The PDO is a long-term climate index based on patterns of variation in sea surface temperatures across the North Pacific Ocean. The PDO index correlates with atmospheric conditions, temperature and precipitation, stream flow, and commercial salmon landings in the Pacific Northwest. Wind patterns over the North Pacific determine the phase. When winds blow more from the southwest they push warmer water from lower latitudes toward the coast, creating warmer conditions. When winds blow more from the north, they drive a process called upwelling, where cooler, deeper water rises to the surface, lowering temperatures. The PDO is classified into two phases: warm, or positive PDO, and cold, or negative PDO. Understanding the phase of the PDO is useful for understanding how environmental changes may impact organisms across multi-year scales.

Unlike short-term weather events, the PDO changes slowly. Each phase can last decades and can vary drastically in duration. For example, a cold phase lasted roughly from 1947 to 1976 and was followed by a warm phase from 1977 to 1998. More recently, the pattern has become less stable, with shorter shifts between phases. Since 2020, the PDO has mostly remained in a negative (cool) phase, reaching some of its most extreme values in 2024 and 2025. When the PDO is in the negative phase, it can counteract sea surface warming, reducing the impact of climate change on marine organisms, in contrast when positive phases coincide with other climate indices and weather phenomena, extreme warming events can occur.

Similar to the PDO, the ONI has two phases, El Niño (positive ONI) and La Niña (negative ONI). The phase of the ONI is determined by sea surface temperatures in equatorial waters. During El Niño, warmer than average ocean temperatures are observed, while La Niña often presents cooler conditions relative to the west coast of the Americas.  When El Niño events occur, warm water is transported north along the west coast of the Americas, eventually reaching the Pacific Northwest. El Niño events cause a cascade of effects regionally: in addition to impacts to sea surface temperature, atmospheric teleconnections strengthen the Aleutian Low, which increases the frequency and intensity of winter storms in the Pacific Northwest. Altered wind patterns that normally drive coastal upwelling (the rise of cold, nutrient-rich water) become disrupted. This can increase sea surface temperature, decrease available nutrients, and majorly impact marine ecosystems.

Ocean temperatures in the northern California Current respond to climate influences at both large and local scales. Broad patterns like the PDO and ONI interact with regional factors such as wind patterns, air pressure, and the position of the jet stream to shape local ocean conditions. There is usually a delay of about 2 to 6 months between a shift in large-scale ocean conditions and the response in coastal waters. Ocean temperatures naturally vary between winter and summer, but these seasonal averages can be significantly altered during climate events. For instance, during strong El Niño events, temperatures can rise by as much as 2 °C above normal. Unusual events can amplify these effects. For example, the 2014–2016 marine heatwave (“the Blob”) caused high temperatures that exceeded even strong El Niño conditions. Temperature anomalies reflect the combined influence of global climate patterns and local ocean dynamics, with important consequences for marine ecosystems.

In general, cool ocean conditions support abundant food and lead to higher survival rates for juvenile salmon, while warm ocean conditions reduce food availability and lead to lower survival rates. Seasonally, local wind patterns drive upwelling and downwelling. Upwelling brings cold, nutrient-rich water to the surface, while downwelling results in warmer, nutrient-poor conditions. These shifting conditions affect the types of plankton and small fish available as food sources for salmon and other predators. When the food web changes, the impacts ripple through the system, influencing all trophic levels including birds, larger fish, and marine mammals.

Wind-driven processes determine the source of water entering the NCC. Water coming from the north tends to be cold, more salty, and nutrient-rich, supporting productive ecosystems. Water coming from the south or offshore is usually warmer, fresher, and nutrient-poor. Scientists track these differences using a “water type indicator,” which helps identify the kind of water reaching coastal ecosystems and what conditions will be present. Observations near the ocean bottom show clear patterns: during positive PDO and ONI phases, deeper coastal waters tend to be warmer and fresher while during negative PDO and ONI, these waters are typically colder and saltier, indicating more nutrient-rich conditions. Near-bottom temperature and salinity provide important clues about ocean circulation and nutrient supply, helping explain changes in marine productivity and species survival.

Coastal upwelling is a key ocean process that supports marine life along the Pacific Northwest, especially during the spring and summer months. From approximately April to September, strong northerly winds blow along the Oregon coast. These winds push surface ocean water away from the shore (offshore) and toward the south. As this surface water moves away, it is replaced by deeper water rising from below. This upwelled water is typically cold, more salty, and rich in nutrients.

When these nutrients reach the sunlit upper ocean (the photic zone), they fuel the growth of phytoplankton - tiny plant-like organisms that form the base of the marine food web. Phytoplankton support zooplankton, small forage fish, and larger predators, including salmon, seabirds, and marine mammals. Scientists measure the strength of upwelling using the Upwelling Index, which estimates how much surface water is being transported offshore based on wind patterns and atmospheric pressure. Positive values indicate stronger than average upwelling, meaning more nutrient rich water reaches the surface while negative values indicate weaker upwelling, which can reduce marine productivity.

