Northern California Current System Research Articles

Subsurface acoustic ducts in the Northern California current system.

This study investigates the subsurface sound channel or acoustic duct that appears seasonally along the U.S. Pacific Northwest coast below the surface mixed layer. The duct has a significant impact on sound propagation at mid-frequencies by trapping sound energy and reducing transmission loss within the channel. A survey of the sound-speed profiles obtained from archived mooring and glider observations reveals that the duct is more prevalent in summer to fall than in winter to spring and offshore of the shelf break than over the shelf. The occurrence of the subsurface duct is typically associated with the presence of a strong halocline and a reduced thermocline or temperature inversion. Furthermore, the duct observed over the shelf slope corresponds to a vertically sheared along-slope velocity profile, characterized by equatorward near-surface flow overlaying poleward subsurface flow. Two potential duct formation mechanisms are examined in this study, which are seasonal surface heat exchange and baroclinic advection of distinct water masses. The former mechanism regulates the formation of a downward-refracting sound-speed gradient that caps the duct near the sea surface, while the latter contributes to the formation of an upward-refracting sound-speed gradient that defines the duct's lower boundary.

A decade-long cruise time series (2008–2018) of physical and biogeochemical conditions in the southern Salish Sea, North America

Abstract. Coastal and estuarine waters of the northern California Current system and southern Salish Sea host an observational network capable of characterizing biogeochemical dynamics related to ocean acidification, hypoxia, and marine heatwaves. Here, we compiled data sets from a set of cruises conducted in estuarine waters of Puget Sound (southern Salish Sea) and its boundary waters (Strait of Juan de Fuca and Washington coast). This data product provides data from a decade of cruises with consistent formatting, extended data quality control, and multiple units for parameters such as oxygen with different end use needs and conventions. All cruises obtained high-quality temperature, salinity, inorganic carbon, nutrient, and oxygen observations to provide insight into the dynamic distribution of physical and biogeochemical conditions in this large urban estuary complex on the west coast of North America. At all sampling stations, conductivity–temperature–depth (CTD) casts included sensors for measuring temperature, conductivity, pressure, and oxygen concentrations. Laboratory analyses of discrete water samples collected at all stations throughout the water column in Niskin bottles provided measurements of dissolved inorganic carbon (DIC), dissolved oxygen, nutrient (nitrate, nitrite, ammonium, phosphate, and silicate), and total alkalinity (TA) content. This data product includes observations from 35 research cruises, including 715 oceanographic profiles, with >7490 sensor measurements of temperature, salinity, and oxygen; ≥6070 measurements of discrete oxygen and nutrient samples; and ≥4462 measurements of inorganic carbon variables (i.e., DIC and TA). The observations comprising this cruise compilation collectively characterize the spatial and temporal variability in a region with large dynamic ranges of the physical (temperature = 6.0–21.8 ∘C, salinity = 15.6–34.0) and biogeochemical (oxygen = 12–481 µmol kg−1, dissolved inorganic carbon = 1074–2362 µmol kg−1, total alkalinity = 1274–2296 µmol kg−1) parameters central to understanding ocean acidification and hypoxia in this productive estuary system with numerous interacting human impacts on its ecosystems. All observations conform to the climate-quality observing guidelines of the Global Ocean Acidification Observing Network, the US National Oceanic and Atmospheric Administration's Ocean Acidification Program, and ocean carbon community best practices. This ongoing cruise time series supports the estuarine and coastal monitoring and research objectives of the Washington Ocean Acidification Center and US National Oceanic and Atmospheric Administration (NOAA) Ocean and Atmospheric Research programs, and it provides diverse end users with the information needed to frame biological impacts research, validate numerical models, inform state and tribal water quality and fisheries management, and support decision-makers. All 2008–2018 cruise time-series measurements used in this publication are available at https://doi.org/10.25921/zgk5-ep63 (Alin et al., 2022).

