Local Physical Indicators
Regional physical conditions experienced by juvenile salmon entering the northern California Current.
Many scientists and managers have noted that marine survival variations of commercially important species such as salmon often correspond with periods of alternating cold and warm ocean conditions. For example, cold conditions are generally good for Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon, whereas warm conditions are not.
Local physical indicators change seasonally in response to shifts in the winds (e.g., upwelling and downwelling) and they also change in response to basin-scale atmospheric forcing captured by indices such as the PDO and ONI.
These local and basin-scale shifts are biologically important because they influence the source waters that feed into the northern California Current. Those, in turn, affect zooplankton and forage fish community types. That then impacts the abundance of salmon predators such as hake and sea birds.
We see a strong association between the PDO and local temperature anomalies. For example, there were four years of negative PDO values from late 1998 until late 2002. These PDO values closely match the negative SST anomalies measured off coastal Oregon and Washington. The timing of the positive PDO values also matches that of the positive SST anomalies.
This connection between PDO and temperature suggests that changes in basin-scale forcing result in local SST changes. Local changes may be due to differences in water transport out of the North Pacific into the northern California Current.
We also observe a time lag between a PDO sign change and a change in local SSTs. In 1998, the PDO changed to negative in July, and SSTs cooled in December. In 2002, we saw the opposite pattern: PDO signal changing to positive in August, followed by warmer SSTs in December. Thus, it takes 5–6 months for a basin-scale signal in the North Pacific to propagate to coastal waters.
We summarize temperature measurements made during surveys conducted every two weeks off Newport, Oregon, at station NH-5 in Figure TA-03. Seasonal averages for winter (Nov-Mar) and summer (May-Sep) can increase by up to 2 °C during El Nino events (1997-98). The seasonal averages have a cyclic pattern reflecting influences of both the PDO and ONI.
Deep–Water Temperature and Salinity
Shifts in the PDO phase can both express itself and be identified by the presence of different water types in the northern California Current. This led us to develop a "water type indicator," the value of which points to the type of water that will upwell at the coast. Cold, salty water of subarctic origin is nutrient-rich, whereas the relatively warm and fresh water of the offshore North Pacific Current is nutrient depleted.
Figure DTS-01 shows the summer average of salinity and temperature and Figure DTS-02 shows the seasonal progression of the average temperature and salinity values from near the bottom (50 m) at station NH-5 (shown in Figure HP-01).
The years 1997-1998 and 2014 were warmer than average. They corresponded to a warm-phase PDO and El Niño conditions during 1997-1998 and positive PDO during 2014 and the onset of a marine heatwave. During 1997-1998 and 2014, the water was also the freshest it has been during the period.
During the contrasting negative-phase PDO years of 1999-2002 and 2007-2008, these waters were cold and relatively salty or intermediate, as in 2009-2012.
Coastal upwelling affects primary productivity during the spring and summer. Upwelling is caused by northerly winds that blow along the Oregon coast from April to September. These winds transport offshore surface water southward and offshore, away from the coastline (orange arrow in Figure CU-01).
This offshore, southward transport of surface waters is balanced by onshore, northward transport of cool, high–salinity, nutrient-rich water (dark blue arrow) that brings nutrient-rich waters to the photic zone. This supports phytoplankton blooms that in turn provide food for zooplankton.
The Upwelling Index is a measure of water volume that upwells along the coast. The index identifies the amount of offshore transport of surface waters due to geostrophic wind fields that 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. Upwelling fluctuates from one year to the next (Figure CU-02) due to atmospheric shifts in low and high pressure systems.
Figure CU-03 illustrates the pattern of upwelling through the use of a cumulative upwelling plot. This method adds the amount of upwelling on one day to that of the next day, and so on. The plot begins on day one, on January first. Due to "downwelling" during winter months, upwelling values are increasingly negative for several weeks after day one. But with the onset of the spring transition and upwelling, the downward trend reverses, and the cumulative line trends upwards.
Physical Spring Transition
Pacific Northwest winters are characterized by their frequent rainfall and southwesterly winds. Southwest winds push water onshore and cause downwelling (the opposite of upwelling).
Downwelling brings relatively warm, nutrient-depleted surface water onshore and results in very low levels of primary production. The seasonal plankton production cycle's most critical time 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 and can occur between March and June. Generally, the earlier upwelling begins, the greater ecosystem productivity will be.
Identifying the Spring Transition
The transition is sharp in some years. We can easily identify the actual day of transition. In other years transition timing is more obscure. 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.
We can index the date of spring transition in several ways (further details can be found in Bakun (1973) and Bograd et al. (2009)). The average date of upwelling is April 13 (Day 103), but can range from early March to early June.
Hydrographic Spring Transition
We developed a new spring transition measure based on temperature measurements taken during our biweekly sampling cruises off Newport, Oregon. We define the spring transition as the date we observe deep water (50 m) colder than 8°C at the mid-shelf (station NH 05). The spring transition indicates 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.
Other measures of the spring transition include:
- 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 to 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. This index is no longer regularly updated and made available on-line.
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. The presence of 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.