In the Pacific Northwest, winter ocean conditions are shaped by frequent storms and southwesterly winds, which push surface water toward the coast. This process, called downwelling, brings relatively warm, nutrient-poor water onshore and results in low biological productivity. The spring transition is the point when the ocean shifts from this winter downwelling state to a summer upwelling state and marks the start of the productive season in the coastal ocean. During this shift, winds reverse direction and become northerly, upwelling begins, bringing cold, nutrient-rich water to the surface and this triggers increased plankton growth, supporting the marine food web. The timing of this transition is critical. An early transition leads to higher ecosystem productivity while a late or interrupted transition can delay or reduce food availability. Short periods of favorable winds may be interrupted by storms, temporarily returning conditions to a winter-like state and making the exact timing hard to identify. Researchers use several approaches to track when the transition occurs: wind-driven upwelling patterns, the arrival of cold nutrient-rich water near the seafloor, ocean current measurements showing when surface waters consistently move southward, and changes in plankton community composition. In addition to indicating the spring transition, the arrival of cold-water zooplankton species during this time provides high-quality food for juvenile salmon.

While coastal upwelling brings nutrient-rich water to the surface and supports plankton growth, it can also introduce low-oxygen water onto the continental shelf. After a phytoplankton bloom, much of the organic material sinks and begins to decompose. This process depletes oxygen in the water, which can lead to the formation of hypoxic conditions, areas where oxygen levels are very low. Off Oregon and Washington, these low-oxygen conditions are most common during the summer months. Hypoxia usually develops near the sea floor and in some cases it can spread across large areas of the continental shelf. It is less common in shallow coastal waters, where wind and waves help mix oxygen back into the water. Low-oxygen conditions can have serious effects on marine life. Hypoxia can be lethal to bottom-dwelling organisms such as crabs and other invertebrates, may force fish to move away from affected areas, and can disrupt food webs by reducing habitat availability.

Copepods are small zooplankton that drift with ocean currents and copepod biodiversity (or species richness) measures the number of unique copepod species. Species richness varies seasonally as different currents influence the region. During summer, cooler sub-Arctic waters dominate the NCC, containing copepod communities with relatively low diversity compared to winter when subtropical waters are transported northward, bringing a more diverse assemblage of copepod species. In addition to seasonal changes, copepod biodiversity responds to larger climate patterns such as PDO and ONI. Species richness tends to be lower during cool conditions, including negative PDO and La Niña events, and higher during warm periods or positive PDO and El Niño. These shifts in part reflect changes in the origin and characteristics of water reaching the Pacific Northwest. Long-term monitoring of copepod biodiversity helps scientists evaluate potential effects of climate change on marine ecosystems. Observations from the late 1990s through 2018 showed a general increase in species richness, a trend that may have been influenced by warming ocean conditions. However, species richness has declined since 2020, returning to levels similar to those observed in the late 1990s.

In addition to overall biodiversity, scientists track changes in the composition of copepod communities by comparing the abundance of northern (cold-water) and southern (warm-water) species. Because these groups are associated with different water masses, they provide valuable information about ocean conditions and the origins of water entering the NCC. Northern copepod species are typically associated with the Gulf of Alaska and Bering Sea while southern species are commonly found in offshore Oregon waters and along the coasts of central and southern California. Under typical conditions, cold-water copepods dominate Oregon and Washington coastal waters during summer, when upwelling brings sub-Arctic water into the region. During winter, warmer subtropical waters transported by the Davidson Current support a greater abundance of southern copepod species.

These shifts are ecologically important because the two copepod groups differ in their nutritional value. Several northern copepod species are rich in lipids that provide an important energy source for fish and other marine organisms. As a result, higher biomass of northern copepods generally indicates better feeding conditions for young salmon and other pelagic fish. Because copepods form a critical link between primary producers and higher trophic levels, changes in the balance between northern and southern species provide a useful indicator of ocean conditions and ecosystem health.

The biological spring transition is defined as the day when the copepod community changes from a (southern, warm-water) community to a summer (northern, cold-water) community. In addition to the transition from downwelling to upwelling outlined in the physical spring transition, the timing of the copepod community shift serves as a meaningful ecological indicator. It marks the onset of the productive season with a transition to favorable food for salmon, sardines, and sablefish in the form of large, lipid-rich copepods, euphausiids, and juvenile forage fish.

Ichthyoplankton are fish eggs and larvae that drift with ocean currents during the earliest stages of life. Young coho salmon, Chinook salmon, and steelhead enter coastal waters in early summer and feed primarily on small fish in their late larval and early juvenile stages (Brodeur et al., 2007; Daly et al., 2009; Daly et al., 2014). By tracking the abundance of fish larvae produced during winter spawning, scientists can estimate the amount of food that may be available to young salmon during this critical period of growth and survival (Daly et al., 2013). This information is used to calculate the Index of Coastal Prey Biomass (ICPB), an indicator of potential prey availability for juvenile salmon.

The ICPB is generally positive during colder ocean conditions, reflecting greater availability of prey for young salmon, and negative during warmer ocean conditions, when prey availability tends to be lower.

Ichthyoplankton community composition is also affected by environmental conditions. The ichthyoplankton community composition anomaly, used to track changes in the types of larval fish present, is also affected by changes in environmental conditions. The anomaly is typically negative during colder ocean conditions, with a higher composition of coastal taxa such as larval Pacific sand lance, smelts, and sculpins; and positive during warmer ocean conditions, when the community shifts to a composition of more offshore taxa such as larval rockfish, northern anchovies, and Pacific sardines.

The number of juvenile salmon captured during surveys can be used as an indicator of early ocean survival for yearling Chinook and coho salmon. The lowest June catches of Chinook and coho salmon have been observed during El Niño events, periods of weak coastal upwelling, and following the 2015-2016 marine heatwave and the highest June catches were recorded during years when the PDO was negative.