and upwelling were Gunsolus (1978) and Nickelson (1986).

Knowledge of upwelling alone does not always provide good predictions of salmon returns.  For example, during the 1998 El Niño event, upwelling was relatively strong, as measured by the CUI; however, plankton production was weak.  This occurred because the deep source waters for upwelling were warm and nutrient–poor.  Low levels of plankton production may have impacted all trophic levels up the food chain.  This observation demonstrates the importance of interpreting the upwelling index in light of the type of source water that upwells in the northern California Current.

An additional focus of concern is the low levels of oxygen present in upwelled source water.  Hypoxia continues to be a potential problem for benthic invertebrates living in continental shelf waters.  Although we have no reason to believe that hypoxia is a problem for juvenile salmon, we include information on this phenomenon here due to a general interest in this topic (Figure 7a). 

	Figure 7a.	Oxygen concentration in bottom waters at a baseline stations 5 miles off Newport.  Hypoxia is observed during the months of June through September. Hypoxia was particularly severe during July 2006; however, we have too few measurements to evaluate if some years were better than other years.  The first record of hypoxia in 2007 was in early June.

The relationship between coho salmon survival and upwelling is shown in Figure 8.  The strongest correlations with survival were found with upwelling in April and upwelling in April and May combined.  A significant, but weaker correlation was also found between upwelling and survival during the months of April, May, and June combined.

	Figure 8.	Scattergram of coho survival vs. CUI anomaliy for 45°N, 1960–2004.  Panel A shows values during April; panel B shows values during April and May combined.

Using regression analysis and the value of the upwelling anomaly for April 2006 (which was –9), the 95% confidence intervals suggest a return of coho salmon between 2.8 and 4.5% in 2007.  Scheuerell and Williams (2005) showed that the upwelling index in April, September, and October is also related to returns of Snake River spring Chinook salmon.  Moreover, they developed a one–year forecast of spring Chinook returns using the CUI.