P. St-Laurent

On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves

AbstractOceanic exchanges across the continental shelves of Antarctica play an important role in biological systems and the mass balance of ice sheets. The focus of this study is on the mechanisms responsible for the circulation of warm Circumpolar Deep Water (CDW) within troughs running perpendicular to the continental shelf. This is examined using process-oriented numerical experiments with an eddy-resolving (1 km) 3D ocean model that includes a static and thermodynamically active ice shelf. Three mechanisms that create a significant onshore flow within the trough are identified: 1) a deep onshore flow driven by the melt of the ice shelf, 2) interaction between the longshore mean flow and the trough, and 3) interaction between a Rossby wave along the shelf break and the trough. In each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of O(1 m yr−1). The third mechanism in particular reproduces several features revealed by moorings from M...

Impact of local winter cooling on the melt of Pine Island Glacier, Antarctica

The rapid thinning of the ice shelves in the Amundsen Sea is generally attributed to basal melt driven by warm water originating from the continental slope. We examine the hypothesis that processes taking place on the continental shelf contribute significantly to the interannual variability of the ocean heat content and ice shelf melt rates. A numerical model is used to simulate the circulation of ocean heat and the melt of the ice shelves over the period 2006–2013. The fine model grid (grid spacing 1.5 km) explicitly resolves the coastal polynyas and mesoscale processes. The ocean heat content of the eastern continental shelf exhibits recurrent decreases around September with a magnitude that varies from year to year. The heat loss is primarily caused by surface heat fluxes along the eastern shore in areas of low ice concentration (polynyas). The cold winter water intrudes underneath the ice shelves and reduces the basal melt rates. Ocean temperatures upstream (i.e., at the shelf break) are largely constant over the year and cannot account for the cold events. The cooling is particularly marked in 2012 and its effect on the ocean heat content remains visible over the following years. The study suggests that ocean-atmosphere interactions in coastal polynyas contribute to the interannual variability of the melt of Pine Island Glacier.

On the modification of tides in a seasonally ice-covered sea

[1] New observations from eight moorings located in Foxe Basin, Hudson Strait, and Hudson Bay are used to study the seasonal variability of the M 2 tide. Significant seasonal variations of the M 2 surface elevation are found in all these regions and at all seasons. The largest variations occur during winter while both elevation increase (Hudson Strait) and decrease (Hudson Bay, Foxe Basin) are observed. These variations are found recurrent at the stations where multiyear observations are available. Observations from a velocity profiler are consistent with a seasonal damping of the tides because of friction under ice. Numerical simulations with a sea ice-ocean coupled model and realistic forcing qualitatively reproduce most of the features of the observed variability. The simulations show that the winter M 2 variations are essentially caused by the under-ice friction, albeit with strong regional differences. Under-ice friction mostly occurs in a limited region (Foxe Basin) and can account for both increased and decreased M 2 elevations during winter.

Impacts of Atmospheric Nitrogen Deposition on Surface Waters of the Western North Atlantic Mitigated by Multiple Feedbacks

The impacts of Atmospheric Nitrogen Deposition (AND) on the chlorophyll and nitrogen dynamics of surface waters in the western North Atlantic (25–45°N, 65–80°W) are examined with a biogeochemical ocean model forced with a regional atmospheric chemistry model (Community Multi-scale Air Quality model, CMAQ). CMAQ simulations with year-specific emissions reveal the existence of a ‘hot-spot’ of AND over the Gulf Stream. The impact of the hot-spot on the oceanic biogeochemistry is mitigated in three ways by physical and biogeochemical processes. First, AND significantly contributes to surface oceanic nitrogen concentrations only during the summer period, when the stratification is maximal and the background nitrogen inventories are minimal. Second, the increase in summer surface nitrate concentrations is accompanied by a reduction in upward nitrate diffusion at the base of the surface layer. This negative feedback partly cancels the nitrogen enrichment from AND. Third, gains in biomass near the surface force a shoaling of the euphotic layer and a reduction of about 5% in deep primary production and biomass on the continental shelf. Despite these mitigating processes, the impacts of AND remain substantial. AND increases surface nitrate concentrations in the Gulf Stream region by 14% during the summer (2% on average over the year). New primary production increases by 22% in this region during summer (8% on average). Although these changes may be difficult to distinguish from natural variability in observations, the results support the view that AND significantly enhances local carbon export.

Impacts of Atmospheric Nitrogen Deposition and Coastal Nitrogen Fluxes on Oxygen Concentrations in Chesapeake Bay

Although rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and coastal DIN concentrations on the continental shelf can also significantly influence hypoxia; however, the relative impact of these additional sources of DIN on Chesapeake Bay hypoxia has not previously been quantified. In this study, the estuarine-carbonbiogeochemistry model embedded in the Regional-Ocean-Modeling-System (ChesROMS-ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN from the atmosphere has roughly the same impact on hypoxia as the same gram-for-gram change in riverine DIN loading, although their spatial and temporal distributions are distinct. DIN concentrations on the continental shelf have a similar overall impact on hypoxia as DIN from the atmosphere (~0.2 mg L ); however, atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, whereas coastal DIN concentrations primarily impact DO via the decomposition of allochthonous organic matter entering the Bay mouth from the shelf. The impacts of atmospheric DIN deposition and coastal DIN concentrations on hypoxia are greatest in summer and occur farther downstream (southern mesohaline) in wet years than in dry years (northern mesohaline). Integrated analyses of the relative contributions of all three DIN sources on summer bottomDO indicate that impacts of atmospheric deposition are largest in the eastern mesohaline shoals, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN concentrations are most influential in the polyhaline region. Plain Language Summary Most organisms living in the Chesapeake Bay, like fish, crabs, and oysters, need adequate oxygen concentrations to survive. However, general increases in the supply of nutrients to estuaries always enhance the production of algae, and the decomposition of these algae takes away oxygen from other organisms, resulting in hypoxic (low-oxygen) conditions or what is commonly referred to as a “dead zone.” Generally, researchers focus on how terrestrial nutrients entering the bay, for example, from fertilizer, wastewater treatment, or sewer runoff, produce the Chesapeake Bay dead zone, since they account for most of the nutrients entering the bay. However, the atmospheric and oceanic nutrients directly impacting the bay are often not accurately considered. In this study the impacts of nutrients from the atmosphere and the open ocean on Chesapeake Bay hypoxia are quantified via the application of a three-dimensional ecosystemmodel. Atmospheric deposition of nitrate is found to have the same gram-for-gram impact on hypoxia as terrestrial nitrate entering via rivers. Overall, these two sources of nutrients have the greatest impact in the summer and have similar impacts on dissolved oxygen, reducing oxygen concentrations by up to 0.2 mg L 1 in the mid-Chesapeake Bay region where oxygen concentrations are lowest.

Carbon Budget of Tidal Wetlands, Estuaries, and Shelf Waters of Eastern North America

Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here, we construct such a budget for Eastern North America using historical data, empirical models, remote-sensing algorithms, and process-based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they respectively make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.