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 CU-01), 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).
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 to geostrophic 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 CU-02. The years of strongest upwelling were 1965–1967.
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).
The relationship of spring and fall adult Chinook salmon returns to the Bonneville dam and coho salmon survival (%) with upwelling for 1996 to present is shown in Figure CU-03. Although there was weak relationship between spring Chinook salmon returns and summed upwelling in April and May, this relationship has fallen apart during the recent warm years of 2015 and 2016.
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 1-year forecast of spring Chinook salmon returns based on this composite upwelling index.
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 upwelling indices; 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.
Upwelling was also strong during summer 2006, yet SST anomalies only averaged −0.3°C. On the other hand, upwelling was relatively weak during the summers of 2007 and 2008, yet these summers had some of the coldest temperatures in the time series, −1.0°C. These observations demonstrate that some care is required when interpreting a given upwelling index. We hypothesize that although upwelling is necessary to stimulate plankton production, its impact is greatest during negative phases of the PDO.
Figure CU-04 illustrates the pattern of upwelling through the use of a cumulative upwelling plot. This method simply adds the amount of upwelling on one day to that of the next day, and so on. The plot begins with day 1, on 1 January. Due to "downwelling" during winter months, upwelling values are increasingly negative for several weeks after day 1. But with the onset of the spring transition and upwelling, the downward trend reverses, and the cumulative line trends upwards.
One can see in Figure CU-04 that the winter downwelling period was close to the climatological mean until the end of January (day 30) when upwelling favorable conditions persisted resulting in weaker than average downwelling for the remainder of the winter. The onset of upwelling on April 15, 2018 followed the long term mean as did the cumulative upwelling during the summer. However, the lack of winter storms prolonged the upwelling season 12 days longer than average.