Characterizing Subsurface Oxygen Variability in the California Current System (CCS) and Its Links to Water Mass Distribution

AbstractThe california current system (CCS) supports a wide array of ecosystem services with hypoxia historically occurring in near‐bottom waters. Limited open ocean data coverage hinders the mechanistic understanding of CCS oxygen variability. By comparing three different models with varying horizontal resolutions, we found that dissolved oxygen (DO) anomalies in the CCS are propagated from shallower coastal areas to the deeper open ocean, where they are advected at a density and velocity consistent with basin‐scale circulation. Since DO decreases have been linked to water mass redistribution in the CCS, we conduct a water mass analysis on two of the models and on biogeochemical Argo floats that sampled multiple seasonal cycles. We found that high variability in biogeochemical variables (DO and nutrients) seen in regions of low variability of temperature and salinity could be linked to water mass mixing, as some of the water masses considered had higher gradients in biogeochemical variables compared to physical variables. Additional DO observations are needed, therefore, to further understand circulation changes in the CCS. We suggest that increased DO sampling north of 35˚N and near the shelf break would benefit model initialization and skill assessment, as well as allow for better assessment of the role of equatorial waters in driving DO in the northern CCS.

Diel Vertical Migrators Respond to Short‐Term Upwelling Events

AbstractPelagic organisms inhabiting coastal upwelling regions face a high risk of advection away from the nearshore productive habitat, potentially leading to mortality. We explored how animals remain in a productive yet highly advective environment in the Northern California Current System using the cabled observatory system located off the Oregon coast. Acoustic scatterers consistent with swimbladder‐bearing fish were only present during the downwelling season as these animals avoided the cold waters associated with strong upwelling conditions in summer and fall. Fish responded to short‐term upwelling events by increasing the frequency of diel vertical migration. Throughout the study, their vertical positions corresponded to the depth of minimum cross‐shelf transport, providing a mechanism for retention. The observed behavioral response highlights the importance of studying ecological processes at short timescales and the abilities of pelagic organisms to control their horizontal distributions through fine‐tuned diel vertical migration in response to upwelling.

Voices from the deep: Odontocete occurrence patterns relative to oceanographic conditions in the northern California Current System

Odontocetes play an important ecological role as apex predators in the northern California Current System (CCS), which is characterized by seasonal rises in primary productivity fueled by wind-driven upwelling of cold nutrient-rich water. This productivity sustains higher trophic-level prey optimal for deep-diving odontocetes including beaked and sperm whales. Due to the cryptic ecology of these whales, there are few systematic studies of their occurrence across seasons in northern CCS waters. The Holistic Assessment of Living marine resources off Oregon (HALO) Project—a collaboration between Oregon State University and Cornell University—addresses this gap via quarterly vessel-based visual surveys and year-round passive acoustic monitoring (PAM). Here, we detect and classify species-specific odontocete acoustic signals using long-term spectral averages, automated click detection, and unsupervised clustering of semi-continuous PAM data from 10/2021 to 12/2022. Site-specific occurrence of beaked and sperm whales is revealed on hourly to seasonal temporal scales at three recording sites along a depth gradient spanning the continental slope (300 and 630 m water depth) and abyssal plain (2860 m). By relating odontocete detections to dynamic oceanographic conditions from nearby profiling moorings, we identify likely drivers of prey availability influencing predator distribution. These results will inform conservation efforts of cryptic odontocetes in the face of rapid environmental change.

Drivers of Physical and Biological Frontal Variability in the Northern California Current System

