The GLOBEC program was tasked with understanding the mechanistic links between climate forcing and the ocean-ecosystem response on the northern Gulf of Alaska (GOA) shelf. To address this task, ...samples were collected five to six times times annually along the Seward Line between 1998 and 2004. However, interpreting Seward-Line field observations in space and time is complicated by the complex circulation on the GOA shelf. The Alaska Current/Alaskan Stream and Alaska Coastal Current produce eddies and meanders which mix the iron-limited small-cell oceanic community with the iron-rich large-cell coastal community. Thus observations at any point in space and time are the result of the degree of mixing of the oceanic and coastal water masses. The ROMS circulation model with an embedded ecosystem model was used to extend GLOBEC observations in space and time on the GOA shelf. The timing of the spring bloom in simulations was related to shallowing of the pycnocline. The spring bloom began in late March–April on the inner shelf and in May on the mid and outer shelf. The simulations suggest that the magnitude of shelf production is a balance between the amount of iron from freshwater runoff and nitrate, with iron limitation on the outer shelf and adjacent ocean and nitrate limitation on the inner shelf. Simulated shelf-break eddies form near Yakutat, have elevated iron concentrations relative to surrounding waters, and propagate westward, influencing production and nitrate concentrations on the outer shelf and in the adjacent ocean during spring and summer. Simulated primary production in the Seward Line region was about 100–130gcm⁻²y⁻¹, but production of up to 300gcm⁻²y⁻¹ is predicted for regions in Lower Cook Inlet and around Kodiak.
The Dungeness crab (Metacarcinus magister) fishery is one of the highest value fisheries in the US Pacific Northwest, but its catch size fluctuates widely across years. Although the underlying causes ...of this wide variability are not well understood, the abundance of M. magister megalopae has been linked to recruitment into the adult fishery four years later. These pelagic megalopae are exposed to a range of ocean conditions during their dispersal period, which may drive their occurrence patterns. Environmental exposure history has been found to be important for some pelagic organisms, so we hypothesized that inclusion of environmental exposure history would improve our ability to predict inter-annual variability in M. magister megalopae occurrence patterns compared to using 'in situ' conditions alone. We combined eight years of local observations of M. magister megalopae and regional simulations of ocean conditions to model megalopae occurrence using a generalized linear model (GLM) framework. The modeled ocean conditions were extracted from J-SCOPE, a high-resolution coupled physical-biogeochemical model. The analysis included variables from J-SCOPE identified in the literature as important for larval crab occurrence: temperature, salinity, dissolved oxygen concentration, nitrate concentration, phytoplankton concentration, pH, aragonite and calcite saturation state. GLMs were developed with either in situ ocean conditions or environmental exposure histories generated using particle tracking experiments. We found that inclusion of exposure history improved the ability of the GLMs to predict megalopae occurrence 98% of the time. Of the five swimming behaviors used to simulate megalopae dispersal, four behaviors generated GLMs with the best fits to the observations, so a biological ensemble of these models was constructed. When the biological ensemble was used for forecasting, the model showed skill in predicting megalopae occurrence (AUC = 0.94). Our results highlight the importance of including exposure history in larval occurrence modeling and help provide a method for predicting pelagic megalopae occurrence. This work is a step towards developing a forecast product to support management of the fishery.
A primitive equation ocean general circulation model is used to investigate climate impacts in the North Pacific Ocean in the 1996 to 2003 period. The objective is to assess the model ability to ...reproduce observed modes of variability and study their impact in the northeast Pacific. This work is done within the framework of the U.S. Global Ecosystem (GLOBEC) Northeast Pacific Program studying the links between climate variability and ecosystem dynamics. Three large‐scale events are considered: The 1997/1998 El Niño, the 1999 “regime shift,” and the 2002 cold/fresh subsurface anomalous water mass that was observed in the Gulf of Alaska and off the coast of Oregon. The circulation model is shown to generate the correct seasonal to interannual large‐scale variability and is able to represent the climatic signals of interest in the eastern Pacific. We show that the influence of the 1997/1998 El Niño reached the coastal Gulf of Alaska and induced an increase in the upper ocean heat content along the coast of North America. An analysis of the sea surface temperature for the model years shows agreement between model and data in the representation of the 1999 shift to a cold phase in the eastern and northern North Pacific. Finally, using the model results, we speculate that the origin of the 2002 cold/fresh anomaly in the northeast Pacific was due to enhanced mixing during the preceding winter in the center of the Alaska gyre. Owing to anomalous changes in the density structure of the upper ocean, this water was able to move geostrophically toward the coast and it persisted in the northeast Pacific below the mixed layer the following year.
