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.