Diagnosing transit times on the northwestern North Atlantic continental shelf

Rutherford, Krysten; Fennel, Katja

doi:10.5194/os-2018-60

Cited by 8 publications

(20 citation statements)

References 22 publications

(31 reference statements)

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“…The total yearly influx of ~211 Gmols N year −1 is comparable in magnitude to the GoM/GB TN inventory (205 Gmols N), suggesting a turnover time of the nitrogen pools of ~1 year. This falls into the range of studies by Rutherford and Fennel (), which reported the mean residence time of 251 days in the GoM, and the observed 1‐ to 2‐year transit time of slope water anomalies through the deep basins of the GoM by Hopkins and Garfield (). Inflows are balanced primarily by net outflows near the Northeast Channel, the southern flank of GB, and across the Nantucket transect.…”

Section: Resultssupporting

confidence: 63%

A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

et al. 2019

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The supply of nitrogen is a primary limiting factor for the productivity of the Northeast United States (NEUS) continental shelf. In this study, a 12‐year (1996–2007) retrospective physical‐biogeochemical simulation over the Northwest Atlantic was used to analyze the mean and seasonal NEUS shelf nitrogen budget, including the connections between shelf subregions: the Gulf of Maine/Georges Bank (GoM/GB), and the Mid‐Atlantic Bight (MAB). The model captures the primary mean and seasonal patterns of shelf circulation, nitrate, and plankton dynamics. Results confirm aspects of previous nitrogen budget analyses, including the dominance of offshore nitrogen influxes into the GoM/GB and the prominent role of riverine influxes and sedimentary denitrification in the MAB. However, detailed spatiotemporal analysis of nitrogen fluxes highlights the importance of dispersed inflows of shallow to intermediate depth waters (0–75 m), which can at times exceed the deep nitrogen influx emphasized in previous studies. A seasonal analysis shows a pronounced shift from the net import of nitrogen to the GoM/GB region during late fall and winter, to the net export of nitrogen from the region in the spring and early summer. The MAB, in contrast, consistently exports nitrogen to offshore waters. The prominence of the 0‐75m nitrogen supply has implications for the roles of Labrador Slope Water and Atlantic Temperate Slope Water on the NEUS ecosystems, as Atlantic Temperate Slope Water has greater nitrate concentrations than Labrador Slope Water at depth but often less at the surface. Results suggest the need for further study of shallow to intermediate depth inflows beyond those from the Scotian Shelf, particularly during the fall/winter of net nitrogen inflow.

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Section: Resultssupporting

confidence: 63%

A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

et al. 2019

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“…Dye and age tracers were implemented to track the transport pathways of FW from the Mississippi and Atchafalaya Rivers similar to previous ROMS applications (Hetland & Zhang, ; Rutherford & Fennel, ; Zhang et al, , ). This requires two additional tracers for each FW source: a passive dye and an active age tracer that keeps track of the “aging” of each dye since it departed from its source.…”

Section: Methodsmentioning

confidence: 99%

Quantifying the Relative Importance of Riverine and Open‐Ocean Nitrogen Sources for Hypoxia Formation in the Northern Gulf of Mexico

2019

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The Mississippi and Atchafalaya River System discharges large amounts of freshwater and nutrients into the northern Gulf of Mexico (NGoM). These lead to increased stratification and elevate primary production in the outflow region. Consequently, hypoxia (oxygen <62.5 mmol/m3), extending over an area of roughly 15,000 km2, forms every summer in bottom waters. High‐resolution models have significantly improved our understanding of the processes controlling hypoxia formation in the NGoM and have strongly implicated riverine nutrients as the dominant nutrient source. However, the relative importance of different nutrient sources (i.e., the Mississippi and Atchafalaya Rivers and offshore) has not been assessed before now. Here, we combine a high‐resolution model with an element tracing method to directly quantify the relative contributions of nitrogen from the two rivers and the open ocean to primary production and sediment oxygen consumption, which is the main oxygen sink contributing to hypoxia in the NGoM. Our results indicate that, averaged over 2001–2011, Mississippi and Atchafalaya nitrogen support 51 ± 9% and 33 ± 9% of summer sediment oxygen consumption, respectively, while open‐ocean nitrogen supports 16 ± 2%. The higher relative impact of Mississippi inputs results from longer transit times compared to those of Atchafalaya inputs. We also analyze the effect of riverine nitrogen load reductions and a larger diversion of discharge to the Atchafalaya River. These scenario simulations show that nutrient load reductions are most effective in mitigating hypoxia.

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“…South of Cape Hatteras, exchange between the shelf and open ocean across the shelf break is impeded by the presence of the Gulf Stream and occurs via baroclinic instabilities in its northern wall (Lee et al, 1991). In the MAB and on the Scotian Shelf, cross-shelf exchange is hindered by shelf-break jets and fronts (Rutherford and Fennel, 2018).…”

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confidence: 99%

Carbon cycling in the North American coastal ocean: A synthesis

Fennel¹,

Alin²,

Barbero³

et al. 2018

Preprint

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Abstract. A quantification of carbon fluxes in the coastal ocean and across its boundaries, specifically the air-sea, land-to-coastal-ocean and coastal-to-open-ocean interfaces, is important for assessing the current state and projecting future trends in ocean carbon uptake and coastal ocean acidification, but is currently a missing component of global carbon budgeting. This synthesis reviews recent progress in characterizing these carbon fluxes with focus on the North American coastal ocean. Several observing networks and high-resolution regional models are now available. Recent efforts have focused primarily on quantifying net air-sea exchange of carbon dioxide (CO2). Some studies have estimated other key fluxes, such as the exchange of organic and inorganic carbon between shelves and the open ocean. Available estimates of air-sea CO2 flux, informed by more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO2. This net uptake is driven primarily by the high-latitude regions. The estimated magnitude of the net flux is 160&#8201;&#177;&#8201;80&#8201;Tg&#8201;C/y for the North American Exclusive Economic Zone, a number that is not well constrained. The increasing concentration of inorganic carbon in coastal and open-ocean waters leads to ocean acidification. As a result conditions favouring dissolution of calcium carbonate occur regularly in subsurface coastal waters in the Arctic, which are naturally prone to low pH, and the North Pacific, where upwelling of deep, carbon-rich waters has intensified and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states during the upwelling season. Expanded monitoring and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.

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Diagnosing transit times on the northwestern North Atlantic continental shelf

Cited by 8 publications

References 22 publications

A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

Quantifying the Relative Importance of Riverine and Open‐Ocean Nitrogen Sources for Hypoxia Formation in the Northern Gulf of Mexico

Carbon cycling in the North American coastal ocean: A synthesis

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