Uptake of anthropogenic carbon dioxide from the atmosphere by the surface ocean is leading to global ocean acidification, but regional variations in ocean circulation and mixing can dampen or accelerate apparent acidification rates. Here we use a regional ocean model simulation for the years 1980 to 2013 and observational data to investigate how ocean fluctuations impact acidification rates in surface waters of the Gulf of Alaska. We find that large-scale atmospheric forcing influenced local winds and upwelling strength, which in turn affected ocean acidification rate. Specifically, variability in local wind stress curl depressed sea surface height in the subpolar gyre over decade-long intervals, which increased upwelling of nitrate- and dissolved inorganic carbon-rich waters and enhanced apparent ocean acidification rates. We define this sea surface height variability as the Northern Gulf of Alaska Oscillation and suggest that it can cause extreme acidification events that are detrimental to ecosystem health and fisheries.
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Abstract. The coastal ecosystem of the Gulf of Alaska (GOA) is especially vulnerable to the effects of ocean acidification and climate change. Detection of these long-term trends requires a good understanding of the system’s natural state. The GOA is a highly dynamic system that exhibits large inorganic carbon variability on subseasonal to interannual timescales. This variability is poorly understood due to the lack of observations in this expansive and remote region. We developed a new model setup for the GOA that couples the three-dimensional Regional Oceanic Model System (ROMS) and the Carbon, Ocean Biogeochemistry and Lower Trophic (COBALT) ecosystem model. To improve our conceptual understanding of the system, we conducted a hindcast simulation from 1980 to 2013. The model was explicitly forced with temporally and spatially varying coastal freshwater discharges from a high-resolution terrestrial hydrological model, thereby affecting salinity, alkalinity, dissolved inorganic carbon, and nutrient concentrations. This represents a substantial improvement over previous GOA modeling attempts. Here, we evaluate the model on seasonal to interannual timescales using the best available inorganic carbon observations. The model was particularly successful in reproducing observed aragonite oversaturation and undersaturation of near-bottom water in May and September, respectively. The largest deficiency in the model is its inability to adequately simulate springtime surface inorganic carbon chemistry, as it overestimates surface dissolved inorganic carbon, which translates into an underestimation of the surface aragonite saturation state at this time. We also use the model to describe the seasonal cycle and drivers of inorganic carbon parameters along the Seward Line transect in under-sampled months. Model output suggests that the majority of the near-bottom water along the Seward Line is seasonally undersaturated with respect to aragonite between June and January, as a result of upwelling and remineralization. Such an extensive period of reoccurring aragonite undersaturation may be harmful to ocean acidification-sensitive organisms. Furthermore, the influence of freshwater not only decreases the aragonite saturation state in coastal surface waters in summer and fall, but it simultaneously decreases the surface partial pressure of carbon dioxide (pCO2), thereby decoupling the aragonite saturation state from pCO2. The full seasonal cycle and geographic extent of the GOA region is under-sampled, and our model results give new and important insights for months of the year and areas that lack in situ inorganic carbon observations.more » « less
Abstract. Although Arctic marine ecosystems are changing rapidly,year-round monitoring is currently very limited and presents multiplechallenges unique to this region. The Chukchi Ecosystem Observatory (CEO)described here uses new sensor technologies to meet needs for continuous,high-resolution, and year-round observations across all levels of theecosystem in the biologically productive and seasonally ice-covered ChukchiSea off the northwest coast of Alaska. This mooring array records a broadsuite of variables that facilitate observations, yielding betterunderstanding of physical, chemical, and biological couplings, phenologies,and the overall state of this Arctic shelf marine ecosystem. While coldtemperatures and 8 months of sea ice cover present challenging conditions forthe operation of the CEO, this extreme environment also serves as a rigoroustest bed for innovative ecosystem monitoring strategies. Here, we presentdata from the 2015–2016 CEO deployments that provide new perspectives on theseasonal evolution of sea ice, water column structure, and physicalproperties, annual cycles in nitrate, dissolved oxygen, phytoplankton blooms,and export, zooplankton abundance and vertical migration, the occurrence ofArctic cod, and vocalizations of marine mammals such as bearded seals. Theseintegrated ecosystem observations are being combined with ship-basedobservations and modeling to produce a time series that documents biologicalcommunity responses to changing seasonal sea ice and water temperatures whileestablishing a scientific basis for ecosystem management.