AbstractOceanic fronts mark the boundary between two water masses and are often sites of complex bio‐physical processes and multi‐trophic level interactions, making them particularly important features in marine ecosystems. As global climate change induces multi‐scale shifts in the driving physical mechanisms of fronts, spatiotemporal tracking of frontal variability can aid in efforts to understand the downstream effects on marine biodiversity and ecosystem structure. Here we focus on fronts within the dynamic northern extent of the California Current System (NCC). We derived mesoscale sea surface temperature (SST) and chlorophyll‐a (chl‐a) fronts across the NCC region from 4‐km MODIS‐Aqua L3 daily fields over 2003–2019. Mesoscale physical (SST) and biological (chl‐a) fronts were often adjacent and coherent in their seasonal and interannual occurrence frequencies, but were spatially decoupled. SST fronts were most frequent and broadly distributed offshore while chl‐a fronts mostly occurred along the continental shelf break, particularly from Vancouver Island to central Oregon. Additionally, we employed a standardized multiple linear regression analysis to quantify the relative influence of local‐ and basin‐scale processes on frontal variability in the NCC. Local wind stress and wind stress curl variability were the most influential drivers of fronts over the shelf, while basin‐scale climate variability (i.e., climate oscillations) significantly drove frontal occurrences along the shelf break and offshore. Given predictions in the intensification of coastal upwelling in systems such as the NCC, our results indicate that oceanic response to climate change driven atmospheric variability will significantly impact the NCC marine ecosystem on the mesoscale.

Subsurface acoustic ducts in the Northern California Current System

A survey of sound speed profiles obtained from archived mooring and glider observations over the continental shelf, slope, and further offshore along the U.S. Pacific Northwest coastline reveals a shallow subsurface duct that appears at depths below the mixed layer. Sound propagation simulations using parabolic equation techniques demonstrate that the presence of the duct has a strong impact on sound propagation at 3.5 kHz. The duct is more prevalent in summer to fall than in winter to spring and extends to at least 200 km offshore from the shelf break. The depth of the minimum sound speed of the duct decreases onshore from between 80 m and 100 m offshore of the continental slope to less than 60 m over the shelf. The duct is also more prevalent offshore of the shelf break than over the shelf. Two processes potentially contribute to the formation of the subsurface duct: (1) differential advection of water masses driven by large-scale circulation within the northern California Current Systemand (2) wintertime surface cooling and convective deepening of the surface mixed layer. Data analysis conducted in this study suggests that the former is likely the dominant mechanism. [Work supported by the Office of Naval Research.]

Influence of Winter Subsurface on the Following Summer Variability in Northern California Current System

AbstractTemperature variations in the North and tropical Pacific contribute to the predictability of temperatures along the 26.4σ isopycnal layer off the Northern California Current System (N‐CCS). Monthly temperature variations at this depth in the N‐CCS are related to a linear combination of factors, including North Pacific spice anomalies, and the PDO and ENSO climate indices. However, the mechanisms for seasonal predictability of subsurface temperatures, are not well explored. While wind and buoyancy driven deep winter mixing influence subsurface temperatures during the following summer in the deep basin of the North Pacific, a coupled atmosphere‐ocean reanalysis (the CFSR) reveals that winter prior surface temperatures explain only 25% of the summer subsurface temperatures in the N‐CCS. A heat budget of the intermediate layer between a temporally varying mixed layer and the 26.4σ level is diagnosed here to explore the possible role of oceanic advection in explaining the remaining variance. Warmer waters from the south near the coast drive temperature changes in ENSO‐neutral winters, thereby preconditioning temperatures for the following summer. During ENSO winters, isopycnal variations associated with propagating coastal kelvin waves and other sources of heaving, along with anomalous alongshore currents, drive convergence/divergence of the advective fluxes, thereby reducing the local memory of the winter subsurface temperatures. Variations in winter advection could account for almost 36% of the summer subsurface temperature variability in the N‐CCS; this exceeds the portion explained by the heat fluxes associated with deep winter mixing.

