Local Physical Indicators
Regional physical conditions experienced by juvenile salmon entering the northern California Current.
Many scientists and managers have noted that the marine survival of commercially important species—such as salmon—often corresponds with physical ocean conditions. For example, cool ocean conditions generally lead to good feeding conditions and increased juvenile survival for Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon. In contrast, warm ocean conditions lead to poor feeding conditions and low juvenile survival.
Local physical indicators (e.g., temperature and salinity) change seasonally in response to shifts in local winds (e.g., upwelling and downwelling), and between years, in response to basin-scale atmospheric forcing (captured by the PDO and ONI indices). The seasonal and interannual shifts in physical forcing are biologically important. They influence the source waters that feed into the northern California Current (NCC), altering zooplankton and forage fish communities that salmon predators (such as hake and sea birds) rely on.
Local variables respond to changes on a broad spectrum of spatial scales. These range from basin-scale changes, indexed chiefly by the PDO and ENSO, to local and regional changes, such as those related to shifts in the jet stream, atmospheric pressure, and surface wind patterns.
We see a strong association between the PDO phase and local sea surface temperature (SST) anomalies (Figure TA-01). For example, we observe positive SST anomalies off Oregon and Washington when the PDO is in a positive phase. We generally observe negative SST anomalies when PDO is negative.
We also observe a time lag between a PDO sign change and a change in local SSTs. The time lag is not consistent each year. It appears to take 2–6 months for a basin-scale signal in the North Pacific to propagate to local coastal waters. The connection between PDO and SST anomalies suggests that shifts in atmospheric forcing result in local SST changes, likely due to differences in water transport out of the North Pacific and into the NCC.
Figure TA-03 summarizes temperature measurements collected at station NH-5 during bi-weekly surveys off Newport, Oregon. The figure shows that seasonal averages for winter (Nov-Mar) and summer (May-Sep) can increase by up to 2 °C during El Niño events (1997-98). Also, it shows that the seasonal averages have a cyclic pattern reflecting influences from both the PDO and ONI. Note the signature of the 2014-16 marine heatwave when winter temperature anomalies in 2015 exceeded the strong 1997-98 El Niño and remained +1.5°C in 2016 and 2017.
Near Bottom Temperature and Salinity
Phase changes of the Pacific Decadal Oscillation are associated with alternating changes in wind speed and direction over the North Pacific. Northerly winds result in upwelling (and a negative PDO), and southerly winds result in downwelling (and a positive PDO) throughout the Gulf of Alaska and the California Current. These winds affect the transport of water into the Northern California Current (NCC). Northerly winds transport water from the north. Southwesterly winds transport water from the south and west (offshore).
We can identify shifts in the PDO phase by the presence of different water types in the northern California Current. This led us to develop a "water type indicator." The value of the water type indicator points to the type of water that will upwell at the coast. For example, cold, salty water of subarctic origin is nutrient-rich, whereas the relatively warm, fresh water from the offshore North Pacific Current is nutrient depleted.
Figure DTS-01 shows the average temperature and salinity values near the bottom (50 m) at station NH-5 during the upwelling season.
During the El Niño event of 1997-1998 and 2014, deep waters on the continental shelf off Newport were warm and relatively fresh throughout the year. During contrasting negative-phase PDO years of 1999-2002, 2007-2008, and 2021, these waters were cold and relatively salty or intermediate, as in 2009-2012.
Coastal upwelling affects primary productivity during the spring and summer. Strong, northerly winds that blow along the Oregon coast from April to September drive upwelling. The wind transports surface water offshore and southward. This water is replaced by the upwelling of deep, cool, high–salinity, nutrient-rich water to the photic zone. This upwelling supports phytoplankton blooms that provide food for zooplankton and higher trophic organisms.
The Upwelling Index identifies the amount of surface water transported offshore using geostrophic wind approximations 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 can fluctuate yearly due to atmospheric shifts in pressure systems (Figure CU-02).
The Spring Transition
Pacific Northwest winters are characterized by their frequent rainfall and southwesterly winds. Southwest winds push water onshore and initiate downwelling. Downwelling brings relatively warm, nutrient-depleted surface water onshore and results in very low levels of primary production. The most critical seasonal plankton production cycle is when the ocean transitions from a winter downwelling state to a summer upwelling state. This time is known as the spring transition.
The spring transition marks the beginning of the upwelling season. Generally, the earlier that upwelling begins, the greater the ecosystem productivity will be. We can easily identify the actual day of transition when the transition to upwelling is sharp, and we see a clear shift to consistent upwelling. However, it is not uncommon for northerly winds (favorable to upwelling) to blow for a few days, followed by southwesterly winds and storms. Intense, late-season storms can erase any upwelling signature and reset the "seasonal clock" to a winter state, making the transition timing more obscure. There are several ways to index the date of the spring transition (Bakun, 1973; Bograd et al., 2009) that we outline below.
Physical Spring Transition
The physical spring transition is based on the cumulative upwelling index and is identified as the spring minima value. The spring transition falls on April 13 (Day 103) on average but can occur from early March to early June (PST-01).
Hydrographic Spring Transition
We developed the hydrographic spring transition to index the local response to wind forcing. This index captures the presence of cold, nutrient-rich water that will upwell at the coast with the onset of strong northerly winds, signaling the potential for high plankton production rates. We define the hydrographic spring transition by the date we observe near bottom water (50 m) colder than eight °C at our mid-shelf station (NH-5) during our fortnightly sampling off Newport, Oregon.
Other Measures of the Spring Transition:
- Dr. Mike Kosro, College of Earth, Ocean and Atmospheric Sciences (CEOAS), Oregon State University, operates an array of coastal radars designed to track the speed and direction of currents at the sea surface. He produces daily charts showing ocean surface current vectors. From those, one can see when surface waters are moving south (due to upwelling) or north (due to downwelling). By scanning progressive images, the user can visualize the date of transition.
- Dr. Steve Pierce and Dr. Jack Barth, CEOAS, Oregon State University, use local wind data from Newport, Oregon, and produce annual plots of the start and end of the upwelling season based on the change in alongshore wind stress.
- Logerwell et al. (2003) indexed the spring transition date based on the first day when the value of the 10–day running average for upwelling was positive and the value of the 10–day running average for sea level was negative.
- Dr. William Peterson, Northwest Fisheries Science Center, NOAA, identified the Biological Spring Transition by the arrival of cold-water copepod communities to the mid-shelf NH-5 station; this indicates the availability of lipid-rich food items for coho and Chinook salmon.
While seasonal upwelling delivers nutrients to the photic zone triggering phytoplankton blooms, the upwelling circulation also delivers water to the continental shelf that has low oxygen concentration. Following a phytoplankton bloom, the decomposing phytoplankton further draw down the oxygen on the continental shelf, creating regions that can be hypoxic.
Hypoxia is common in bottom waters across the continental shelf off Oregon and Washington during the summer months (Figure HYP-01). We define hypoxia as dissolved oxygen concentrations < 1.4 ml/L (Diaz et al. 2001). Hypoxic waters can be lethal to benthic invertebrates and may displace bottom-dwelling fish species (Grantham et al. 2004).
Along the Newport Hydrographic Line, hypoxic waters—that usually occupy the lower 10-30 meters of the water column—are most common during August through September (Figure HYP-01). Hypoxic bottom waters can cover the entire width of the shelf (Figure HYP-02) but are less common in shallower areas (< 30 m depth) where wind and wave action help mix and oxygenate the water column.