High‐latitude and subpolar regions like the Gulf of Alaska (GOA) are more vulnerable than equatorial regions to rising carbon dioxide (CO2) levels, in part due to local processes that amplify the ...global signal. Recent field observations have shown that the shelf of the GOA is currently experiencing seasonal corrosive events (carbonate mineral saturation states Ω, Ω < 1), including suppressed Ω in response to ocean acidification as well as local processes like increased low‐alkalinity glacial meltwater discharge. While the glacial discharge mainly influences the inner shelf, on the outer shelf, upwelling brings corrosive waters from the deep GOA. In this work, we develop a high‐resolution model for carbon dynamics in the GOA, identify regions of high variability of Ω, and test the sensitivity of those regions to changes in the chemistry of glacial meltwater discharge. Results indicate the importance of this climatically sensitive and relatively unconstrained regional freshwater forcing for Ω variability in the nearshore. The increase was nearly linear at 0.002 Ω per 100 µmol/kg increase in alkalinity in the freshwater runoff. We find that the local winds, biological processes, and freshwater forcing all contribute to the spatial distribution of Ω and identify which of these three is highly correlated to the variability in Ω. Given that the timing and magnitude of these processes will likely change during the next few decades, it is critical to elucidate the effect of local processes on the background ocean acidification signal using robust models, such as the one described here.
Key Points
Alkalinity of glacial meltwater is an important driver of the variability in aragonite saturation state of the nearshore waters, and is fairly unconstrained
Local winds, biological processes, and freshwater forcing all contribute to the spatial distribution of aragonite saturation state in the Gulf of Alaska
A set of spatially nested circulation models is used to explore interannual change in the northeast Pacific (NEP) during 1997–2002, and remote vs. local influence of the 1997–1998 El Niño on this ...region. Our nested set is based on the primitive equations of motion, and includes a basin-scale model of the north Pacific at ∼40-km resolution (NPac), and a regional model of the Northeast Pacific at ∼10-km resolution. The NEP model spans an area from Baja California through the Bering Sea, from the coast to ∼2000-km offshore. In this context, “remote influence” refers to effects driven by changes in ocean velocity and temperature outside of the NEP domain; “local influence” refers to direct forcing by winds and runoff within the NEP domain. A base run of this model using hindcast winds and runoff for 1996–2002 replicates the dominant spatial modes of sea-surface height anomalies from satellite data, and coastal sea level from tide gauges. We have performed a series of sensitivity runs with the NEP model for 1997–1998, which analyze the response of coastal sea level to: (1) hindcast winds and coastal runoff, as compared to their monthly climatologies and (2) hindcast boundary conditions (from the NPac model), as compared to their monthly climatologies. Results indicate penetration of sea-surface height (SSH) from the basin-scale model into the NEP domain (e.g., remote influence), with propagation as coastal trapped waves from Baja up through Alaska. Most of the coastal sea-level anomaly off Alaska in El Niño years appears due to direct forcing by local winds and runoff (local influence), and such anomalies are much stronger than those produced off California. We quantify these effects as a function of distance along the coastline, and consider how they might impact the coastal ecosystems of the NEP.
Although the Gulf of Alaska is subjected to intense downwelling through much of the year, during early spring and summer, upwelling due to local wind stress curl can occur over major portions of the ...shelf, resulting in high production. Satellite observations indicate that shallow banks may have substantially elevated chlorophyll concentrations relative to surrounding waters during much of the summer. We use the Regional Ocean Modeling System (ROMS) and ocean observation data to examine circulation and stratification around Portlock Bank in the Gulf of Alaska, and to explore mechanisms contributing to interannual variability in the supply of iron and nitrate onto the bank in spring and summer. ROMS at 3-km resolution is coupled to a lower trophic level biology model for the Gulf of Alaska; the coupled model is driven by tidal forcing, sub-daily atmospheric forcing, freshwater runoff, and boundary and initial conditions from Simple Ocean Data Assimilation (SODA) products. Hydrographic observations were made as part of six surveys undertaken by the GLOBEC/NEP (Global Ocean Ecosystem Dynamics/Northeast Pacific) program. Modeling results suggest that iron supply to the shallow layer around Portlock Bank is controlled by both advection and vertical diffusion processes, while nitrate supply is dominated by tidally-induced vertical diffusion. Overall, higher chlorophyll concentration in summer around Portlock Bank is attributed to strong vertical mixing, which pumps nutrients onto the bank from the flanks on either side, and from the top of the bank into the euphotic zone. Recirculation attributed to tidal effects increases residence time over the bank, further enhancing potential production, but intense mixing atop the bank in early spring can lead to light limitation of phytoplankton production. We used July 2004 cruise data from Portlock Bank to help verify model results.