Particle trajectories in an eastern boundary current using a regional ocean model at two horizontal resolutions

Lagrangian particle tracking (LPT) models are used to study the transport and dispersal of marine organisms. In LPT studies, the accuracy of the circulation is essential for nearshore habitats of Eastern Boundary Current (EBC) regions that are areas of high productivity and economically important fisheries. We used the California Current System as an example of an EBC region, specifically the Oregon coast located in the northern California Current System because it has distinct upwelling and downwelling regimes and variable shelf width. More specifically, we developed and applied a LPT model to compare and contrast particle drift patterns during the spring transition as it is an important period for spawning. We contrasted years (2016–18) using Regional Ocean Modeling System (ROMS) with different horizontal spatial resolutions (2 km, 250 m). Lagrangian particles experience stronger downward velocities and displacements to greater depths in the 250 m ROMS simulations that used a finer resolution bathymetry. Consequently, retention along the Oregon coast increases in the 250 m ROMS compared to the 2 km ROMS. After 10 days, 37%–83% of particles forced with the 2 km ROMS remain in the model domain, compared to 61%–86% of particles remaining when using the 250 m ROMS. Particles in the 250 m ROMS are advected to depth at specific times and locations for each simulated year, coinciding with the location and timing of a strong and shallow alongshore undercurrent that is not present in the 2 km ROMS. Additionally, ageostrophic dynamics close to shore, in the bottom boundary layer, and around headlands emerge in the 250 m resolution model, while they are at best poorly resolved in the 2 km resolution case. We conclude that the higher horizontal model resolution and bathymetry used in the 250 m ROMS generates well-resolved mesoscale and submesoscale features (e.g., surface, subsurface, and nearshore jet) that vary annually. These physical features are significantly different than those modeled by the 2 km model and may be responsible for these differences in particle dispersal. These results have implications for modeling the dispersal, growth, and development of coastal organisms with dispersing early life stages. • We compare ROMS of different horizontal resolutions in particle tracking models. • The higher resolution ROMS (250 m) better resolves mesoscale and submesoscale features. • The resolved mesoscale and submesoscale features may be due to higher resolution and bathymetry. • An increase in particle retention is linked to more resolved mesoscale features.

Subseasonal‐to‐Seasonal Forecast Skill in the California Current System and Its Connection to Coastal Kelvin Waves

AbstractAccurate dynamical forecasts of ocean variables in the California Current System (CCS) are essential decision support tools for advancing ecosystem‐based marine resource management. However, model and dynamical uncertainties present a significant challenge when attempting to incorporate these forecasts into a formal decision‐making process. To provide guidance on the reliability of dynamical forecasts, previous studies have suggested that deterministic climate processes associated with atmospheric or oceanic teleconnections may provide opportunities for enhanced forecast skill. Recent computational advances have led to the availability of subseasonal‐to‐seasonal (S2S) forecasts of key oceanic variables such as sea surface height (SSH), which may be leveraged to identify such “forecast opportunities.” In this study, we conduct a S2S forecast skill assessment of SSH anomalies in the CCS using an ensemble of 46‐day reforecasts made by the European Center for Medium‐range Weather Forecasting (ECMWF) model for the period 2000–2018. We find that the ECMWF model consistently produces skillful dynamical forecasts of SSH, particularly in both the southern and northern CCS at leads of 4–7 weeks. Using a high‐resolution ocean reanalysis, we develop a new index designed to characterize the location and intensity of coastally trapped waves propagating through the CCS. We then show that the S2S dynamical forecasts have enhanced skill in forecasts of SSH in weeks 4–7 when initialized with strong or extreme coastally trapped wave conditions, explaining 30–40% more SSH variance than the corresponding persistence forecast.

Influence of the 2014–2016 marine heatwave on seasonal zooplankton community structure and abundance in the lower Cook Inlet, Alaska

