Coastal Upwelling

An important process affecting primary productivity during the spring and summer 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 (orange arrow in Figure 8), with a component transported away from the coastline (to the right of the wind, light green 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 8.	Forces affecting coastal upwelling.

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. 

The Upwelling Index 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 togeostrophic wind fields.  Geostrophic wind fields are calculated from surface atmospheric pressure fields measured and reported by the U.S. Navy Fleet Numerical Meteorological and Oceanographic Center (FNMOC) in Monterey, California. 

The Upwelling Index is calculated in 3–degree intervals from 21°N to 60°N latitude, and data are 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 anomalies of the upwelling index Figure 9.  The years of strongest upwelling were 1965–1967.

Figure 9.	Anomalies of the coastal Upwelling Index during May to September each year, 1946 - present

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.

http://www.nwfsc.noaa.gov/research/divisions/fed/oeip/db-coastal-upwelling-index.cfm