Seasonal variation of the buoyancy‐ and downwelling‐wind‐forced Alaska Coastal Current (ACC) and the fate of freshwater contained in it is considered using idealized analytical and numerical models ...of the ACC formed from a half‐line source of buoyant inflow. The coastal current initially develops two‐dimensionally but becomes three‐dimensional from a balance between coastal influx of buoyancy and its downstream transport, which leads to a coastal current depth limit Hmax = ()1/2, where x is along‐shelf distance, Q is the line source strength for unit length, f is the Coriolis frequency, and g′ is the reduced gravity of the buoyant inflow. This limit is unchanged under downwelling wind stress and is reached on timescales of less than 1 month for the ACC. The coastal current width is roughly constant in x and increases in time at the same rate as the two‐dimensional solution. Imposition of a downwelling wind stress τ results in an approximate balance among wind stress and along‐ and cross‐shelf momentum advection so that the current width is reduced to Ywind ≈ LD ()1/2, where LD is the Rossby radius of deformation, τ is the wind stress and ρ0 is a reference density. Waves/eddying motions eventually grow in the half‐line source coastal current with wavelengths proportional to the coastal current width and with a downstream phase speed slower than the maximum current speed. These features cause an offshore flux of buoyant water, a broader coastal current, and further accumulation of buoyancy on the shelf. Increasing downwelling wind stress reduces the effects of the instabilities. Continual accumulation of buoyancy on the shelf occurs during all model runs but is nearly absent under maximum winter downwelling wind stress. It is suggested that freshwater accumulation on the shelf during spring, summer, and fall may be largely lost downstream during winter.
•Impacts of ocean acidification change with latitude in the California Current.•Vulnerable species (e.g., calcifying invertebrates) and their predators decline most.•Decline in revenue projected, ...mainly from lower Dungeness crab catch in the north.
Marine ecosystems are experiencing rapid changes driven by anthropogenic stressors which, in turn, are affecting human communities. One such stressor is ocean acidification, a result of increasing carbon emissions. Most research on biological impacts of ocean acidification has focused on the responses of an individual species or life stage. Yet, understanding how changes scale from species to ecosystems, and the services they provide, is critical to managing fisheries and setting research priorities. Here we use an ecosystem model, which is forced by oceanographic projections and also coupled to an economic input-output model, to quantify biological responses to ocean acidification in six coastal regions from Vancouver Island, Canada to Baja California, Mexico and economic responses at 17 ports on the US west coast. This model is intended to explore one possible future of how ocean acidification may influence this coastline. Outputs show that declines in species biomass tend to be larger in the southern region of the model, but the largest economic impacts on revenue, income and employment occur from northern California to northern Washington State. The economic consequences are primarily driven by declines in Dungeness crab from loss of prey. Given the substantive revenue generated by the fishing industry on the west coast, the model suggests that long-term planning for communities, researchers and managers in the northern region of the California Current would benefit from tracking Dungeness crab productivity and potential declines related to pH.
Three different global earth system models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) were used to explore anticipated changes in the Bering Sea under high (SSP126) and low ...(SSP585) carbon mitigation scenarios (i.e. low and high emission scenarios), via dynamical downscaling. A multivariate pattern analysis, based on Empirical Orthogonal Functions applied to monthly time series, reveals strong coupling of changes across several biophysical variables and the global forcing itself, on both yearly and multidecadal time scales. Rising air and ocean temperatures from the global models are strongly coupled with rising regional temperatures and reduced ice cover/thickness, as well as strong changes to the phenology of the plankton food chain, including reduced biomass of large zooplankton in the fall. This method ultimately provides a compact way to estimate the changes to many regional attributes under a variety of global change scenarios. Application of this method to a broad ensemble of the CMIP6 global model air temperatures suggests that compared to present conditions, the Bering Sea shelf bottom temperatures in July will warm by an average of ∼4 degrees C by the end of the 21st century under SSP585, as compared with ∼1 degrees C under SSP126, with greatest warming focused on the outer northern shelf.
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