During the 2014–2016 Pacific marine heatwave (MHW), seabird die-offs, declines in forage fish populations, and the appearance of subtropical marine taxa (e.g. ocean sunfish, skipjack tuna) occurred in the northern Gulf of Alaska; however, the response of the zooplankton community to the MHW remains poorly understood. As part of the Gulf Watch Alaska program in Cook Inlet, zooplankton, phytoplankton, and a suite of oceanographic variables were collected monthly from 2012 to 2018. Surface zooplankton were collected via vertical tows (0–50 m) with a 333 μm bongo net along with CTD data and chlorophyll a concentration. Generalized additive models showed that seasonal zooplankton abundance peaked mid-June and declined into late January. During the MHW, monthly temperature anomalies ranged from +0.2 to +2.8 °C in 2015 and 2016, but monthly means of large lipid-rich calanoid copepods showed no clear declines in abundance compared to before and after the MHW suggesting that these copepods had some resiliency. An indicator species analysis using 88 of the most common zooplankton taxa produced five groups of zooplankton based on season. A BIO-ENV Best model showed that the environmental variables best explaining the observed zooplankton community structure included SST, temperature of the top 50 m of the water column, mixed layer depth (MLD), MLD temperature, and integrated chl a (ρ = 0.3491, Mantel Test: r = 0.3491, α < 0.05). Seasonal transitions of community groups displayed a phenological shift in 2016 when the late spring group of meroplanktonic larvae and the fall group of warm water copepods (WWC) typical of the Northern California Current system appeared earlier and persisted for longer compared to before and after the MHW. WWC abundance increased during the fall seasons of the MHW. Throughout 2015–2016, the elevated abundance of WWC compared to previous years and before/after the MHW suggested that warmer than average oceanographic conditions in the winter of 2015/2016 may have been sufficient to allow for an overwintering population in this area. A potential overwintering population of a warm water predatory copepod, Corycaeus anglicus, was observed and may have direct fine scale predator/prey impacts on zooplankton in this region.

State of the California Current 2019–2020: Back to the Future With Marine Heatwaves?

The California Current System (CCS) has experienced large fluctuations in environmental conditions in recent years that have dramatically affected the biological community. Here we synthesize remotely sensed, hydrographic, and biological survey data from throughout the CCS in 2019–2020 to evaluate how recent changes in environmental conditions have affected community dynamics at multiple trophic levels. A marine heatwave formed in the north Pacific in 2019 and reached the second greatest area ever recorded by the end of summer 2020. However, high atmospheric pressure in early 2020 drove relatively strong Ekman-driven coastal upwelling in the northern portion of the CCS and warm temperature anomalies remained far offshore. Upwelling and cooler temperatures in the northern CCS created relatively productive conditions in which the biomass of lipid-rich copepod species increased, adult krill size increased, and several seabird species experienced positive reproductive success. Despite these conditions, the composition of the fish community in the northern CCS remained a mixture of both warm- and cool-water-associated species. In the southern CCS, ocean temperatures remained above average for the seventh consecutive year. Abundances of juvenile fish species associated with productive conditions were relatively low, and the ichthyoplankton community was dominated by a mixture of oceanic warm-water and cosmopolitan species. Seabird species associated with warm water also occurred at greater densities than cool-water species in the southern CCS. The population of northern anchovy, which has been resurgent since 2017, continued to provide an important forage base for piscivorous fishes, offshore colonies of seabirds, and marine mammals throughout the CCS. Coastal upwelling in the north, and a longer-term trend in warming in the south, appeared to be controlling the community to a much greater extent than the marine heatwave itself.

Projected Shifts in 21st Century Sardine Distribution and Catch in the California Current

Predicting changes in the abundance and distribution of small pelagic fish species in response to anthropogenic climate forcing is of paramount importance due to the ecological and socioeconomic importance of these species, especially in eastern boundary current upwelling regions. Coastal upwelling systems are notorious for the wide range of spatial (from local to basin) and temporal (from days to decades) scales influencing their physical and biogeochemical environments and, thus, forage fish habitat. Bridging those scales can be achieved by using high-resolution regional models that integrate global climate forcing downscaled from coarser resolution earth system models. Here, “end-to-end” projections for 21st century sardine population dynamics and catch in the California Current system (CCS) are generated by coupling three dynamically downscaled earth system model solutions to an individual-based fish model and an agent-based fishing fleet model. Simulated sardine population biomass during 2000–2100 exhibits primarily low-frequency (decadal) variability, and a progressive poleward shift driven by thermal habitat preference. The magnitude of poleward displacement varies noticeably under lower and higher warming conditions (500 and 800 km, respectively). Following the redistribution of the sardine population, catch is projected to increase by 50–70% in the northern CCS and decrease by 30–70% in the southern and central CCS. However, the late-century increase in sardine abundance (and hence, catch) in the northern CCS exhibits a large ensemble spread and is not statistically identical across the three downscaled projections. Overall, the results illustrate the benefit of using dynamical downscaling from multiple earth system models as input to high-resolution regional end-to-end (“physics to fish”) models for projecting population responses of higher trophic organisms to global climate change.

