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Ocean Science An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 5 | Copyright
Ocean Sci., 14, 1207-1221, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Oct 2018

Research article | 12 Oct 2018

Diagnosing transit times on the northwestern North Atlantic continental shelf

Krysten Rutherford and Katja Fennel Krysten Rutherford and Katja Fennel
  • Department of Oceanography, Dalhousie University, 1355 Oxford Street, Halifax B3H 4R2, Nova Scotia, Canada

Abstract. The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a high-resolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf ( ∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high along-shelf transport to the southwest, and on the Grand Banks ( ∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence ( ∼ 12 months) and the Gulf of Maine ( ∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.

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Short summary
Using a regional model of the northwestern North Atlantic shelves, we calculate transport timescales and pathways in order to understand the transport processes that underlie the rapid oxygen loss, air–sea CO2 flux, and supply of plankton seed populations on the Scotian Shelf. Study results highlight the limited connectivity between the Scotian Shelf and adjacent slope waters; instead, the dominant southwestward currents bring Grand Banks and Gulf of St. Lawrence waters to the Scotian Shelf.
Using a regional model of the northwestern North Atlantic shelves, we calculate transport...