Coastal processes modify projections of some climate-driven stressors in the California Current System

Abstract. Global projections for ocean conditions in 2100 predict that the North Pacific will experience some of the largest changes. Coastal processes that drive variability in the region can alter these projected changes but are poorly resolved by global coarse-resolution models. We quantify the degree to which local processes modify biogeochemical changes in the eastern boundary California Current System (CCS) using multi-model regionally downscaled climate projections of multiple climate-associated stressors (temperature, O2, pH, saturation state (Ω), and CO2). The downscaled projections predict changes consistent with the directional change from the global projections for the same emissions scenario. However, the magnitude and spatial variability of projected changes are modified in the downscaled projections for carbon variables. Future changes in pCO2 and surface Ω are amplified, while changes in pH and upper 200 m Ω are dampened relative to the projected change in global models. Surface carbon variable changes are highly correlated to changes in dissolved inorganic carbon (DIC), pCO2 changes over the upper 200 m are correlated to total alkalinity (TA), and changes at the bottom are correlated to DIC and nutrient changes. The correlations in these latter two regions suggest that future changes in carbon variables are influenced by nutrient cycling, changes in benthic–pelagic coupling, and TA resolved by the downscaled projections. Within the CCS, differences in global and downscaled climate stressors are spatially variable, and the northern CCS experiences the most intense modification. These projected changes are consistent with the continued reduction in source water oxygen; increase in source water nutrients; and, combined with solubility-driven changes, altered future upwelled source waters in the CCS. The results presented here suggest that projections that resolve coastal processes are necessary for adequate representation of the magnitude of projected change in carbon stressors in the CCS.

Biogeochemical variability in the California Current System

The biological productivity and diversity of the California Current System (CCS) is at the leading edge of major emerging climate trends, including hypoxia and acidification. We present results from a hindcast simulation (reanalysis) of an eddy-resolving oceanic physical-biogeochemical model of the CCS, to characterize its mean state and its patterns and drivers of variability in marine biogeochemical and ecosystem processes from 1995 to 2010. This is a companion paper to a physical analysis in Renault et al. (2021). The model reproduces long-term mean distributions of key ecosystem metrics, including surface nutrients and productivity and subsurface O2 and carbonate undersaturation. The spatial patterns of Net Primary Productivity (NPP) are broadly consistent with measured and remotely sensed rates, and they reflect a predominant limitation by nitrogen, with seasonal and episodic limitation by Fe nearshore in the central CCS, and in the open ocean northern CCS. The vertical distribution of NPP is governed by the trade-off between nutrient and light limitation, a balance that reproduces and explains the observed spatial variations in the depth of the deep Chl maximum. The seasonal to interannual variability of biogeochemical properties and rates is also well captured by model simulations. Because of the prevailing nutrient limitation, fluctuations in the depth of the pycnocline and associated nutricline are the leading single factor explaining interannual variability in the interior biogeochemical state, and the relationships between density and biogeochemical rates and tracers are consistent between model and observations. The magnitude and relationship between density structure and biogeochemical processes is illustrated by the 1997–98 El Niño event, which faithfully reproduces the single largest deviation from the mean state in the simulated period. A slower decadal shoaling of the pycnocline also accounts for the concomitant trends in hypoxic and corrosive conditions on the shelf. The resulting variability is key to understanding the vulnerability of marine species to oceanic change, and to the detection of such changes, soon projected to exceed the range of conditions in the past century.

Host-specific symbioses and the microbial prey of a pelagic tunicate (Pyrosoma atlanticum)

Pyrosomes are widely distributed pelagic tunicates that have the potential to reshape marine food webs when they bloom. However, their grazing preferences and interactions with the background microbial community are poorly understood. This is the first study of the marine microorganisms associated with pyrosomes undertaken to improve the understanding of pyrosome biology, the impact of pyrosome blooms on marine microbial systems, and microbial symbioses with marine animals. The diversity, relative abundance, and taxonomy of pyrosome-associated microorganisms were compared to seawater during a Pyrosoma atlanticum bloom in the Northern California Current System using high-throughput sequencing of the 16S rRNA gene, microscopy, and flow cytometry. We found that pyrosomes harbor a microbiome distinct from the surrounding seawater, which was dominated by a few novel taxa. In addition to the dominant taxa, numerous more rare pyrosome-specific microbial taxa were recovered. Multiple bioluminescent taxa were present in pyrosomes, which may be a source of the iconic pyrosome luminescence. We also discovered free-living marine microorganisms in association with pyrosomes, suggesting that pyrosome feeding impacts all microbial size classes but preferentially removes larger eukaryotic taxa. This study demonstrates that microbial symbionts and microbial prey are central to pyrosome biology. In addition to pyrosome impacts on higher trophic level marine food webs, the work suggests that pyrosomes also alter marine food webs at the microbial level through feeding and seeding of the marine microbial communities with their symbionts. Future efforts to predict pyrosome blooms, and account for their ecosystem impacts, should consider pyrosome interactions with marine microbial communities.

A Dynamically Downscaled Ensemble of Future Projections for the California Current System

Given the ecological and economic importance of eastern boundary upwelling systems like the California Current System (CCS), their evolution under climate change is of considerable interest for resource management. However, the spatial resolution of global earth system models (ESMs) is typically too coarse to properly resolve coastal winds and upwelling dynamics that are key to structuring these ecosystems. Here we use a high-resolution (0.1°) regional ocean circulation model coupled with a biogeochemical model to dynamically downscale ESMs and produce climate projections for the CCS under the high emission scenario, Representative Concentration Pathway 8.5. To capture model uncertainty in the projections, we downscale three ESMs: GFDL-ESM2M, HadGEM2-ES, and IPSL-CM5A-MR, which span the CMIP5 range for future changes in both the mean and variance of physical and biogeochemical CCS properties. The forcing of the regional ocean model is constructed with a “time-varying delta” method, which removes the mean bias of the ESM forcing and resolves the full transient ocean response from 1980 to 2100. We found that all models agree in the direction of the future change in offshore waters: an intensification of upwelling favorable winds in the northern CCS, an overall surface warming, and an enrichment of nitrate and corresponding decrease in dissolved oxygen below the surface mixed layer. However, differences in projections of these properties arise in the coastal region, producing different responses of the future biogeochemical variables. Two of the models display an increase of surface chlorophyll in the northern CCS, consistent with a combination of higher nitrate content in source waters and an intensification of upwelling favorable winds. All three models display a decrease of chlorophyll in the southern CCS, which appears to be driven by decreased upwelling favorable winds and enhanced stratification, and, for the HadGEM2-ES forced run, decreased nitrate content in upwelling source waters in nearshore regions. While trends in the downscaled models reflect those in the ESMs that force them, the ESM and downscaled solutions differ more for biogeochemical than for physical variables.

Fog and low-level stratus in coupled ocean-atmosphere simulations of the northern California Current System upwelling season

AbstractFog and low-level stratus during April through September 2009 are examined in a set of coupled ocean-atmosphere numerical simulations of the northern California Current System (CCS). The model configurations differ only in the choice of planetary boundary layer (PBL) parameterization scheme and, in one case, surface flux scheme. The results suggest that fog formation in this region primarily occurs through condensation at the surface induced locally by surface cooling, when moist offshore air is advected over cold upwelled waters and the shallow coastal marine PBL is further stabilized by warm, dry, continental air that extends offshore above the PBL inversion. These results are consistent with some but not all prior hypotheses for fog formation in the CCS region. Fog formation by downward growth of a pre-existing stratus layer is also found in the simulations but dominates only in those simulations with PBL schemes that produce an extensive and evidently unphysical stratus layer at 200-m height, which serves as the source for the downward growth. The stronger fog response in later summer months arises from seasonal warming of offshore SST, which increases the moisture content and temperature of the upstream air mass, while cool coastal SSTs are maintained by upwelling. On synoptic timescales, a similar influence of fog response on upstream conditions is found but controlled instead by changes in wind direction. These results suggest that the critical factors determining the evolution of the coastal fog regime in a warming climate are likely the temperature of upwelling source waters and the offshore flow of continental air.

Long-term patterns of mass stranding of the colonial cnidarian Velella velella: influence of environmental forcing

Velella velella is a pleustonic cnidarian noted worldwide for mass stranding of the colonial phase. Utilizing a 20 yr dataset (2000-2019; 23265 surveys) collected by the COASST citizen science program, we examined the spatio-temporal occurrence of mass strandings of V. velella along the Pacific Northwest coast from Washington to northern California, USA. V. velella mass strandings were documented in 14 years, with expansive events in 2003-2006 and 2014-2019. Events predominantly occurred in spring and were synchronous (April) among years, concurrent with shifts to prevailing onshore winds. Autumn mass stranding events occurred infrequently, with no consistent phenology (2005: November; 2014: August). In stranding years, reports of V. velella were mostly synchronous throughout the surveyed area, and events consistently spanned &gt;400 km of coastline, with highest reporting rates in the vicinity of the Columbia River plume, collectively suggesting extensive V. velella blooms throughout the northern California Current system in some years. Annual metrics of spring V. velella reporting rate (proportion of beaches; January-June) were modeled as a function of indices representing sea surface temperature anomaly (SSTa), easterly (onshore) wind speed, and regional upwelling. The best models (based on Akaike’s information criterion corrected for small sample size) indicated that SSTa averaged over the preceding winter (December-February) was positively correlated with spring reporting rate, suggesting that mass strandings of V. velella may be more prevalent in warmer years. As planetary warming continues, and V. velella strandings are easily recorded by citizen science programs globally, we suggest that stranding prevalence may be one relatively easy measure providing evidence for epipelagic ecosystem response.

Wintertime particulate organic matter distributions in surface waters of the northern California current system

Semi-automated sampling via the surface underway systems of research vessels was used to explore the distribution and composition of particulate organic matter in surface waters of the northern California Current ecosystem during winter, a poorly studied period that is characterized by downwelling favorable winds and elevated discharge by coastal rivers. New wintertime observations were compared to those measured during a summer cruise characterized by strong upwelling and highly reduced coastal river flows. Particulate organic carbon (POC) concentrations in surface waters along the Oregon shelf during winter periods were significantly lower (7.0 ± 5.0 μM) than those measured in the same region during summer (21 ± 25 μM) with similar seasonal contrasts in chlorophyll (Chl) concentrations (1.7 ± 1.7 mg Chl m−3 in winter and 8.7 ± 3.8 mg Chl m−3 in summer). The combination of POC, Chl, and particle beam attenuation (cp) measurements revealed spatial and temporal distributions that confirm the importance of physical drivers such as wind, waves and river discharge in influencing the biogeochemistry of eastern boundary current systems during non-upwelling conditions. Elevated contributions of allocthonous particulate materials with distinct compositional characteristics, including low Chl:POC and POC:cp ratios, were measured during winter in low-density, low-salinity surface waters influenced by coastal river discharge. In contrast, mid-salinity, intermediate-density surface waters exhibited higher concentrations of POM with elevated Chl:POC and POC:cp ratios, which approached those measured during highly productive summer upwelling periods. These results are solid evidence for wintertime phytoplankton productivity along this margin under elevated buoyancy and nutrient contributions from the discharge of coastal rivers.